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Multiple small beamlets in the delivery of highly modulated single-isocenter HyperArc VMAT plan can lead to dose delivery errors associated with small-field dosimetry, which can be a major concern for stereotactic radiosurgery for multiple brain lesions. Herein, we describe and compare a clinically valuable dynamic conformal arc (DCA)-based VMAT (DCA-VMAT) approach for stereotactic radiosurgery of multiple brain lesions using flattening filter free beams to minimize this effect. Original single-isocenter HyperArc style VMAT and DCA-VMAT plans were created on 7 patients with 2 to 8 brain lesions (total 35 lesions) for 10 MV- flattening filter free beam. 20 Gy was prescribed to each lesion. For identical planning criteria, DCA-VMAT utilizes user-controlled field aperture shaper before VMAT optimization. Plans were evaluated for conformity and target coverage, low- and intermediate dose spillages to brain volume that received more than 30% (V30%) and 50% (V50%) of prescription dose. Additionally, mean brain dose, V8, V12 and maximal dose to adjacent organs-at-risk (OAR) including hippocampi were reported. Total monitor units, beam modulation factor, treatment delivery efficiency, and accuracy were recorded. Comparing with original VMAT, DCA-VMAT plans provided similar tumor dose, target coverage and conformity, yet tighter radio-surgical dose distribution with lower dose to normal brain V30% (p = 0.009), V50% (p = 0.05) and other OAR including lower dose to hippocampi. Lower total number of monitor units and smaller beam modulation factor reduced beam on time by 2.82 min (p0.001), on average (maximum up to 3.8 min). Beam delivery accuracy was improved by 8%, on average (p0.001) and maximum up to 13% in some cases for DCA-VMAT plans. This novel DCA-VMAT approach provided excellent plan quality, reduced dose to normal brain, and other OAR while significantly reducing beam-on time for radiosurgery of multiple brain lesions-improving patient compliance and clinic workflow. It also provided less MLC modulation through the targets-potentially minimizing small field dosimetry errors as demonstrated by quality assurance results. Incorporating DCA-based VMAT optimization in HyperArc module for radiosurgery of multiple brain lesions merits future investigation.

When Delivery Quality Assurance (DQA) results do not coincide with specific criteria, re-measurement and re-treatment planning are required because incorrect beam delivery to the patient may occur. Therefore, an analysis of the plan parameters to find reasons for failure is essential. In this paper, we examine the effect of plan parameters on the DQA passing rate and clarify the plan parameters that should be adjusted mainly for each Tomotherapy when DQA fails and re-planning is required. Two versions of Tomotherapy treatment machines, Tomotherapy Hi-art and Radixact X9, were used. An independent t-test was performed to confirm the significant difference between the DQA passing rates of Tomotherapy Hi-art and Radixact X9. Also, a regression analysis was performed to analyze the correlation between plan parameters and passing rates. The average passing rate for Radixact X9 was 99.82 (standard deviation [SD] = 0.32) and that for Tomotherapy Hi-art was 98.46 (SD = 1.76). The passing rate of Radixact X9 was significantly higher than that of Tomotherapy Hi-art (P = 0.000). Statistically significant independent variables adopted from Tomotherapy Hi-art were IEC Yf (p = 0.025), the modulation factor (p = 0.003), and the number of control points (p = 0.006). The statistically significant parameter for Radixact X9 was IEC Xf (p = 0.000). According to the results, the new version of Tomotherapy(Radixact X9) has a relatively higher DQA pass rate than the old version (Tomotherapy Hi-art), and if the DQA fails and re-planning is required, each of the plan parameters can be adjusted to achieve a higher pass rate.

Introduction Helical TomoTherapy allows a highly conformal dose distribution to complex target geometries with a good protection of organs at risk. However, the small field sizes associated with this method are a possible source of dosimetrical uncertainties. The IAEA has published detector-specific field output correction factors for static fields of the TomoTherapy in the TRS483. This work investigates the average subfield size of helical TomoTherapy plans. Material and methods A new parameter for helical TomoTherapy was defined – the fluence-weighted average subfield size. The subfield sizes were extracted from the leaf-opening time sinograms in the RT-plan files for 30 clinical prostate and head and neck plans and were put in relation to Delat4 Phantom+ measurement results. Additionally the influence of planning parameters on the subfield size was studied by varying the modulation factor, number of iterations and pitch in the dose optimization and calculation for three different clinical indications H&N, prostate and rectum cancer . Selected plans were dosimetrically verified by Delta4 measurements to examine the reliability in dependence of the average subfield size. Furthermore, the impact of the planning parameters on a) the dose distribution, with regard to the planning target volume and regions at risks, and b) machine characteristics such as delivery time, actual modulation factor and leaf-opening times were evaluated. Results The average equivalent square subfield lengths ( s ¯ e q ) of the two investigated indications did not differ significantly – prostate plans: 2.75 ± 0.14 cm and H&N plans: 2.70 ± 0.16 cm, both with a jaw width of 2.5 cm. No correlation between field size and measured dose deviation was detected. The number of iterations and the modulation factor have a considerable influence on the average subfield size. The higher the planned modulation factor and the more iterations are used during optimization, the smaller is the subfield size. In our pilot study plans were calculated with field sizes s ¯ e q between 4.2 cm and 1.7 cm, with a jaw width of 2.5 cm. Again, the measurement results of Delta4 showed no significant deviation from the doses calculated by the TomoTherapy planning system for the whole range of subfield sizes, and no significant correlation between field sizes and dose deviations was found. As expected, the clinical dose distribution improved with increasing modulation factor and an increasing number of iterations. The compromise between an improved dose distribution and smaller s ¯ e q was shown. Conclusion In this work, a method was presented to determine the average subfield size for helical TomoTherapy plans. The response of the Delta4 did not show any dependence on field size in the range of the field sizes covered by the studied plans. The influence of the subfield sizes on other dosimetry systems remains to be investigated.

Objectives: Hippocampus-sparing whole-brain radiotherapy (HS-WBRT) using tomotherapy is known to provide a better dose distribution than volumetric-modulated arc therapy but requires an extended irradiation time. The present study aimed to investigate whether irradiation time can be shortened by reducing the modulation factor (MF) without losing the target dose distribution. Methods: Using six tilted computed tomography images in the head area, the planning target volume (PTV) and hippocampal doses, and the irradiation time was investigated with a jaw width of 1 cm, a pitch of 0.200, and the MF changed from 3.0 to 2.6, 2.2, 1.8, and 1.4. Results: No significant changes in the PTV or hippocampus were found with MF in the range from 3.0 to 1.8, but marked deterioration was found with that of 1.4. The irradiation time showed a linear relationship with the MF within the range from 3.0 to 1.8, with 1334, 1158, 986, and 817 s at modulation factors of 3.0, 2.6, 2.2, and 1.8, respectively. However, when the MF was 1.4, the irradiation time was 808 s. Conclusions: When HS-WBRT is performed with a tilted body position and a jaw width of 1 cm, with a MF of 1.8, a favorable balance between dose parameters and irradiation time is achieved, whereas with a MF of 1.4, the quality of the radiotherapy plan deteriorates, and the irradiation time is approximately the same as that with a MF of 1.8.

Purpose Since the modulation factor (MF) impacts both plan quality and delivery efficiency in tomotherapy Intensity Modulated Radiation Therapy (IMRT) treatment planning, the purpose of this study was to demonstrate a technique in determining an efficient MF from the Multileaf Collimator (MLC) leaf-open time (LOT) distribution of a tomotherapy treatment delivery plan. Methods Eight clinical plans of varying complexity were optimized with the highest allowed MF on the Accuracy Precision treatment planning system. Using a central limit theorem argument a range of reduced MFs were then determined from the first two moments of the LOT distribution. A step down approach was used to calculate the reduced-MF plans and plan comparison tools available on the Precision treatment planning system were used to evaluate dose differences with the reference plan. Results A reduced-MF plan that balanced delivery time and dosimetric quality was found from the set of five MFs determined from the LOT distribution of the reference plan. The reduced-MF plan showed good agreement with the reference plan (target and critical organ dose-volume region of interest dose differences were within 1% and 2% of prescription dose, respectively). Discussion Plan evaluation and acceptance criteria can vary depending on individual clinical expectations and dosimetric quality trade-offs. With the scheme presented in this paper a planner should be able to efficiently generate a high-quality plan with efficient delivery time without knowing a good MF beforehand. Conclusion A methodology for deriving a reduced MF from the LOT distribution of a high MF treatment plan using the central limit theorem has been presented. A scheme for finding a reduced MF from a set of MFs that results in a plan balanced in both dosimetric quality and treatment delivery efficiency has also been presented.

TomoTherapy® is a modern radiation treatment technique in which intensity-modulated radiation therapy (IMRT) is delivered in helical fashion. A two-dimensional (2D) array which has been existing for IMRT patient-specific quality assurance (PSQA) verifications for many years is I'MatriXX. Our objectives were to validate this I'MatriXX and to evaluate it for different patient sites and fractionation schedules of TomoTherapy treatment. Twenty-five plans were created with virtual target for different possible pitch values and field widths for validation. Gamma index criteria of 3%/2% dose differences and 3/2 mm distance to agreement were used. QA plans of 26 different treatment sites and different fractionation schedules were used. Results indicated that the matrix response is independent of field width, pitch, and modulation factor of TomoTherapy with 3%, 3 mm criteria. High passing rate ranging from 99.7% to 90.7% was observed for selected patient plans. We found that I'MatriXX 2D array can be utilized for easy and quick TomoTherapy PSQA.

There is no ideal detector-phantom combination to perform patient specific quality assurance (PSQA) for Total Marrow (TMI) and Lymphoid (TMLI) Irradiation plan. In this study, 3D dose reconstruction using mega voltage computed tomography detectors measured Leaf Open Time Sinogram (LOTS) was investigated for PSQA of TMI/TMLI patients in helical tomotherapy. The feasibility of this method was first validated for ten non-TMI/TMLI patients, by comparing reconstructed dose with (a) ion-chamber (IC) and helical detector array (ArcCheck) measurement and (b) planned dose distribution using 3Dγ analysis for 3%@3mm and dose to 98% (D98%) and 2% (D2%) of PTVs. Same comparison was extended for ten treatment plans from five TMI/TMLI patients. In all non-TMI/TMLI patients, reconstructed absolute dose was within ± 1.80% of planned and IC measurement. The planned dose distribution agreed with reconstructed and ArcCheck measured dose with mean (SD) 3Dγ of 98.70% (1.57%) and 2Dγ of 99.48% (0.81%). The deviation in D98% and D2% were within 1.71% and 4.10% respectively. In all 25 measurement locations from TMI/TMLI patients, planned and IC measured absolute dose agreed within ± 1.20%. Although sectorial fluence verification using ArcCHECK measurement for PTVs chest from the five upper body TMI/TMLI plans showed mean ± SD 2Dγ of 97.82% ± 1.27%, the reconstruction method resulted poor mean (SD) 3Dγ of 92.00% (± 5.83%), 64.80% (± 28.28%), 69.20% (± 30.46%), 60.80% (± 19.37%) and 73.2% (± 20.36%) for PTVs brain, chest, torso, limb and upper body respectively. The corresponding deviation in median D98% and D2% of all PTVs were 98% and D2%

Aim: The aim of the study was to find the most balanced plan with an optimal combination of planning parameters in helical tomotherapy (HT) for bilateral breast irradiation by evaluating dosimetric indices and time factors. In particular, we investigated the best combination of field width (FW), pitch, and modulation factor (MF). Materials and Methods: A total of 90 plans (18 plans for each patient) was created in this study, with different combination of planning parameters (FW: 2.5 cm [F1] and 5 cm [F2]; pitch: 0.215 [P1], 0.287 [P2], and 0.43 [P3]; and MF: 2.0 [M1], 2.5 [M2], and 3.0 [M3]). Plans were analyzed using several dosimetric indices: homogeneity index, conformity index, dose near minimum D98%, dose near maximum D2%, and the coverage by D95% of the target. Organ at risk (OAR) doses were evaluated by mean dose, V5Gy and V25Gy for the heart and mean dose V5Gy and V20Gy for both the lungs. Treatment time was also reported for all plans. Results: Reducing FW from 5 cm to 2.5 cm increased the treatment time by 40%–50% and improved homogeneity of the target. Tightening the pitch value from 0.43 to 0.215 improved target as well as OAR doses without increasing the treatment time. Increasing MF from 2 to 3 improved all the dosimetric indices and also increased treatment time. Conclusions: On the basis of our analysis, a plan with FW 5 cm, pitch 0.215, and MF 2.5 can be considered as an optimal combination of planning parameters for bilateral breast irradiation in HT technique.

Several studies have reported changes in dose distribution and delivery time based on the value of specific planning parameters [field width (FW), pitch, and modulation factor (MF)] in tomotherapy. However, the variation in the parameters between different facilities is unknown. The purpose of this study was to determine standard values of the above parameters for cases of head and neck cancer (HNC) and prostate cancer (PC) in Japan. In this survey, a web-based questionnaire was sent to 48 facilities performing radiation therapy with tomotherapy in March 2016. The deadline for data submission was April 2016. In the questionnaire, the values of the planning parameters usually used were requested and 23 responses were received, representing a response rate of 48% (23/48). The FW selected was 2.5 cm in most facilities, and facilities with a tomoEDGE license used dynamic FW rather than fixed FW. Facilities changed the pitch based on FW, dose per fraction, or target offset more frequently in HNC than in PC. In contrast, >50% of the facilities used the magic number proposed by Kissick et al. Median preset MFs (range, min to max) in HNC and PC were 2.4 (1.8-2.8) and 2.0 (1.8-3.0), respectively, and MF values showed large variations between the facilities. Our results are likely to be useful to several facilities designing treatment plans in tomotherapy.

A low modulation factor (MF) maintaining a good dose distribution contributes to the shortening of the delivery time and efficiency of the treatment plan in helical tomotherapy. The purpose of this study was to reduce the delivery time using initial values and the upper limit values of MF. First, patients with head and neck cancer (293 cases) or prostate cancer (181 cases) treated between June 2011 and July 2015 were included in the analysis of MF values. The initial MF value (MF initial) was defined as the average MF actual value, and the upper limit of the MF value (MFUL) was defined according the following equation: MFUL = 2 × standard deviation of MF actual value + the average MF actual Next, a treatment plan was designed for patients with head and neck cancer (62 cases) and prostate cancer (13 cases) treated between December 2015 and June 2016. The average MF actual value for the nasopharynx, oropharynx, hypopharynx, and prostate cases decreased from 2.1 to 1.9 (p = 0.0006), 1.9 to 1.6 (p < 0.0001), 2.0 to 1.7 (p < 0.0001), and 1.8 to 1.6 (p = 0.0004) by adapting the MF initial and the MFUL values, respectively. The average delivery time for the nasopharynx, oropharynx, hypopharynx, and prostate cases also decreased from 19.9 s cm−1 to 16.7 s cm−1 (p < 0.0001), 15.0 s cm−1 to 13.9 s cm−1 (p = 0.025), 15.1 s cm−1 to 13.8 s cm−1 (p = 0.015), and 23.6 s cm−1 to 16.9 s cm−1 (p = 0.008) respectively. The delivery time was shortened by the adaptation of MF initial and MFUL values with a reduction in the average MF actual for head and neck cancer and prostate cancer cases.

The workload for shielding purposes of modern linear accelerators (linacs) consists of primary and scatter radiation which depends on the dose delivered to isocenter (cGy) and leakage radiation which depends on the monitor units (MUs). In this study, we report on the workload for 10 treatment vaults in terms of dose to isocenter (cGy), monitor units delivered (MUs), number of treatment sessions (Txs), as well as, use factors (U) and modulation factors (CI) for different treatment techniques. The survey was performed for the years between 2006 and 2015 and included 16 treatment machines which represent different generations of Varian linear accelerators (6EX, 600C, 2100C, 2100EX, and TrueBeam) operating at different electron and x‐ray energies (6, 9, 12, 16 and 20 MeV electrons and, 6 and 15 MV x‐rays). An institutional review board (IRB) approval was acquired to perform this study. Data regarding patient workload, dose to isocenter, number of monitor units delivered, beam energies, gantry angles, and treatment techniques were exported from an ARIA treatment management system (Varian Medical Systems, Palo Alto, Ca.) into Excel spreadsheets and data analysis was performed in Matlab. The average (± std‐dev) number of treatment sessions, dose to isocenter, and number of monitor units delivered per week per machine in 2006 was 119 ± 39 Txs, (300 ± 116) × 102 cGys, and (78 ± 28) × 103 MUs respectively. In contrast, the workload in 2015 was 112 ± 40 Txs, (337 ± 124) × 102 cGys, and (111 ± 46) × 103 MUs. 60% of the workload (cGy) was delivered using 6 MV and 30% using 15 MV while the remaining 10% was delivered using electron beams. The modulation factors (MU/cGy) for IMRT and VMAT were 5.0 (± 3.4) and 4.6 (± 1.6) respectively. Use factors using 90° gantry angle intervals were equally distributed (~0.25) but varied considerably among different treatment techniques. The workload, in terms of dose to isocenter (cGy) and subsequently monitor units (MUs), has been steadily increasing over the past decade. This increase can be attributed to increased use of high dose hypo‐fractionated regimens (SBRT, SRS) and the increase in use of IMRT and VMAT, which require higher MUs per cGy as compared to more conventional treatment (3DCRT). Meanwhile, the patient workload in terms of treatment sessions per week remained relatively constant. The findings of this report show that variables used for shielding purposes still fall within the recommendation of NCRP Report 151.

TomoDirect has been reported to have some advantages over TomoHelical in delivering total body irradiation (TBI). This study aimed to investigate the relationships between the number of ports and the dose evaluation indices in low‐dose TBI in TomoDirect mode using 2–12 ports and to compare these data with those for the TomoHelical mode in a simulation study. Thirteen patients underwent low‐dose TBI in TomoHelical mode from June 2015 to June 2016. We used the same computed tomography data sets for these patients to create new treatment plans for upper‐body parts using TomoDirect mode with 2–12 beam angles as well as TomoHelical mode. The prescription was 4 Gy in two equal fractions. For the TomoDirect data, we generated plans with 2–12 ports with approximately equally spaced angles; the modulation factor, field width, and pitch were 2.0, 5.0 cm, and 0.500, respectively. For the TomoHelical plans, the modulation factor, field width, and pitch were 2.0, 5.0 cm, and 0.397, respectively. D2, D98, D50, and the homogeneity index (HI) were evaluated to compare TomoDirect plans having 2–12 ports with the TomoHelical plan. Using TomoDirect plans, D2 with four ports or fewer, D98 with 10 ports or fewer, D50 with four ports or fewer and HI with five ports or fewer showed statistically significantly worse results than the TomoHelical plan. With the TomoDirect plans, D2 with seven ports or more, D50 with eight ports or more, and HI with eight ports or more showed statistically significant improvement compared with the TomoHelical plan. All of the dose evaluation indices of the TomoDirect plans showed a tendency to improve as the number of ports increased. TomoDirect plans showed statistically significant improvement of D2, D50, and HI compared with the TomoHelical plan. Therefore, we conclude that TomoDirect can provide better dose distribution in low‐dose TBI with TomoTherapy.

Background This study investigated planned MLC distribution and treatment region specific plan parameters to recommend optimal delivery parameters based on statistical process techniques. Methods A cohort of 28 head and neck, 19 pelvic and 23 brain pre-treatment plans were delivered on a helical tomotherapy system using 2.5 cm field width. Parameters such as gantry period, leaf open time (LOT), actual modulation factor, LOT sonogram, treatment duration and couch travel were investigated to derive optimal range for plans that passed acceptable delivery quality assurance. The results were compared against vendor recommendations and previous publications. Results No correlation was observed between vendor recommended gantry period and percentage of minimum leaf open times. The range of gantry period (min–max) observed was 16–21 s for head and neck, 15–22 s for pelvis and 13–18 s for brain plans respectively. It was also noted that the highest percentage (average ( X - ) ± SD) of leaf open times for a minimum time of 100 ms was seen for brain plans (53.9 ± 9.2%) compared to its corresponding head and neck (34.5 ± 4.2%) and pelvic (32.0 ± 9.4%) plans respectively. Conclusions We have proposed that treatment site specific delivery parameters be used during planning that are based on the treatment centre and have detailed recommendations and limitations for the studied cohort. This may enable to improve efficiency of treatment deliveries by reducing inaccuracies in MLC distribution.

Aim: To study the effect of various planning parameters viz field width (FW), pitch factor (PF) on breast tumor and evaluate the relation among these parameters and their influence on the quality of plan and the treatment time. Material and method: Nine plans for each of the ten breast cancer patients were created taking various combinations of field width (FW) and pitch keeping the Modulation factor (MF) constant 3. The reference plan selected for the study is field width of 1, pitch 0.215. Other plans were created keeping the same constraints and penalty as the reference plan and 1000 iterations. Target coverage was evaluated in terms of conformity index (CN) and homogeneity index (HI) while the sparing of organs at risk (OARs) were evaluated in terms of mean, maximum and relevant dose volumes. Low dose spillages in normal tissues were also evaluated. The plans are compared with the reference plan using the Wilcoxon Signed Ranks Test. Concept of plan Quality index is also used to evaluate the plans qualitatively. Results: The mean treatment delivery time for 5, 2.5 and 1.0 cm FW plans were 5.04 ± 0.4, 8.51 ± 0.87 and 20.67 ± 2.02 minutes respectively. There was no significant variation in doses for OARs and COIN and HI of targets for 1 cm and 2.5 cm plans except for 5 cmplans. But the low dose spillage increased by 9-14% and 23-43% for 2.5 and 5 cm respectively. Conclusion: Dose to the OARs was not significantly higher for 2.5_FW plans from the reference plan but, significantly higher for 5_FW. Larger the FW and higher the pitch, spillage were more. On the basis of our analysis, 2.5_0.215 plan with MF=3, can be considered as the optimum plan for unilateral breast treatment using Tomotherapy.

Purpose Hippocampal avoidance during irradiation is hypothesized to reduce the risk of memory function decline. This study is designed to evaluate the dosimetric and clinical results of Helical Tomotherapy (HT) whole brain radiotherapy with hippocampal avoidance (WBHA) and Simultaneous Integrated Boost (SIB). Methods A total of 19 patients with 1 to 4 brain metastases (BM), diameter ⩽3 cm, absent or controlled extracranial disease were planned for WBHA with SIB using HT, from September 2014 to November 2017. The hippocampus was contoured on diagnostic T1-weighted magnetic resonance imaging (MRI) which had been registered with the planning CT image set. The hippocampal Planning Risk Volume was generated using a 5 mm uniform margin around the hippocampi. The BM were contoured on T1-weighted MRI registered with the planning computed tomography (CT), and the BM-PTV was the BM expanded 3 mm in all directions. The whole-brain planning target volume (WB-PTV) was defined as the whole-brain tissue volume, expanded 3 mm in all directions, minus hippocampal PRV and BM-PTVs. HT plans were generated with the following parameters: field width set at 1.05 cm, pitch 0.287, modulation factor (MF) set in the range 2–3 leading to a final value of 2 on average (range 1.3–2.8). Prescription dose was 30 Gy for WB-PTV and 40 Gy for each BM-PTVs in 10 fractions. Results Dosimetric result were reported in Table 1: all the parameter analysed were very promising and in accord to the RTOG 0933 trial [1] . The mean treatment time was 8.7 min (range 6.5–12.6 min). Conclusions HT plans achieved three objectives: quite homogeneous whole brain dose distribution; conformal hippocampal avoidance; and high coverage to the BM. Download : Download high-res image (323KB) Download : Download full-size image

Purpose Classical Kaposi’s sarcoma (KS) was usually treated with large parallel and opposed photon beam. To spare muscles and bones, a patient with an extended KS (from limbs to fits on both legs) was treated with Helical Tomotherapy (HT). Because of the skin involvement two polyurethane foam cushions (PFC) were applied as immobilization device and as bolus material, in a “shell” configuration. Methods The patient was positioned inside a PFC shell: the inner surface of the shell fitted the skin, in order to act as a bolus material. The CTV extended at least 5 mm inside legs; the PTV was the CTV expanded of 5 mm in all directions. A core structure distant 2 mm from the PTV was created for each leg and completely blocked in order to force HT to deliver only tangential beams to the PTV. The prescription was 60 Gy in 30 fraction; because of the length of target (1 m) the larger field width (5.02 cm), a pitch of 0.430, and a modulation factor of 2.0 were used. The irradiation time resulted in 15 min and the time required for the positioning and the MVCT was about 20 min. The surface doses was measured on the first session by means of EBT3 films: 24 films (2 cm × 2.5 cm) were placed all over the skin of the legs and feet. Results The dosimetric results were reported in Table 1. The density of PFC was measured from the CT and resulted in 0.0066 g/cm3. The measured doses ranged from 83% to 94% with a mean value of 88% (1.75 Gy). Conclusions In KS irradiation, the combination of a core blocked HT and the PSC shell offered an adequate dose coverage on the whole target, including the skin, spearing at the same time the muscles and skeleton. Download : Download high-res image (169KB) Download : Download full-size image

Quality assurance (QA) of the image quality for image‐guided localization systems is crucial to ensure accurate visualization and localization of regions of interest within the patient. In this study, the temporal stability of selected image parameters was assessed and evaluated for kV CBCT mode, planar radiographic k V, and MV modes. The motivation of the study was to better characterize the temporal variability in specific image‐quality parameters. The CATPHAN, QckV‐1, and QC‐3 phantoms were used to evaluate the image‐quality parameters of the imaging systems on a Varian Novalis Tx linear accelerator. The planar radiographic images were analyzed in PIPSpro with high‐contrast spatial resolution (f30,f40,f50lp/mm) being recorded. For OBI kV CBCT, high‐quality head full‐fan acquisition and pelvis half‐fan acquisition modes were evaluated for uniformity, noise, spatial resolution, HU constancy, and geometric distortion. Dose and X‐ray energy for the OBI were recorded using the Unfors RaySafe Xi system with the R/F High Detector for kV planar radiographic and the CT detector for kV CBCT. Dose for the MV EPID was recorded using a PTW975 Semiflex ion chamber, PTW UNIDOS electrometer, and CNMC Plastic Water. For each image‐quality parameter, values were normalized to the mean, and the normalized standard deviations were recorded to evaluate the parameter's temporal variability. For planar radiographic modes, the normalized standard deviations of the spatial resolution (f30,f40,& f50) were 0.015, 0.008, 0.004 lp/mm and 0.006, 0.009, 0.018 lp/mm for the kV and MV, respectively. The normalized standard deviation of dose for kV and MV were 0.010 mGy and 0.005 mGy, respectively. The standard deviations for full‐and half‐fan kV CBCT modes were averaged together. The following normalized standard deviations for each kV CBCT parameter were: 0.075 HU (uniformity), 0.071 HU (noise), 0.006 mm (AP‐geometric distortion), 0.005 mm (LAT‐geometric distortion), 0.058 mm (slice thickness), 0.124 (f50), 0.031 (HU constancy – Lung), 0.063 (HU constancy – Water), 0.020 (HU constancy – Bone), 0.006 mGy (Dose – Center), 0.004 mGy (Dose –Periphery). Using control chart analysis, institutional QA tolerances were reported as warning and action thresholds based on 1σ and 2σ thresholds. A study was performed to characterize the stability of image‐quality parameters recommended by AAPM Task Group‐142 for the Varian OBI and EPID imaging systems. Both imaging systems show consistent imaging and dosimetric properties over the evaluated time frame. PACS number: 87.10.‐e

Background and purpose To determine the optimum combination of treatment parameters between pitch, field width (FW) and modulation factor (MF) for extremity sarcomas in tomotherapy. Materials and methods Six patients previously treated for extremity sarcomas (3 arms and 3 legs) with tomotherapy were included in this study. 288 treatment plans were recalculated, corresponding to all combinations between 2 FW (2.5 and 5 cm), 4 MF (1.5, 2, 2.5 and 3) and 6 pitches (0.215, 0.287, 0.43 and 3 off-axis pitches). The treatment parameters (MF, FW or pitch) are modified between each plan, and the calculation is relaunched for 400 iterations, without modifying the optimisation constraints of the plan under which the patient has been treated. Results We suggest eliminating the 0.43 pitch and never combining a 0.215 pitch with an MF ≤ 2. We also do not recommend using an MF = 1.5 unless treatment time is an absolute priority over plan quality. We did not see any advantage in using Chen off-axis pitches, except for targets far from the axis (>15 cm) treated with a high pitch. A combination of MF = 2/FW = 5 cm/pitch = 0.287 gives plans of acceptable quality, combined with reduced treatment times. These conclusions are true only for extremity sarcomas treated in 2 Gy/fraction. Conclusions We have shown that the choice of pitch/MF/FW combination is crucial for the treatment of extremity sarcomas in tomotherapy: some produce good dosimetric quality with a reduced irradiation time, while others may increase the time without improving the quality.

A 55-year-old hypertensive patient presents atrial fibrillation after vasovagal syncope. Non-invasive cardiac workup is normal. Without antiarrhythmic therapy, the patient has no recurrence for the next 3years, then presents with a stroke. Echocardiography eventually reveals left atrial dilation. This sequence of events illustrates the well-known links between age, arterial hypertension, atrial fibrillation, atrial neuromyopathy and stroke. A frequently neglected common denominator in this equation is impaired sympathovagal balance. Contrary to what is often stated, autonomic imbalance is not a simple modulation factor of atrial fibrillation; both the trigger and the substrate of atrial fibrillation can be influenced by abnormal cardiac innervation. Here, we review the neurogenic theory of atrial fibrillation, based on literature and original data. We also provide evidence that this concept may help to improve atrial fibrillation prediction, early diagnosis and therapy.

The influence on target motion resulted from the respiratory process, the pitch and modulation factor presented in Tomotherapy. This study was aimed to investigate the effect of motion management using CT-images obtained from the fusion process between the static and dynamic scanning mode on the CIRS Thorax Phantom. The images were created using axial and helical modes which amplitudes were varied from 10 mm, 15 mm, and 20 mm. Then, the organ structures were planned using TomoPlan TPS with the variation of pitch ranging from 0.25 to 0.5, and the modulation factor (MF) in the range 2 to 3. Furthermore, evaluation of radiotherapy planning was performed using dose parameters on target, the organ at risk, homogeneity index (HI), and mean leaf open time (LOT). Target volume for static mode was 3.53 cc, whereas target volume has changed to 10.9 cc, 10.3 cc, 16.1 cc for axial and 6.6 cc, 8.5 cc, 14.9 cc for helical scanning. The HI value obtained was less than 0.07 and lower HI was resulted for a larger MF. It means that the higher modulation factor values will provide a uniform dose distribution to the target and lower dose to the OAR. The greater value of pitch gave the greater mean LOT. The optimum parameters were at the pitch of 0.5 and MF of 3 based on the evaluation of mean LOT and HI. Axial scanning mode provides a larger target volume compared to a helical scanning mode. Therefore, the increment of the target volume in delineating organ should be considered.

Background and aimThis study proposed a method to estimate the beam-on time for prostate cancer patients treated on Tomotherapy when FW (field width), PF (pitch factor), modulation factor (MF) and treatment length (TL) were given.Material and methodsThe study was divided into two parts: building and verifying the model. To build a model, 160 treatment plans were created for 10 patients. The plans differed in combination of FW, PF and MF. For all plans a graph of beam-on time as a function of TL was created and a linear trend function was fitted. Equation for each trend line was determined and used in a correlation model. Finally, 62 plans verified the treatment time computation model – the real execution time was compared with our estimation and irradiation time calculated based on the equation provided by the manufacturer.ResultsA linear trend function was drawn and the coefficient of determination R2 and the Pearson correlation coefficient r were calculated for each of the 8 trend lines corresponding to the adequate treatment plan. An equation to correct the model was determined to estimate more accurately the beam-on time for different MFs. From 62 verification treatment plans, only 5 disagreed by more than 60s with the real time from the HT software. Whereas, for the equation provided by the manufacturer the discrepancy was observed in 16 cases.ConclusionsOur study showed that the model can well predict the treatment time for a given TL, MF, FW and it can be used in clinical practice.

Helical tomotherapy plans using a combination of pitch and jaw width settings were developed for 3 patients previously treated for head and neck cancer. Three jaw widths (5, 2.5, and 1 cm) and 4 pitches (0.86, 0.43, 0.287, and 0.215) were used with a (maximum) modulation factor setting of 4. Twelve plans were generated for each patient using an identical optimization procedure (e.g., number of iterations, objective weights, and penalties, etc.), based on recommendations from TomoTherapy (Madison, WI). The plans were compared using isodose plots, dose volume histograms, dose homogeneity indexes, conformity indexes, radiobiological models, and treatment times. Smaller pitches and jaw widths showed better target dose homogeneity and sparing of normal tissue, as expected. However, the treatment time increased inversely proportional to the jaw width, resulting in delivery times of 24 ± 1.9 min for the 1-cm jaw width. Although treatment plans produced with the 2.5-cm jaw were dosimetrically superior to plans produced with the 5-cm jaw, subsequent calculations of tumor control probabilities and normal tissue complication probabilities suggest that these differences may not be radiobiologically meaningful. Because treatment plans produced with the 5-cm jaw can be delivered in approximately half the time of plans produced with the 2.5-cm jaw (5.1 ± 0.6 min vs. 9.5 ± 1.1 min), use of the 5-cm jaw in routine treatment planning may be a viable approach to decreasing treatment delivery times from helical tomotherapy units.

The purpose of present study is to investigate the decrease of delivery time for prostate cancer patient by using the helical type accelerator, Hi-Art System. The delivery time for Hi-Art System depends on planning parameters [pitch, modulation factor (MF) and field width (FW)], which are set by the operator at the beginning of the treatment planning. If you can allow for the deterioration of the dose distribution, the delivery time is able to decrease by increasing of FW and/or by decreasing of MF. On the other hands, as the use of 5.0 cm FW tends to increase the dose for the penile bulb, enough consideration for the dose distribution is needed. In addition, pitch should be set for the gantry rotation period not to become 15 s or less to prevent the increase of delivery time.

Helical tomotherapy (HT) can be used for the delivery of cranio-spinal axis irradiation (CSAI) without the need for beam matching of conventional linac-based external beam irradiation. The aim of this study is to retrospectively evaluate HT plans used for treatment in nine patients treated with CSAI. Helical tomotherapy cranio-spinal axis irradiation (HT-CSAI) plans were created for each patient. Average length along the cranio-spinal axis of the PTV was 65.6 cm with a range between 53 and 74 cm. Treatment planning optimization and plan evaluation parameters were obtained from the HT planning station for each of the nine patients. PTV coverage by the 95% isodose surface ranged between 98.0 to 100.0% for all nine patients. The clinically acceptable dose variation within the PTV or tolerance range was between 0.7 and 2.5% for all nine patients. Doses to the organs at risk were clinically acceptable. An increasing length along the longitudinal axis of the PTV did not consistently increase the beam-on time indicating that using a larger jaw width had a greater impact on treatment time. With a larger jaw width it is possible to substantially reduce the normalized beam-on treatment time without compromising plan quality and sparing of organs at risk. By using a larger jaw width or lower modulation factor or both, normalized beam-on times were decreased by up to 61% as compared to the other evaluated treatment plans. From the nine cases reported in this study the minimum beam-on time was achieved with a jaw width of 5.0 cm, pitch of 0.287 and a modulation factor of 2.0. Large and long cylindrical volumes can be effectively treated with helical tomotherapy with both clinically acceptable dose distribution and beam-on time.

determining the most desirable and achievable target dose and organ at risk (OAR) sparing using helical TomoTherapy planning system for mesothelioma treatment plans. A range of planning parameters was used. The reviewers' ranking assessment (Ranking in Groups: 1 = Good, 2 = Above Average, 3 = Average, 4 = Poor). The overall rankings revealed that a plan with a balanced trade-off among all planning objectives was preferred by most participants and reviewers. Other studies found low doses to the contralateral lung to be limiting. This was not the case in our study, with TomoTherapy we found the dose to contra lateral lung to be as low as V5Gy=0.87%. A pitch value of 0.287 or 0.43 would provide better result. A delivered modulation factor of above 1.7 and a treatment time around 500 sec will be beneficial consideration in planning.

Background: Modulation factor (MF) and pitch have an impact on Helical TomoTherapy (HT) plan quality and HT users mostly use vendor-recommended settings. This study analyses the effect of these two parameters on both plan quality and treatment time for plans made with TomoEdge planning software by using the concept of Pareto optimal fronts. Methods: More than 450 plans with different combinations of pitch [0.10-0.50] and MF [1.2-3.0] were produced. These HT plans, with a field width (FW) of 5 cm, were created for five head and neck patients and homogeneity index, conformity index, dose-near-maximum (D2), and dose-near-minimum (D98) were analysed for the planning target volumes, as well as the mean dose and D2 for most critical organs at risk. For every dose metric the median value will be plotted against treatment time. A Pareto-like method is used in the analysis which will show how pitch and MF influence both treatment time and plan quality. Results: For small pitches (≤0.20), MF does not influence treatment time. The contrary is true for larger pitches (≥0.25) as lowering MF will both decrease treatment time and plan quality until maximum gantry speed is reached. At this moment, treatment time is saturated and only plan quality will further decrease. Conclusion: The Pareto front analysis showed optimal combinations of pitch [0.23-0.45] and MF > 2.0 for a FW of 5 cm. Outside this range, plans will become less optimal. As the vendor-recommended settings fall within this range, the use of these settings is validated., SCOPUS: ar.j, info:eu-repo/semantics/published

Purpose: The overall aim was to conduct an analytical study of the impact of the modulation factor (MF) on the quality of the head and neck treatment plans and their execution time on Tomotherapy. Materials and Methods: In-phantom (RANDO® Alderson) planning study of the head and neck cancer was performed. Thirteen different plans in terms of MF were prepared. Other optimization parameters were the same for all plans. Results: Analysis of treatment plans in terms of quality shows that MF < 1.4 does not provide an accepted dose distribution (physician decision). Statistically significant differences were observed for plans with an MF < 1.6. No differences were obtained for plans with MF from 6.0 to 1.8. Decreasing of MF leads to a shorter time of irradiation. The maximum rotational speed has been reached for an MF = 3.0. Further reducing this however produces no decrease in the time of irradiation. The actual and planned values of the MF were compared. The optimal range of MF for head and neck was determined as 3.0 > MF > 1.8. The lower limit increases to 2.4 when hard reduction of the dose in critical organs is required. Conclusions: It was showed that the final MF value is less than the value calculated after each loop of optimization. The computer system reduces MF by shortening the longest time and increasing the average time of leaves opening. Increase in the average time is obtained by eliminating the use of leafs with the shortest times of opening, thereby reducing the dose in critical organs that are outside the direct irradiation area.

Purpose To report potential dose heterogeneity leading to underdosing at different skeletal sites in total marrow irradiation (TMI) with helical tomotherapy due to the thread effect and provide possible solutions to reduce this effect. Methods and Materials Nine cases were divided into 2 groups based on patient size, defined as maximum left-to-right arm distance (mLRD): small mLRD (≤47 cm) and large mLRD (>47 cm). TMI treatment planning was conducted by varying the pitch and modulation factor while a jaw size (5 cm) was kept fixed. Ripple amplitude, defined as the peak-to-trough dose relative to the average dose due to the thread effect, and the dose–volume histogram (DVH) parameters for 9 cases with various mLRD was analyzed in different skeletal regions at off-axis (eg, bones of the arm or femur), at the central axis (eg, vertebrae), and planning target volume (PTV), defined as the entire skeleton plus 1-cm margin. Results Average ripple amplitude for a pitch of 0.430, known as one of the magic pitches that reduce thread effect, was 9.2% at 20 cm off-axis. No significant differences in DVH parameters of PTV, vertebrae, or femur were observed between small and large mLRD groups for a pitch of ≤0.287. Conversely, in the bones of the arm, average differences in the volume receiving 95% and 107% dose (V95 and V107, respectively) between large and small mLRD groups were 4.2% ( P =.016) and 16% ( P =.016), respectively. Strong correlations were found between mLRD and ripple amplitude ( r s=.965), mLRD and V95 ( r s=−.742), and mLRD and V107 ( r s=.870) of bones of the arm. Conclusions Thread effect significantly influences DVH parameters in the bones of the arm for large mLRD patients. By implementing a favorable pitch value and adjusting arm position, peripheral dose heterogeneity could be reduced.

We examined the adequacy of existing shielding guidelines using five‐year clinical data from a busy CyberKnife center. From June 2006 through July 2011, 1,370 patients were treated with a total of 4,900 fractions and 680,691 radiation beams using a G4 CyberKnife. Prescription dose and total monitor units (MU) were analyzed to estimate the shielding workload and modulation factor. In addition, based on the beam's radiation source position, targeting position, MU, and beam collimator size, the MATLAB program was used to project each beam toward the shielding barrier. The summation of the projections evaluates the distribution of the shielding load. On average, each patient received 3.6 fractions, with an average 9.1 Gy per fraction prescribed at the 71.1% isodose line, using 133.7 beams and 6,200 MU. Intracranial patients received an average of 2.7 fractions, with 8.6 Gy per fraction prescribed at the 71.4% isodose line, using 133 beams and 5,083 MU. Extracranial patients received an average of 3.94 fractions, with 9.2 Gy per fraction prescribed at the 71% isodose line, using 134 beams and 6,514 MU. Most‐used collimator sizes for intracranial patients were smaller (7.5 to 20 mm) than for extracranial patients (20 to 40 mm). Eighty‐five percent of the beams exited through the floor, and about 40% of the surrounding wall area received no direct beam. For the rest of the wall, we found “hot” areas that received above‐average MU. The locations of these areas were correlated with the projection of the nodes for extracranial treatments. In comparison, the beam projections on the wall were more spread for intracranial treatments. The maximum MU any area received from intracranial treatment was less than 0.25% of total MU used for intracranial treatments, and was less than 1.2% of total MU used for extracranial treatments. The combination of workload, modulation factor, and use factor in our practice are about tenfold less than recommendations in the existing CyberKnife shielding guidelines. The current guidelines were found to be adequate for shielding, even in a busy center. There may be a potential to reduce shielding in areas with no or few direct beams in the current G4 model. Since a newer model CyberKnife (M6) has recently been introduced, the patterns of usage reported here may be changed in the future. The uneven distribution of use factor we have found may, however, be considered in the vault design. PACS number: 87.55.N

Purpose: To report cranio-spinal irradiation (CSI) planning experience, compare dosimetric quality and delivery efficiency with Tomotherapy from different institutions, and to investigate effect of planning parameters on plan quality and treatment time. Methods: Clinical helical tomotherapy IMRT plans for thirty-nine CSI cases from three academic institutions were retrospectively evaluated. The planning parameters: field width (FW), pitch, modulation factor (MF), and achieved dosimetric endpoints were cross-compared. A fraction-dose-delivery-timing index (FDTI), defined as treatment time per fraction dose per PTV length, was utilized to evaluate plan delivery efficiency. A lower FDTI indicates higher delivery efficiency. We studied the correlation between planning quality, treatment time and planning parameters by grouping the plans under specific planning parameters. Additionally, we created new plans using 5cm jaw for a subset of plans that used 2.5cm jaw to exam if treatment efficiency can be improved without sacrificing plan quality. Results: There were significant dosimetric differences for organ at risks (OARs) among different institutions (A,B,C). Using the lowest average MF (1.9±0.4) and 5cm field width, C had the highest lung, heart, kidney, liver mean doses and maximum doses for lens. Using the same field width of 5cm, but higher MF (2.6±0.6), B had lower doses to the OARs in the thorax and abdomen area. Most of A's plans were planned with 2.5cm jaw, the plans yielded better PTV coverage, higher OAR doses and slightly shorter FDTI compared to institution B. The replanned 5cm jaw plans achieved comparable PTV coverage and OARs sparing, while saving up to 44.7% treatment time. Conclusion: Plan quality and delivery efficiency could vary significantly in CSI planning on Tomotheapy due to choice of different planning parameters. CSI plans using a 5cm jaw, with proper selection of pitch and MF, can achieve comparable/ better plan quality with shorter delivery time compared to 2.5cm jaw plans.

Purpose: A new development in linac-based intracranial radiosurgery is treatment of multiple targets using single isocenter VMAT, which dramatically reduces treatment time while maintaining high conformality and dose gradient. Our purpose is to translate this technique to extracranial SBRT of oligometastases and address additional challenges such as interplay from motion and setup uncertainties. Methods: We propose SIDO (Single Isocenter with Distinct Optimizations) planning in which all VMAT arcs share an isocenter but each arc treats only one target. Setup uncertainties from rotations and deformations are mitigated by applying a couch translation between VMAT arcs, while interplay is minimized by using dynamic conformal arcs (DCAs) as the starting point for VMAT optimization. We compared SIDO to single (VMAT) and dual (VMAT & DCA) isocenter plans for phantom and patient (N=2) cases. Dose statistics included conformity index (CI), gradient index (GI), and modulation factor (MLC opening & total MU). Finally, we determined likelihood of needing a translational shift between SIDO arcs to correct rotational uncertainties using CBCTs from eleven previous SBRT cases. Results: For phantoms with target separations of 5–15cm, CI was 0.87±0.02, 0.89±0.05, and 0.91±0.02, for SIDO, single, and dual-isocenter VMAT respectively. GI was 6.83±1.0, 7.45±1.0 and 5.94±0.7 respectively. SIDO conformity did not trend with increasing distance between PTVs, while gradient decreased for all planning techniques. Using DCAs as the starting point of VMAT optimization decreased modulation, with a 39.4% reduction in monitor units. SIDO had 4.9% and 39% less MU than single and dual-isocenter VMAT respectively. A translational shift between SIDO arcs would be required in 10–28% of cases due to rotational uncertainties to stay within a 5mm margin, for targets separated by ≥10cm. Conclusion: SIDO for extracranial oligometastases has comparable dosimetry to traditional VMAT with low modulation similar to DCAs. Future work will quantify decrease in dosimetric interplay.

Helical TomoTherapy(HT) planning depends on optimize parameters including field width (FW), pitch factor (PF) and modulation factor (MF). These optimize parameters are effect to quality of plans and treatment time. The aim of this study was to find the optimized parameters which compromise between plan quality and treatment times. Six nasopharyngeal cancer patients were used. For each patient data set, 18 treatment plans consisted of different optimize parameters combination (FW=5.0, 2.5, 1.0 cm; PF=0.43, 0.287, 0.215; MF2.0, 3.0) were created. The identical optimization procedure followed ICRU83 recommendations. The average D50 of both parotid glands and treatment times per fraction were compared for all plans. The study show treatment plan with FW1.0 cm showed the lowest average D50 of both parotid glands. The treatment time increased inversely to FW. The FW1.0 cm the average treatment time was 4 times longer than FW5.0 cm. PF was very little influence on the average D50 of both parotid glands. Finally, MF increased from 2.0 to 3.0 the average D50 of both parotid glands was slightly decreased. However, the average treatment time was increased 22.28%. For routine nasopharyngeal cancer patients with HT, we suggest the planning optimization parameters consist of FW=5.0 cm, PF=0.43 and MF=2.0.

In bacteria, 70S ribosomes (consisting of 30S and 50S subunits) dimerize to form 100S ribosomes, which were first discovered in Escherichia coli. Ribosome modulation factor (RMF) and hibernation promoting factor (HPF) mediate this dimerization in stationary phase. The 100S ribosome is translationally inactive, but it dissociates into two translationally active 70S ribosomes after transfer from starvation to fresh medium. Therefore, the 100S ribosome is called the ‘hibernating ribosome’. The gene encoding RMF is found widely throughout the Gammaproteobacteria class, but is not present in any other bacteria. In this study, 100S ribosome formation in six species of Gammaproteobacteria and eight species belonging to other bacterial classes was compared. There were several marked differences between the two groups: (i) Formation of 100S ribosomes was mediated by RMF and short HPF in Gammaproteobacteria species, similar to E. coli, whereas it was mediated only by long HPF in the other bacterial species; (ii) RMF/short HPF-mediated 100S ribosome formation occurred specifically in stationary phase, whereas long HPF-mediated 100S ribosome formation occurred in all growth phases; and (iii) 100S ribosomes formed by long HPF were much more stable than those formed by RMF and short HPF.

Purpose: To investigate the effect on PTV target coverage of liver VMAT SBRT treatment plans by varying the IMRT modulation factor (MF) and length of delivery time in the presence of induced respiratory motion. Methods: Four different VMAT SBRT plans (MFs 2.1 to 3.5) were created for a liver phantom within the treatment planning system (Eclipse, Varian Oncology Systems). For each plan, the dose prescription was set to 5 different values (1000cGy to 5000cGy in 5 fractions); these variations force a change in the treatment delivery time due to reduce gantry speed required to deliver higher doses. To simulate respiratory motion, an in-house program was developed that shifts the positions of the MLC leaves at each control point according to population-based respiratory curves. This was done for 4 respiratory traces of comparable peak-to-peak amplitudes and breathing cycle lengths, with one trace exhibiting a baseline drift. A PTV margin of 5mm was used for all treatment plans. Results: A two-way repeatedmeasures ANOVA looked at changes in PTV V95 and GTV V95 coverage, based on IMRT factor and treatment time. The GTV V95 coverage and mean liver dose showed no significant difference (p values >0.05). The PTV V95 coverage showed a significant decrease with increasing IMRT factor and decreasing treatment time (p values < 0.01 for both). The patient curve with baseline drift exhibited a decrease in PTV coverage with increasing delivery time. Conclusion: Decreased PTV coverage was seen with increasing modulation factor and decreasing treatment times for patients with regular breathing curves. With the advent of faster treatment times it is recommended that lower modulation factors for SBRT treatment plans be utilized to mitigate the possibility of interplay. Additionally, longer treatment times could be advantageous to blur out the interplay effect for SBRT treatments. Further investigation is required in patient plans.

Purpose: The purpose of this study is to come up with the most desirable and achievable target dose and OAR sparing using helical TomoTherapy planning system based on the earlier conducted studies. Methods: Patient having mesothelioma with mediastinal nodes was planned using helical TomoTherapy with Gross tumor volumes (GTVs) outlined were investigated: The goal was (1) keeping the prescribed dose to the targets while reducing the dose to the OARs and (2) escalating the target dose while maintaining the original level of healthy tissue sparing. Results: The primary lesion and nodal mass were kept at 98.7% of the dose and planning target volume (PTV) margins of 3 mm inner and 1cm outer with the planned mean of 94% volume to 54 Gy. Mean lung dose is 15.6 Gy, V5Gy:33%, V20Gy:30.1%, with contralateral lung V5Gy:0.87% Heart V45:26.2%, PTV V54Gy:90.5%, Pitch: 0.43 and Modulation factor: 1.66 Conclusion: Based on many studies on the comparison of 3DCRT, IMRT and TomoTherapy, it is recommended that helical TomoTherapy provide better target coverage and sparing of OARs. Our data proved better PTV coverage. We planned a target dose of 54Gy but our institute would recommend re-evaluation at 45Gy. Helical TomoTherapy is a promising technique in the multimodality treatment of malignant pleural mesothelioma. Unlike other studies where low doses received by the contralateral lung appear to be the limiting factor, we found using helical TomoTherapy the contralateral to be minimal V5Gy:0.87%.

Purpose: Important limitations for dose painting are due to treatment planning and delivery constraints. The purpose of this study was to determine the sensitivity of dose conformity to optimization parameters using the clinical TomoTherapy Hi‐Art treatment planning system (TPS). Materials and Methods: Uptake data from a patient who underwent a [ 61 Cu ]Cu‐ATSM PET/CT (surrogate of hypoxia) scan was retrospectively extracted for treatment planning. Extracted data was reformatted from voxel‐based to level‐based for compatibility with the Hi‐Art TPS. Optimized treatment plans were generated with physical objectives for each prescription sub‐volume modifying pitch, jaw width, modulation factor and iteration number. Effects of variations in plan parameters were simulated varying one physical parameter while keeping the other parameters unchanged. Avoidance structures were not used. The conformity of treatment plans to their non‐uniform prescriptions was evaluated via quality‐volume histograms (QVH) and percent receiving planned dose within 2 percent of prescription (Q0.98–1.02). Results: In general, the conformity of treatment plans to dose prescriptions was found to be adequate for delivery of dose painting plans. The conformity was better as the jaws decreased in width from 2.5 cm to 1.0 cm (Q0.98–1.02:64% vs 73%) and as the pitch increased from 0.122 to 0.443 (Q0.98–1.02:71% vs 73%). The effects on dose conformity of modulation factor and iteration number seem to be insensitive to change. Treatment delivery times varied from 5–11 minutes for changes in field width (2.5 cm to 1.0 cm) and from 11–25 minutes for changes in pitch (0.443 to 0.122). Conclusions: This investigation demonstrates the ability of the helical tomotherapy to create and deliver plans with non‐uniform dose distributions. Results indicate that agreement in prescription dose and planned dose distributions for all treatment plans are sensitive to physical parameter changes such as pitch and field width but insensitive to changes in modulation factor and iteration number.

Purpose: To determine the feasibility of reducing Tomotherapy treatment delivery time by planning with increased pitch and reduced modulation factor (MF). Method and Materials: Twelve patients with Tomotherapy treatment sites (head/neck, prostate, prostate/nodes, lung) and fraction size of 1.8–2.5 Gy originally planned with a low pitch (< 0.3) and fast gantry period (< 20 s) were replanned. The first replan increased the pitch to 0.43 and used the original MF (MF0). If the first replan was acceptable and the gantry period increased, the patient was replanned again with pitch = 0.43 and a lower MF to reduce treatment time. This was repeated until the plan (DVH) became unacceptable or the gantry period reached its minimum (15 s). If the first replan was not satisfactory or if the gantry period did not increase, the MF was increased by 0.5 to improve the plan and to test if the treatment time was still reduced. Results: For all patients, we were able to produce a DVH comparable to the original DVH using pitch = 0.43 and MF0. When comparing the original DVH to the accelerated plan DVH that most closely matched it, treatment times were reduced by 13–66%. Comparing these accelerated plans to the original plans showed the PTV median dose increase 0.4±0.5 Gy, insignificant changes in target coverage and homogeneity index, an average change in critical structure median dose of −0.8±1.3 Gy, and a maximum critical structure median dose increase of 1.3 Gy. Conclusion: Faster treatment times can be achieved by planning with increased pitch and reduced modulation. Treatments planned with pitch = 0.43 and MF0 showed little or no plan degradation and most had faster treatment times. Further reduction in treatment time was possible by reducing the MF. Conflict of Interest: Some of the authors have financial interest in TomoTherapy, Inc.

Purpose: To investigate the influence of pitch and jaw width selection on helical tomotherapy treatment plans for head and neck. Method and Materials: Helical tomotherapy plans using a combination of pitch and jaw width settings were developed for three patients previously treated for head and neck cancer. Three jaw widths (5, 2.5 and 1 cm) and six pitches (0.86, 0.7, 0.43, 0.34, 0.287 and 0.215) were used with a (maximum) modulation factor setting of 4. The 18 plans per patient were generated using an identical optimization procedure (e.g., number of iterations, objective weights and penalties, etc.), based on recommendations from TomoTherapy. The plans were compared using isodose plots, dose volume histograms, dose homogeneity indices, conformity indices, radiobiological models, and treatment times. Results: Smaller pitches and jaw widths showed better target dose homogeneity and sparing of normal tissue, as expected. The conformity index had a maximum for pitches between 0.287 and 0.43. The tumor control probabilities were greater than 99% for the 2.5‐ and 1‐cm jaw widths for all pitches except 0.86. The treatment time increased inversely proportional to the jaw width. Conclusion: Our study indicates that the smaller jaw widths (2.5 and 1 cm) and pitches near the range of 0.3–0.4 produce better plans for helical tomotherapy head and neck treatment. However, because of the large increase in treatment time with only slight improvement in sparing of the critical structures for the 1‐cm jaw width, the 2.5‐cm jaw width, currently used in our clinic, remains our preference. Supported in part by a research agreement with TomoTherapy, Inc.

Purpose: The planning process for helical tomotherapy plans is governed by three basic planning parameters: field length, pitch ratio, and modulation factor. The purpose of this work is to evaluate the influence of each parameter on the achievable plan. Method and Materials: Using commercial treatment planning software (TomoTherapy, Inc.) the same head and neck case was re‐planned by the same person using different planning parameters. The standard settings (field length of 25 mm, pitch of 0.287 and modulation factor of 2.5) were used to generate a baseline plan. Using the standard settings for two parameters the third parameter was varied for different plans. Three field lengths (10, 25, 50 mm), four pitch ratios (0.2, 0.287, 0.5, 1) and seven modulation factors were used (1.5, 2, 2.5, 4, 6, 8, 10). The plans were evaluated in terms of treatment time, target coverage, conformity, and sensitive structure sparing. Results: Increasing the pitch ratio leads to an increase in the volume of unspecified tissues that receives prescription dose (less conformal plans) and an increase in the dose to sensitive structures. The treatment time did not significantly reduce with increasing pitch (there are fewer gantry rotation but the gantry rotation period increases). Similarly, an increased modulation reduces the dose to unspecified tissue and sensitive structures. However the cost is an increased treatment time. Using a larger field length increases the dose to unspecified tissue and sensitive structures while reducing the treatment time. All plans achieved acceptable target coverage and homogeneity. Conclusions: Changing the planning parameters mainly affects plan conformity. It appears that with respect to our standard settings a slight reduction of the pitch or a slight increase in the modulation factor should improve plan quality with a modest increase in treatment time. Conflict of Interest: One co‐author is employed by TomoTherapy, Inc.

difference in event-free survival due to increased mortality in the high dose arm. This was thought to be a consequence of toxicity from the high dose per fraction. Total marrow irradiation (TMI) with helical tomotherapy (HT) may be a solution to the problem of dose escalation in TBI. This treatment planning study investigated the feasibility of TBI with simultaneous total red marrow (rTMI) boost using HT. Materials/Methods: For treatment planning, a partial whole body CTimage was obtained from a previously HT-TBI treated patient. Organs at risk (OAR) were outlined. The CTV-TBI represented the whole body less the kidneys, lungs and red marrow distribution.Theredmarrow CTV(CTV-rTMI)wasthe axialskeleton(skull,spine,sternum,flat bones)andtheproximalendsof the humeri and femurs. The prescription (15.75 Gy, 2.25 Gy/fraction, 7 fractions) was set to cover 80% of CTV-rTMI. Simultaneously, a second prescription (12.25 Gy, 1.75 Gy/fraction, 7 fractions) was set to cover 80% of CTV-TBI. The prescription to the CTV-TBI is biologically equivalent to 12 Gy in 6 fractions assuming an alpha-beta of 10 Gy 1 . Based on our clinical HT-TBI experience, a pitch of 0.287, a jaw width of 5 cm, a normal dose calculation grid and a modulation factor of 2.0 were selected. Constraints to OAR were set such that the lungs and kidneys received # 9 Gy and # 10 Gy, respectively. A dose homogeneity index or DHI (defined as the dose ratio received by 90% of the volume to the minimum dose to 10% of the volume) $ 0.85 was considered clinically acceptable based on our HT-TBI experience. Constraints were adjusted accordingly during the optimization process. Initial beamlet dose calculation time was 7 hours. Time between iterations was approximately 40 seconds. About 20 iterations were required to view the impact of constraints’ changes. Optimization timewas approximately 120 minutes. Results: Eighty percent of the CTV-TBI and CTV-rTMI received 13.3 Gy and 15.4 Gy, respectively. Ninety-five percent of the CTV-TBIand CTV-rTMI received 12.6Gy and12.7 Gy, respectively. Average doses (Gy) to the CTV-TBI,CTV-rTMI,lungs,left kidney andrightkidneywere 13.9,15.7, 8.7, 10.0and10.0, respectively. DHIwas 0.87 and0.86for the CTV-rTMI andCTV-TBI, respectively. Beam on time was 1190 seconds. Conclusions: Treatment planning for TBI with simultaneous rTMI boost is feasible. Prescription and DHI goals were met with areductionofdosetoOARsofupto45%.WeproposeaclinicaltrialtotestthepotentialbenefitsofrTMIboostusingthisapproach. Author Disclosure: J.A. Penagaricano: None. E.G. Moros: None. P.M. Corry: None. V. Ratanatharathorn: None.

Purpose: To perform an inter‐comparison of TPS results for different IMRT delivery systems. Methods: Patient data with prostate cancer was selected. Same geometries and energy were employed for fixed gantry angle plans. Treatment includes simultaneous irradiation, prostate to 82Gy and seminal vesicles to 64Gy (41 fractions). Plans were normalized to achieve 82Gy minimum dose to prostate. Plan 1 : step&shoot with 1cm leaf‐width (MLC‐Optifocus), TPS Konrad v2.2 (Siemens). Plan 2: same plan 1 but with TPS iPlan v4.1 (BrainLAB). Plan 3: step&shoot with 0.25cm leaf‐width (mMLC‐Moduleaf, Siemens), TPS iPlan v4.1. Plan 4: dynamic with 0.5cm leaf‐width (MLC‐Millenium), TPS Eclipse v8.1 (Varian). Plan 5: Tomotherapy TPS v3.1.4.7. Longitudinal field size 2.5cm, pitch 0.277 and modulation factor 1.783, leaf width 6.25mm Results: Total numbers of MU were 508, 488, 570, 577 and 3219 for Plan 1 to 5. D98% (V82Gy) for PTV prostate were 81.6Gy (95.7%), 80.7Gy (94.9%), 81.8Gy (97.4%), 80.1Gy (84.6%) and 79.9Gy (82.6%) for Plan 1 to 5. V64Gy for PTV seminal vesicle were 98.2%, 99.9%, 100%, 92.1% and 97.8% for Plan 1 to 5. Rectum EUD (V40Gy) were 65.1Gy (56.6%), 62.7Gy (53.1%), 61.9Gy (44.3%), 63.3Gy (55.7%) and 61.9Gy (39%) for Plan 1 to 5. Rectum wall EUD was 66.4Gy, 65.2Gy, 64.8Gy, 64.9Gy and 65.2Gy for Plan 1 to 5. Bladder EUD (V65Gy) were 47.9Gy (27.4%), 47.4Gy (24.8%), 46.6Gy (24.6%), 46.2Gy (24.4%) and 45.8Gy (20.1%) for Plan 1 to 5. Femoral head D10% were 31.8Gy, 34.5Gy, 33.9Gy, 39.3Gy and 26.1Gy for Plan 1 to 5. Conclusions: MUs are lower with step&shoot MLC. MicroMLC produces slightly better conformation of PTVs. Better sparing of rectum and bladder with mMLC and Tomotherapy. For rectum wall EUD shows that mMLC produce the better results. Better sparing of femoral head is obtained with Tomotherapy. All techniques give the OAR's similar EUD (+/−2Gy), except for femoral heads.

Purpose: To analyze the sensitivity of a plan‐class specific correction factor to dose distribution in nonstandard field dosimetry. Methods: Ten different TomoTherapy‐based IMRT fields were created on a water‐filled PMMA cylindrical phantom at the center of which the absorbed dose to water was measured. Dose distribution of each IMRT field was evaluated using three parameters: modulation factor (MF), modified conformal index (COIN) and homogeneity index (HI). The relative dose in each IMRT field was measured using a PTW micro liquid ion chamber. Based on the new dosimetry formalism, the plan‐class specific correction factor was measured for two 0.6 cm3 Farmer‐type chambers (Exradin A12 and NE2571) and a 0.06 cm3 Exradin A1SL chamber. We investigated the dependence of the measured correction factor on the three plan evaluation parameters. Results: Uncertainty on the measurement of the plan‐class specific correction factor was 0.2–0.4 % and 0.2–0.7 % for the Farmer‐type chambers and Exradin A1SL, respectively. When the dose homogeneity was better than 5 % along with values of MF and modified COIN smaller than 2.0 and 0.025, respectively, the correction factor of the IMRT field was not different from unity by more than 0.7 %. For more heterogeneous IMRT fields, the correction factor deviated from unity by up to 2.4 % for the Farmer‐type chambers because of increasing residual gradient effects. For the Exradin A1SL, while it was closer to unity, the correction factor was highly variable in different IMRT fields due to a more significant impact of positioning uncertainties on the response of the smaller chamber. Conclusions: We have shown that the plan‐class specific correction factor can be characterized as a function of plan evaluation parameters, especially for Farmer‐type chambers. This work provides a recipe based on quantifying dose distribution to accurately select air‐filled ionization chamber correction factors for nonstandard fields.

Purpose: To compare plan quality between fixed‐gantry IMRT plans generated by a treatment planning system (TPS) based on biological constraints (Monaco, Elekta CMS) and rotational IMRT plans produced by a TPS based on dose‐volume constraints (TomoTherapy) for prostate irradiation. Methods: Five previously treated TomoTherapy (Ver. 3.1.4.7) plans with a prescription dose of 7560 cGy in 42 fractions to prostate PTV were selected. TomoTherapy plans were generated using 2.5 cm field width, a typical pitch of 0.43, and a mean modulation factor of 2.20. Monaco (Ver. 2.03) plans were generated for these five cases using 9 equispaced beams, 2 mm grid spacing, 2 mm beamlet width, and a Monte Carlo algorithm for final dose calculation with 3% variance. Because greater PTV dose heterogeneity was anticipated, Monaco plans that did not meet a “ 110% of the prescribed dose” criterion were rejected. Two sets of Monaco plans were generated for 58‐ and 160‐leaf Siemens MLC beam models. Various plan quality metrics were compared by the paired t‐test. Results: In all five cases, it was possible to obtain Monaco plans that met the PTV dose homogeneity criterion and equaled or exceeded the TomoTherapy plans in terms of rectal and bladder sparing. A small, but statistically significant, improvement in rectal V45 (−1.3%) and V70 (−2.4%) and in bladder V45 (−3.7%) was observed in Monaco 58‐leaf MLC plans compared to the TomoTherapy plans. In Monaco 160‐leaf MLC plans, a trend toward further improvement in critical structure sparing and PTV dose homogeneity was observed. Conclusions: Fixed‐gantry IMRT prostate plans generated by the Monaco TPS have identical or slightly improved critical structure sparing in comparison with the TomoTherapy plans, but are characterized by greater, albeit clinically acceptable, heterogeneity in target volumes.

Purpose: To develop a methodology for creating voxel‐based dose painting plans that are clinically deliverable using a commercialized, structure‐based treatment planning system (TPS). Methods and Materials: Uptake data from head and neck patients who underwent [61Cu]Cu‐ATSM (hypoxia surrogate) PET/CT scans were retrospectively extracted for treatment planning. Non‐uniform voxel‐based prescriptions were converted to structure‐based prescriptions for compatibility with the TomoTherapy Hi‐ART II™ TPS. Optimized treatment plans were generated using fixed pitch, jaw width, and modulation factor delivery parameters. The effects of structure importance, prescription point normalization, iteration number, DVH volume, Min/Max dose, and dose penalty were investigated by varying these constraints one at a time in each plan. Avoidance structures were constrained to clinical tolerances. Quality‐volume histograms (QVH) as percent receiving within 5 percent of the prescription (Q0.95–1.05) were used to evaluate treatment plan conformity. Results: Conformity of the plans to their non‐uniform dose prescription was insensitive to the optimization parameters when the prescription DVH constraint and the dose normalization point were set to the mean PTV volume and dose. Plan conformity decreased as the prescription DVH constraint (Q0.95–1.05: 88% vs. 84%) or the normalization point (Q0.95–1.05: 88% vs. 85%) deviated from the means. Plan conformity was also negatively impacted by increasing integral boost doses from 10 Gy to 30 Gy (Q0.95–1.05: 97% vs 78%), decreasing iteration number from 1000 to 100 (Q0.95–1.05: 86% vs 80%), and applying conventional planning strategies (Q0.95–1.05: 88% vs 59%). Conclusions: This investigation demonstrates the ability of a structure‐based TPS to create voxel‐based dose painting plans using a relaxed set of optimization parameters and a dose normalization point set to the mean PTV dose. Under these conditions, plan conformity is dominated by few optimization parameters. However, tightening constraints on target structures via conventional planning strategies results in decreased plan conformity.

Purpose:IMRT has become a standard technique for the treatment of many tumors. It is often important to know the out‐of‐field dose associated with such treatments, such as for fetal dose evaluation, or estimating organ doses during epidemiologic study of second cancer risk. While evaluating such doses is routine for conventional therapies (e.g., Task Group 36 data), no simple technique is available to estimate doses from IMRT treatments. This work develops and evaluates such a technique. Method and Materials: A simple equation was developed to calculate the out‐of‐field dose (Dimrt) from an IMRT treatment based on treatment parameters and the out‐of‐field dose from conventional therapy (Dconv) with the same jaw setting (which is readily estimated from such sources as TG‐36). The accuracy of this equation was tested by comparing out‐of‐field doses calculated with the developed equation to those calculated for IMRT treatments using Monte Carlo. Results: The following equation was developed to estimate the out‐of‐field dose from IMRT: Dimrt=Dcon[P×CF + (L+C)×MF], where P, L, and C are the fraction of the out‐of‐field dose originating from patient scatter, head leakage, and collimator scatter respectively, CF is the fraction of the open field covered by the CIAO field (continuous irradiated area opening), and MF is the IMRT MU modulation factor. These values are tabulated for typical treatments. Compared to the out‐of‐field dose from IMRT, doses calculated with this equation generally agreed within 25%. Larger variations were sometimes observed, particularly within 5cm of the edge of the treatment field where large variations in the out‐of‐field dose are expected due to its dependence on the particular treatment plan and planning objectives. Conclusion: A simple and suitably accurate equation was derived to calculate the out‐of‐field dose from IMRT treatments. This technique represents a viable approach for estimating the out‐of‐field dose for clinical or epidemiological applications.

Purpose: To investigate whether the overshoot effect or “ballooning” of isodose lines into the rectum observed in tomotherapy patient treatment plans is caused by the sharp inclination of the posterior border of the prostate. Method and Materials: Cylindrical target and OAR volumes were segmented in a solid water phantom and placed adjacent to one another in an anterior‐posterior orientation at the center of the phantom. Individual treatment plans were created for the following angular orientations with respect to the y‐axis (IEC coordinates): Target0/OAR0: 0° above y‐axis, Target15/OAR15: 15° above y‐axis, Target30/OAR30: 30° above y‐axis and Target45/OAR45: 45° above y‐axis. Separation between the target and OAR remained fixed at 5 mm for all angles. Treatment plans were generated with a field width of 2.5cm, pitch of 0.287, and modulation factor of 2.5. For all plans, dose prescription was 98% of the target volume to receive at least 6.0Gy. Four additional treatment plans were created in which the target extended into the OAR. Results: None of the phantom plans showed any overshoot of the isodose lines into the adjacent OAR. In the phantom plans with the target extended into the OAR, the effect was not clearly seen however a slight indication of minimal ballooning was indeed observed. Additionally, it was noted that as the inclination angle increased the mean dose to the OAR increased. Conclusion: This study served to investigate the dependency of the target inclination on the isodose ballooning effect seen in prostate patients. Our phantom study results show that the angle of inclination does not produce a ballooning of the isodose lines but does lead to an increase in mean dose to the OAR as the angle is incremented. Future studies will investigate whether the extension of the target volume into the OAR is the cause.

Purpose:Intensity modulated radiation therapy(IMRT) can be delivered using several different techniques. The present study compares dosimetric characteristics of three commercial IMRT planning and delivery systems, i.e., helical tomotherapy (TOMO), sequential tomotherapy with MIMiC (MIMiC) and dynamic multileaf collimated IMRT (DMLC‐IMRT). Method and Materials:IMRT plans for three common clinical cases, head‐and‐neck (HN), brain and prostate cancers, were generated using respective RTOG protocols 9406, 9803 and H0022 as guidelines. For each case, multiple (more than four) plans were produced for each technique using different strategies by adjusting constraints and/or parameters such as beam width, modulation factor, the number of fields, etc. Dosimetric indices were used as figures‐of‐merit to evaluate the plans. These indices include coverage index (CI), overflow index (OI), sparing index (SI), and overall conformal index (COIN), where COIN=CI×OI×SI. The plans for each technique were compared among themselves to determine the best plan of each technique which in turn was used for the comparison of different techniques. Results: For the HN case, the COINs for TOMO, MIMiC and DMLC‐IMRT were 0.333, 0.064 and 0.305, respectively. TOMO had highest SI, but DMLC‐IMRT had highest CI and OI. MIMiC plan performance was poorer than TOMO and DMLC‐IMRT for the HN case. The COINs for TOMO, MIMiC and DMLC‐IMRT were 0.471, 0.616 and 0.685 for the brain case, and 0.304, 0.487 and 0.554 for the prostate case, respectively. DMLC‐IMRT had the highest COIN for both the brain and prostate cases. TOMO had best coverage for the brain case while DMLC‐IMRT had best coverage for the prostate case. Conclusion:IMRT plan optimization is a process to balance and compromise between target coverage and normal tissue sparing. The three techniques may result in different plans for similar constraints. Using different strategies, all three techniques produced clinically acceptable treatment plans for all cases.

Purpose: To determine the fetal dose arising from a typical helical tomotherapy IMRT treatment delivered at anatomical sites distant from the embryo/fetus. Method and Materials: An anthropomorphic phantom (Rando™ slices 1–25) with the pelvis section replaced by a plastic water™ phantom was used to develop a treatment sinogram with the Hi‐Art™ device using typical parameters: 6 MV photons, 2.5 cm field width, pitch of 0.3, and modulation factor of 2. Target dimensions were 8 cm (W) × 7 cm (H) × 11 cm (L) and was centrally located in the abdominal region of Rando™, which was planned and treated to a dose of 5 Gy. Fetal dose measurements were made at a midplane depth of 10 cm in the plastic water using TLD and a calibrated diode. Four sites in Rando™ were treated using the same sinogram allowing the fetal dose to be determined as a function of distance from the treatment site center, which varied from 23 cm to 75 cm. To express results as a percent of the delivered dose for the same sinogram, an approximate correction was made to account for differences in tissue thicknesses at the various treatment sites. Results: For the same sinogram delivery (of approximately 5 Gy), fetal dose varied from 3.53 cGy for a lower abdominal treatment at a distance of 23 cm to 0.23 cGy for a brain/head treatment at a distance of 75 cm. A power law fit indicates fetal dose is proportional to the distance to the −2.3 power. Conclusion: Typical helical tomotherapy treatments at distant sites may yield fetal doses approaching 1 percent of delivered dose; and fetal dose is reduced by more than the square of the distance to the treatment site. Conflict of Interest: A collaborative research relationship is currently being developed with TOMOTHERAPY, Inc.

Interaction modulation factors from murine lymphocytes were first described by Curtis & De Sousa (1973, 1975). These factors are low molecular weight glycoproteins secreted by lymphocytes in short term culture and which can be detected in body fluids and adsorbed to certain cell types, Curtis (1978). In earlier work it was shown that these substances specifically diminish the adhesion of lymphocytes of the opposite type. Thus the T interaction modulation factor from thymocytes or T cells decreases the adhesion of B lymphocytes, not only to each other but also to other types of cell surface. Conversely the B interaction modulation factor diminishes the adhesion of thymocytes and of peripheral T cells. It was suggested by Curtis & De Sousa (1975) that these substances might play a role in the control of lymphocyte circulation and a certain amount of evidence supporting this interpretation was described by Curtis (1978).