Search

Purpose: To develop a real time dose monitoring and dose reconstruction tool to identify and quantify sources of errors during patient specific VMAT delivery and QAMethods: The VMAT delivery monitor tool called Linac Data Monitor (LDM) has been developed that connects to the linac in clinical mode and displays, records and compares real‐time machine parameters to the planned parameters. A new quantity called integral error keeps a running total of leaf overshoot and undershoots errors in each leaf pair multiplied by leaf width and the amount of time during which error exists in MU delivery. Another tool reconstructs pinnacle format delivered plan based on the saved machine logfile and recalculates actual delivered dose in patient anatomy.Delivery characteristics of various standard and hypofractionation VMAT plans delivered on Elekta Axesse and Synergy linacs were quantified. Results: The MLC and gantry errors for all the treatment sites were 0.00±0.59mm and 0.05±0.31°, indicating a good MLC gain calibration. Standard fractionation plans had a larger gantry error than hypofractionation plans due to frequent dose rate changes. On average the MLC errors were negligible but larger errors of 4–6 mm and 2.5° were seen when dose rate varied frequently. Large gantry errors occurred during the acceleration and deceleration process, and correlated well with MLC errors (p

Purpose: To reduce the potential of late toxicity (tissue necrosis and cognitive function loss) from brainradiotherapy, we studied a novel planning technique for MLC‐equipped linear accelerators(LINACS) that reduces the high doses delivered outside the PTV, enhancing radiationdose conformity. Methods: The feasibility of improving dose conformity by using variable source‐to‐tumor distances was tested on brain CT datasets with tumor contours representing solitary lesions and an example with two lesions. For the single tumor cases, different tumor volumes from 2 to 83 cc were explored, while an example double lesion had a total volume of 13 cc. Three different source‐to‐axis distances (SADs), 100, 80 and 65 cm, were used to create separate 5‐field IMRT plans with the same beam angles for all SADs. For each example, the PTV dose coverage and critical organdoses were constrained to be the same, except for the normal braindoses that were determined by the dose conformity. High dose spillage outside the PTV was quantified by R50 (the ratio between the 50% isodose volume and the PTV), and the V12 (volume of normal brain receiving 12Gy or higher). Results: R50 decreased monotonically with shorter SAD for all examples. Compared with 100 cm SAD, average R50 reductions of 13% and 19% were observed with 80 and 65 cm SAD respectively. Improved conformity was more notable on smaller lesions and the multiple lesion case. V12 also decreases significantly with shorter SAD. Conclusions: Improvement in dose conformity can be achieved on existing LINACS by reducing the treatment distance for each IMRT field. Sharper beam penumbra and smaller projected MLC leaf width contribute to the improvement. Precise delivery of non‐ isocentric beams can be challenging on traditional LINACS, but the problem is much more manageable on machines with precise robotic gantry and couch capabilities.

Concurrent treatment of the prostate and the pelvic lymph nodes encounters the problem of the prostate gland moving independently from the pelvic lymph nodes on a daily basis. The purpose of this study is to develop a leaf-tracking algorithm for adjustment of IMRT portals without requirement of online dose calculation to account for daily prostate position during concurrent treatment with pelvic lymph nodes. A leaf-shifting algorithm was developed and programmed to adjust the positions of selected MLC leaf pairs according to prostate movement in the plane perpendicular to each beam angle. IMRT plans from five patients with concurrent treatment of the prostate and pelvic lymph nodes were selected to test the feasibility of this algorithm by comparison with isocenter-shifted plans, using defined dose endpoints. When the prostate moved 0.5, 1.0, and 1.5 cm along the anterior/posterior direction, the average doses to 95% of the prostate (D{sub 95%}) for the iso-shift plans were similar to the MLC-shift plans, (54.7, 54.4, and 54.1 Gy versus 54.5, 54.3, and 53.9 Gy, respectively). The corresponding D{sub 95%} averages to the pelvic lymph nodes were reduced from the prescription dose of 45 Gy to 42.7, 38.3, and 34.0 Gy for iso-shift plans (p=0.04 for eachmore » comparison), while the D{sub 95%} averages for the MLC-shift plans did not significantly differ from the prescription dose, at 45.0, 44.8, and 44.5 Gy. Compensation for prostate movement along the superior/inferior direction was more complicated due to a limiting MLC leaf width of 1.0 cm. In order to concurrently treat the prostate and pelvic lymph nodes with the prostate moving independently, shifting selected MLC leaf pairs may be a more practical adaptive solution than shifting the patient.« less

This paper systematically studied the dosimetric difference between a 3 mm micro multileaf collimator (MLC), a 5 mm MLC, and a 10 mm MLC for stereotactic radiosurgery using the Brainscan treatment planning system. Thirty-four cases treated with the dynamic conformal arcs technique and 20 cases treated with the intensity modulated radiosurgery/fractionated radiotherapy (IMRS/ IMRT) technique were retrospectively studied. The conformity index, the percentage target coverage, and the dose-volume histogram (DVH) for organs-at-risk (OARs) were used for dosimetric analysis and comparison for different treatment techniques, target volumes, and treatment sites. For the dynamic conformal arcs technique, there were statistically significant differences in the conformity indices between different leaf-width MLCs. The ratio of the conformity indices between different MLCs depended on the target volume. The average conformity index ratios between the 5 mm MLC and the 3 mm MLC were 1.37+/-0.09, 1.12+/-0.04, 1.08+/-0.02 and 1.04+/-0.01, respectively, for patients with the target volume (V) in groups: (1) V 27 CC. The average conformity index ratios between the 10 and 3 mm MLCs were 2.00+/-0.33, 1.45+/-0.09, 1.28+/-0.09, and 1.18+/-0.05 for patients in these four volume groups, respectively. No statistically significant difference was found for the target coverage among different MLCs. For the IMRS/IMRT technique, the average conformity index and target coverage ratios were 1.01+/-0.05 and 1.00+/-0.02, respectively, between the 5 and 3 mm MLCs, and were 1.04+/-0.07 and 0.97+/-0.02, respectively, between the 10 and 3 mm MLCs. The 3 mm MLC showed slightly better overall OAR DVHs than the 5 and 10 mm MLCs, especially for the cranial site with small-volume OARs defined. The results suggest that for the dynamic conformal arcs technique, the narrower leaf-width MLC provides better dose conformity than the wider leaf-width MLCs. This advantage decreases when the target volume increases. For the IMRS/IMRT technique, the narrower leaf-width MLC could have better sparing of small OARs than the wider leaf-width MLC.

This study investigates the influence of multileaf collimator (MLC) leaf width on intensity modulated radiation therapy (IMRT) plans delivered via the segmented multileaf collimator (SMLC) technique. IMRT plans were calculated using the Corvus treatment planning system for three brain, three prostate, and three pancreas cases using leaf widths of 0.5 and 1 cm. Resulting differences in plan quality and complexity are presented here. Plans calculated using a 1 cm leaf width were chosen over the 0.5 cm leaf width plans in seven out of nine cases based on clinical judgment. Conversely, optimization results revealed a superior objective function result for the 0.5 cm leaf width plans in seven out of the nine comparisons. The 1 cm leaf width objective function result was superior only for very large target volumes, indicating that expanding the solution space for plan optimization by using narrower leaves may result in a decreased probability of finding the global minimum. In the remaining cases, we can conclude that we are often not utilizing the objective function as proficiently as possible to meet our clinical goals. There was often no apparent clinically significant difference between the two plans, and in such cases the issue becomes one of plan complexity. A comparison of plan complexity revealed that the average 1 cm leaf width plan required roughly 60% fewer segments and over 40% fewer monitor units than required by 0.5 cm leaf width plans. This allows a significant decrease in whole body dose and total treatment time. For very complex IMRT plans, the treatment delivery time may affect the biologically effective dose. A clinically significant improvement in plan quality from using narrower leaves was evident only in cases with very small target volumes or those with concavities that are small with respect to the MLC leaf width. For the remaining cases investigated in this study, there was no clinical advantage to reducing the MLC leaf width from 1 to 0.5 cm. In such cases, there is no justification for the increased treatment time and whole body dose associated with the narrower MLC leaf width.

Purpose: To compare the impact of MLC width on the dose conformity, doses received by the normal brain tissue, and other critical structures for single isocenter (SI) and multiple isocenter (MI) stereotactic IMRT (SRT) treatments for patients with at least four brain tumors. Methods: Six patients (three with seven and three with four lesions) planned in BrainLab iPlan were evaluated using Varian HD MLC (2.5mm leaf-width) and Millennium-120 MLC (5mm leaf-width). Prescribed doses in all plans were 25 Gy delivered in five fractions normalized to 98% dose covering 98% of the target volume. Results: All plans were judged clinically acceptable. The average Paddick conformity index for 2.5mm vs 5mm SI and MI plans were (0.48±0.22 vs. 0.38±0.21, p

Purpose: We have performed a systematic review and meta-analysis in order to compare MLC leaf width used for different treatment techniques such as conformal and IMRT. Methods: Medline and Google scholar databases have been searched using the “MLC leaf width” keywords with a cut-off date of 31st December 2015, considering only peer-reviewed papers. We have defined as intervention the MLC leaf width of 5 and 2.5 mm. We used the Conformity Index (CI) as the outcome subjected to the analysis. The mean and standard deviation of each group has been either extracted or calculated. The association between MLCs and Conformity Index reduction across the selected studies was then computed as Pooled Mean Difference (PMD) with 95% CI. Results: From the literature search 43 studies were selected, 12 of them compared MLC 2.5 or 3 mm vs. 5 mm in terms of Conformity Index. On the whole there is a slight significant difference between MLC2.5 and MLC5.0 in favour of the former. The pooled mean difference is −0.036 with a 95% confidence interval ranging between −0.068 and −0.005 (p=0.026). The analysis highlights a very large heterogeneity (I2=87.7%) suggesting the results could be different in specific subgroups. Therefore a subgroup analysis has been performed by comparing techniques (IMRT/VMAT vs 3DCRT/DCA). In the subgroup of IMRT/VMAT the difference between MLC2.5 and MLC5.0 appears to be negligible (mean diff:-0.006; 95%CI: −0.013–0.001) and not significantly different from zero (p=0.064). On the contrary in the subgroup of studies which used the conformal 3DCRT/ DCA techniques, the MLC2.5 resulted significantly different from the MLC5.0 (mean difference: −0.054; 95% CI: −0.096–0.012; p=0.02). Conclusion: A systematic review of TPS studies have been performed. In this analysis, using IMRT/VMAT shows identical capability of achieving good target coverage ability (CI) with 2.5 and 5mm MLC, differently from 3DCRT/DCA. Marco Luzzara and Kevin Brown are Elekta employees. Luigi Santoro has a scientific collaboration with Elekta

Purpose: To investigate the effect of multi-leaf collimators (MLCs) with leaf width of 1.25 mm on the plan quality of volumetric modulated arc therapy (VMAT) for prostate cancer. Methods: A total of 20 patients with prostate cancer were retrospectively selected. Using a high definition MLC (HD MLC), primary and boost VMAT plans with two full arcs were generated for each patient (original plan). After that, by shifting patient CT images by 1.25 mm in the cranio-caudal direction between the 1st and the 2nd arc, we simulated fluences made with MLCs with leaf width of 1.25 mm. After shifting, primary and boost plans were generated for each patient (shifted plan). A sum plan was generated by summation of the primary and boost plan for each patient. Dose-volumetric parameters were calculated and compared. Results: Both homogeneity index (HI) and conformity index (CI) of the shifted plans were better than those of the original plans in primary plans (HI = 0.044 vs. 0.040 with p < 0.001 and CI = 1.056 vs. 1.044 with p = 0.006). Similarly, the shifted plans for boost target volume showed better homogeneity and conformity than did the original plans (HI = 0.042 vs. 0.037 with p = 0.006 and CI = 1.015 vs. 1.009 with p < 0.001). The total body volumes of the original plans irradiated by the prescription dose were larger than those of the shifted plans in sum plans (60.9 cc vs. 49.0 cc with p = 0.007). Conclusion: Use of extremely narrow MLCs could increase dose homogeneity and conformity of the target volume for prostate VMAT. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1C1A1A02036331).

Purpose: To compare the plan quality between Eclipse (Varian) and Monaco (Elekta) TPS. To ascertain, if SBRT lung treatment could be delivered in a single coplanar arc (360 degrees) with both Elekta and Varian platforms. To assess if the smaller leaf width in Varian Millennium and Elekta Agility MLC heads have a dosimetric advantage over Elekta MLCi2 head Methods: Ten SBRT lung patients (PTV volumes ranging from 11 cc to 103cc) who were previously treated on Varian Linac with non-coplanar arcs and received 50Gy in 5 fractions were chosen for this study. The patients were replanned in Eclipse TPS (AAA algorithm) using a 360 degree coplanar single arc (SA) delivery technique and 2 partial complimentary 180 degree arcs (PA). Treatment planning using single coplanar arc (360 degree arc) was also done on Monaco TPS (Montecarlo) for both Agility (160 leaf) and MLCi2 (80 leaf) Elekta MLC heads Results: The average monitor units to deliver 10 Gy across all delivery methods were 3000 ± 474 MU and did not vary with PTV size. Coplanar single arc and partial arc techniques did not compromise either the RTOG 0813 or 0915 low dose spillage criteria for R50% or the maximum dose to any point 2cm away from the PTV. OAR doses to spinal cord, heart, great vessels, esophagus, rib and lung were comparable on both Eclipse (Varian) and Monaco (Elekta) platforms regardless of the delivery method. Conclusion: SBRT lung tumors can be treated with a single coplanar 360 degree arc in both Varian and Elekta platforms. Non coplanar arcs and increasing arc degrees more than 360 degrees had no benefit in this study regardless of the volume of PTV. 0.5 cm leaf width used in Millennium and Agility MLC heads had no significant dosimetric improvement over 1 cm leaves in the MLCi2 head.

Purpose: To determine the effects of multileaf collimator (MLC) leaf width on normal-brain-tissue doses and dose conformity of SRS RapidArc treatment plans for brain tumors. Methods: Ten patients with 24 intracranial tumors (seven with 1–2 and three with 4–6 lesions) were planned using RapidArc for both Varian Millennium 120 MLC (5 mm leaf width) and high definition (HD) MLC (2.5 mm leaf width). Between 2 and 8 arcs were used with two full coplanar arcs and the rest non-coplanar half arcs. 6 MV beams were used and plans were optimized with a high priority to the Normal Tissue Objective (to achieve dose conformity and sharp dose fall-off) and normal brain tissue. Calculation was done using AAA on a 1 mm grid size. The prescription dose ranged from 14–22 Gy. Plans were normalized such that 99% of the target received the prescription dose. Identical beam geometries, optimizations, calculations, and normalizations were used for both plans. Paddick Conformity Index (PCI), V4, V8 and V12 Gy for normal brain tissue and Integral Dose were used for analysis. Results: In all cases, HD MLC plans performed better in sparing normal brain tissue, achieving a higher PCI with a lower Integral Dose. The average PCI for all 24 targets was 0.75±0.23 and 0.70±0.23 (p ≤0.0015) for HD MLC and Millennium MLC plans, respectively. The average ratio of normal brain doses for Millennium MLC to HD MLC plans was 1.30±0.16, 1.27±0.15, and 1.31±0.18 for the V4, V8, and V12, respectively. The differences in normal brain dose for all criteria were statistically significant with p-value < 0.02. On average Millennium MLC plans had a 16% higher integral dose than HD MLC plans. Conclusion: Significantly better dose conformity with reduced volume of normal brain tissue and integral dose was achieved with HD MLC plans compared to Millennium MLC plans.

In this study we focus on how the intensity level and multileaf collimator (MLC) resolution affect the quality of IMRT plans using the static MLC delivery technique. The planning process is based on a least-square dose-based quadratic function and uses a simulated annealing algorithm to sample the discrete variables. Three clinical cases are studied empirically: a medulloblastoma, a prostate, and an oropharyngeal carcinoma. The intensity levels used are 3, 5, 10, 20, and continuous; the map resolution varies from 0.15-1.5 cm, with the leaf width equal to the step size. The influence of these two parameters are studied by comparing the cost value and the cost of delivery time from a trade-off point of view. An "efficient frontier" is drawn by connecting the plans with the lowest cost value at any given resolutions. For each case, a practical delivery region is defined by doubling the delivery time needed at a normal setting (five levels, 1.0 cm). Within this region, the "efficient frontier" demonstrates that the plans with five intensity levels are the most efficient comparing with plans with higher levels. This is a confirmation of the conclusion from Keller-Reichenbecher et al. [Int. J. Radiat. Oncol., Biol., Phys. 45, 1315-1324 (1999)]. It indicates that to further improve the plan quality with the minimal cost of extra delivery time, the most economical way is to improve the resolution rather than using higher intensity levels.

Purpose: There have been controversial reports on the comparison of dosimetric quality of TomoTherapy (Tomo), VMAT and IMRT. One of the main reasons is the sampled cases are often not dosimetrically challenging enough to test the limit of optimization/delivery modalities. We chose difficult re-irradiation cases when certain organ at risk (OAR) requires extremely low dose to examine the ability of OAR sparing of three main intensity modulated techniques. Methods: Three previous treated patients with disease site on head and neck (HN), brain and lung are planned for reirradiation treatment. The Tomo planning used jaw 2.5cm and pitch 0.3. VMAT and IMRT were planned on Pinnacle for a Varian 21iX Linac with MLC leaf width 5mm. VMAT plan used 2 Arcs and IMRT plan had beams 11–13. The dosimetric endpoints and treatment time were compared for each technique of each patient. Results: Plans of three techniques cover PTV similarly. The HN case requires PTV dose 60Gy but to limit dose of cord which is 8mm away

Purpose: To study the dosimetric impact of MLC leaf width in patients treated with Volumetric Modulated Arc Therapy (VMAT) for spine Stereotactic Body radiation Therapy (SBRT). Methods: Twelve spine SBRT patients were retrospectively selected for this study. The patients were treated with IMRT following the RTOG-0631 of spine metastasis. The prescription dose was 16 Gy in one fraction to 90% of the target volume (V16 > 90%). The maximum spinal cord dose of 14 Gy and 10% of the cord receiving < 10 Gy (V10) were set as dose constraints. For purpose of this study, three dual arc VMAT plans were created for each patient using three different MLC leaf widths: 2.5 mm, 4mm, and 5mm. The compliance to RTOG 0631, conformal index (CI), dose gradient index (DGI), and number of monitor units (MUs) were compared. Results: The average V16 of the target was 91.91±1.36%, 93.73±2.38%, and 92.25±2.49% for 2.5 mm, 4 mm, and 5 mm leaf widths, respectively (p=0.39). Accordingly, the average CI was 1.36±0.39, 1.36±0.34, and 1.41±0.3 (0.96), respectively. The average DGI was 0.24 ± 0.05, 0.22 ± 0.05, and 0.23 ± 0.04, respectively (p=0.86). The average spinal cord maximum dose was 12.10 ± 0.88 Gy, 12.52 ± 1.15 Gy, and 12.05 ± 1.12 (p=0.75) and V10 was 2.69 ± 1.71 cc, 5.43 ± 2.16 cc, and 3.71 ± 2.34 cc (p=0.15) for 2.5 mm, 4 mm, and 5 mm leaf widths, respectively. According, the average number of MUs was 4255 ± 431 MU, 5049 ± 1036 MU, and 4231 ± 580 MU respectively (p=0.17). Conclusion: The use of 2.5 mm, 4 mm, and 5 mm MLCs achieved similar VMAT plan quality as recommended by RTOG-0631. The dosimetric parameters were also comparable for the three MLCs.

Purpose: To assess the impacts that multileaf collimator (MLC) leaf width has on the dose conformity and normal brain tissue doses of single and multiple isocenter stereotactic IMRT (SRT) plans for multiple intracranial tumors. Methods: Fourteen patients with 2–3 targets were studied retrospectively. Patients treated with multiple isocenter treatment plans using 9 to 12 non-coplanar beams per lesion underwent repeat planning using single isocenter and 10 to 12 non-coplanar beams with 2.5mm, 3mm and 5mm MLC leaf widths. Brainlab iPlan treatment planning system for delivery with the 2.5mm MLC served as reference. Identical contour sets and dose-volume constraints were applied. The prescribed dose to each target was 25 Gy to be delivered over 5 fractions with a minimum of 99% dose to cover ≥ 95% of the target volume. Results: The lesions and normal brains ranged in size from 0.11 to 51.67cc (median, 2.75cc) and 1090 to 1641cc (median, 1401cc), respectively. The Paddick conformity index for single and multiple isocenter (2.5mm vs. 3mm and 5mm MLCs) was (0.79±0.08 vs. 0.79±0.07 and 0.77±0.08) and (0.79±0.09 vs. 0.77±0.09 and 0.76±0.08), respectively. The average normal brain volumes receiving 15 Gy for single and multiple isocenter (2.5mm vs. 3mm and 5mm MLCs) were (3.65% vs. 3.95% and 4.09%) and (2.89% vs. 2.91% and 2.92%), respectively. Conclusion: The average dose conformity observed for the different leaf width for single and multiple isocenter plans were similar, throughout. However, the average normal brain volumes receiving 2.5 to 15 Gy were consistently lower for the 2.5mm MLC leaf width, especially for single isocenter plans. The clinical consequences of these integral normal brain tissue doses are still unknown, but employing the use of the 2.5mm MLC option is desirable at sparing normal brain tissue for both single and multiple isocenter cases.

Purpose: The authors evaluated the effects of multileaf collimator (MLC) leaf width (2.5 vs. 5 mm) on dosimetric parameters and delivery efficiencies of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) for head and neck (H&N) cancers. Methods: The authors employed two types of mock phantoms: large-sized head and neck (LH&N) and small-sized C-shape (C-shape) phantoms. Step-and-shoot IMRT (S&S_IMRT) and VMAT treatment plans were designed with 2.5- and 5.0-mm MLC for both C-shape and LH&N phantoms. Their dosimetric characteristics were compared in terms of the conformity index (CI) and homogeneity index (HI) for the planning target volume (PTV), the dose to organs at risk (OARs), and the dose-spillage volume. To analyze the effects of the field and arc numbers, 9-field IMRT (9F-IMRT) and 13-field IMRT (13F-IMRT) plans were established for S&S_IMRT. For VMAT, single arc (VMAT1) and double arc (VMAT2) plans were established. For all plans, dosimetric verification was performed using the phantom to examine the relationship between dosimetric errors and the two leaf widths. Delivery efficiency of the two MLCs was compared in terms of beam delivery times, monitor units (MUs) per fraction, and the number of segments for each plan. Results: 2.5-mm MLC showed better dosimetric characteristics in S&S_IMRT and VMAT for C-shape, providing better CI for PTV and lower spinal cord dose and high and intermediate dose-spillage volume as compared with the 5-mm MLC (p 0.05). Further, beam delivery efficiency was not observed to be significantly associated with leaf width for either C-shape or LH&N. However, MUs per fraction were significantly reduced for the 2.5-mm MLC for the LH&N. In dosimetric error analysis, absolute dose evaluations had errors of less than 3%, while the Gamma passing rate was greater than 95% according to the 3%/3 mm criteria. There were no significant differences in dosimetric error between the 2.5- and 5-mm MLCs. Conclusions: As compared with MLC of 5-mm leaf widths, MLC with finer leaf width (2.5-mm) can provide better dosimetric outcomes in IMRT for C-shape. However, the MLC leaf width may only have minor effects on dosimetric characteristics in IMRT for LH&N. The results of the present study will serve as a useful assessment standard when assigning or introducing equipment for the treatment of H&N cancers.

Purpose: To identify persistent characteristics of IMRT, helical tomotherapy and IMPT across the range of potential applications. Method and Materials: The comparison of optimized treatment plans from different planning systems is difficult because treatment objective definitions and dose algorithms differ. Here, the techniques were implemented in the same optimization algorithm and Monte Carlo dose computation was available. Each of the techniques may offer benefits for certain cases and be less suitable for others. Five cases of different classes were selected including prostate, pediatric, lung and head‐and‐neck. The optimization employs biologically‐based objectives and enforces constraints for normal tissue doses. For each case, the same prescription and constraints were used. Results: For the clinical dose levels of this study, no treatment modality produced significantly superior plans. IMPT spared larger parts of the OAR. However, the EUD was mostly similar to that reached by photon techniques as it is mostly determined by the extent of the high dose volume. Tomotherapy provided generally better target coverage and higher homogeneity compared to fixed‐beam IMRT. However, the difference was mainly caused by the translation of the fully modulated fluence into static MLC segments. OAR irradiation was equivalent for both photon modalities, with higher mean doses for the tomo. Thus, the advantage of helical irradiation was mostly offset by the finer resolution of the MLC leaves (4 mm), once sufficiently chosen beams were used. On the other hand, noncoplanar beams did not provide a clear benefit. Conclusion: All modalities were optimized with the same planning system, thereby eliminating differences caused by the TPS. The differences between modalities were rarely significant. The quality of the dose distribution is governed by the particle type (mean dose) and the ability to deliver the ideal dose accurately (MLC leaf width, sequencing, number of beams, scanning grid).

Purpose: Intensity modulated arc therapy (IMAT) is commonly used to treat prostate cancer. The purpose of this study was to evaluate the impact of leaf width and plan complexity on dynamic multileaf collimator (DMLC) tracking for prostate motion management during IMAT treatments. Methods: Prostate IMAT plans were delivered with either a high-definition MLC (HDMLC) or a Millennium MLC (M-MLC) (0.25 and 0.50 cm central leaf width, respectively), with and without DMLC tracking, to a dosimetric phantom that reproduced four prostate motion traces. The plan complexity was varied by applying leaf position constraints during plan optimization. A subset of the M-MLC plans was converted for delivery with the HDMLC, isolating the effect of the different leaf widths. The gamma index was used for evaluation. Tracking errors caused by target localization, leaf fitting, and leaf adjustment were analyzed. Results: The gamma pass rate was significantly improved with DMLC tracking compared to no tracking (p < 0.001). With DMLC tracking, the average gamma index pass rate was 98.6% (range 94.8%–100%) with the HDMLC and 98.1% (range 95.4%–99.7%) with the M-MLC, using 3%, 3 mm criteria and the planned dose as reference. The corresponding pass rates without tracking were 87.6% (range 76.2%–94.7%) and 91.1% (range 81.4%–97.6%), respectively. Decreased plan complexity improved the pass rate when static target measurements were used as reference, but not with the planned dose as reference. The main cause of tracking errors was leaf fitting errors, which were decreased by 42% by halving the leaf width. Conclusions: DMLC tracking successfully compensated for the prostate motion. The finer leaf width of the HDMLC improved the tracking accuracy compared to the M-MLC. The tracking improvement with limited plan complexity was small and not discernible when using the planned dose as reference.

Purpose: The Agility multileaf collimator (MLC) mounted on an Elekta Synergy linear accelerator for 6 MV was modeled for IMRT and VMAT calculations using the BEAMnrc Monte Carlo (MC) code and verified versus measurements. Methods: To describe the Agility MLC in BEAMnrc, the available Component Module code was modified to include its characteristics; 5 mm leaf width, flat leaf sides with a focus point shifted from the radiation source. The MLC model was verified by comparison of the calculated interleaf leakage and tongue‐and‐groove effect for a closed MLC field and an irregular field to measurements with EBT2 film in a solid water phantom and diode measurements in a water phantom, respectively. We have developed a time dependent phase space data (PSD), which include a parameter based on MU index. Because leaf, jaw, collimator and gantry positions of each segment are controlled by MU index, this PSD enabled to simulate dynamic motions by interpolating positions between each segment. IMRT and VMAT calculations were compared with film measurements in a solid water phantom to validate the accuracy of the overall MLC model. MC statistical uncertainty was below 2% for all simulations. Results: We found a good agreement with our measurements on interleaf leakage. Agreement between mean calculated and measured leaf transmissions with fully opened jaws normalized to the center of a 10×10 cm2 field at the same depth was within 0.1%. Discrepancy between MC calculation and measurement for the irregular field was below 2%/2 mm. The gamma analysis of the comparison of MC and EBT2 film measurements in IMRT and VMAT fields showed 99.1%, 99.5% pass rates with 3%/3 mm criteria, respectively. Conclusion: The Agility MLC produced by Elekta could be accurately MC modeled with an adaptation in BEAMnrc. The MC model proved to be applicable for IMRT and VMAT calculations. Japan Society for the Promotion of Science (JSPS) Core‐to‐Core Program

Purpose: To evaluate the variation of dosimetric leaf‐gap (DLG) along the travel path of each MLC leaf pairs. Furthermore, dosimetric implication of this variation is studied for several dynamic MLC fields. Methods: During dose calculation, the treatment planning systems (TPS) retracts the MLC leaf positions by half of the DLG value (measured at CA) for all leaf positions in a Dynamic MLC plan to accurately model the rounded leaf ends. The 6 MV DLG values were measured, for all leaf pairs, in increment of 0.5 cm at Dmax, in the direction of leaf motion. The measurements were performed on three Varian Linacs, employing the Millennium 120‐leaf MLCs. Measurements were performed using both the Sun Nuclear MapCheck device and the PTW pinpoint ion chamber. Measurements with an ion chamber were also performed with varying leaf gaps to determine % change in dose with leaf gap discrepancy. Results: The measured DLGs for all leaf pairs were very similar along the in‐plane direction; varying maximally only by 0.3 mm. But in the in the leaf travel direction, the DLG increases gradually from CA to 6.0 cm off‐axis, reaching to a maximum variation of 1.6 mm for a particular leaf pair. Varying gap measurements indicate that DLG deviation of about 1 mm can cause a dose difference of about 5.0 % for a fluence with 1.5 cm leaf width sweeps and significantly increases to about 12.5 % difference for 0.5 cm sweeps. Conclusions: The spatial variation in DLG is caused by the variation in beam spectrum as well as MLC motion across the field. Fields with dynamic MLC centered on the CAX would not be affected since DLG does not vary as much; but any field residing significantly off‐axis with narrow sweeping leafs can exhibit significant dose differences due to large DLG differences.

Purpose: Our previous study showed that adjusting selected MLC leaf pairsto follow prostate movement is an effective strategy to account for daily prostate displacement during concurrent treatment with pelvic lymph nodes. MLC leaf width affects the quality of MLC shifting plans for longitudinal prostate motion compensation. This study is to investigate the effect of the MLC leaf width in compensation of the prostate movement. Methods: Fifty‐one daily CT on‐rail scans from three patients were available for this study. On these CTs, the prostate, bladder and rectum were manually contoured, and the lymph nodes contours were transferred from the planning CT after rigid bony registration. For each patient, three different IMRT plans were created based on a planning CT using leaf width of 2.5, 5, and 10 mm, respectively. For each CT, the prostate displacement was determined by dual imaging registration and compensated by shifting MLC resulting in a total of 153 MLC shifted plans. Results: Among 51 daily CTs, the average prostate movement along the superior/inferior direction was 1.1±3.7 mm (range: −6 to 6.5 mm). The differences in D99 of the prostate between the dose of the day and dose of the plan were 2.3±3.3%, 1.3±2.0%, and 4.4±5.1% for 2.5, 5, and 10 mm leaf width plans, respectively (p<

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: The purpose was to analyze the influence of the step size during head&neck IMRT treatments. Methods: Ten patients with head&neck IMRT treatment plan were selected. Treatment plans were done using nine 6MV photon‐beams on a Primus linac (Siemens) with Optifocus MLC (10mm leaf width) and a dose rate of 200MU/min. The TPS used was Konrad v2.2 (Siemens) with leaves step size of 10, 5 and 3mm. For each step‐size, plans were evaluated in terms of MU, segments number, treatment time, PTV's (66Gy and 50Gy) dose uniformity and OAR (parotids and spinal cord) doses. Variations between measured and calculated total plan doses were obtained using solid water phantom and PTW PinPoint ionization chamber. Results: The MU number for 10mm step‐size plans was 624 [495,770] and increased 51% [36%, 87%] for 5mm and 124% [93%, 171%] for 3mm. The segments number for 10mm step‐size plans was 111 [99,129] and increased 41% [27%, 52%] for 5mm and 102% [71%, 125%] for 3mm. The treatment time for 10mm step‐size plans was 11.47min [11.08min, 12.45min] and increased 24% [9%, 33%] for 5mm and 65% [27%, 74%] for 3mm. Reducing the step‐size from 10mm to 5mm leads to better PTV_66Gy dose uniformity within ICRU50 tolerance (no difference observed between 5mm and 3mm). Step‐size diminution from 10mm to 3mm leads to a better coverage of PTV_50Gy especially in patient surface regions. For all the studied plans there are no differences in the parotids mean doses and spinal cord maximum dose. The variation between measured and calculated doses was 1.1% for 10mm step‐size. It increased 1.7% and 2.4% for 5mm and 3mm respectively. Conclusions: Using 10mm step‐size produces plans with adequate OAR doses. PTV Dose uniformity and volume coverage could be improved by 5mm step‐size whereas 3mm step‐size produces larger treatment times without advantages in dose distribution.

Introduction Reducing the beamlet size during step&shoot IMRT planning may lead to improve dose conformity. The beamlet length can be adjusted in the treatment planning system (TPS) with the leaf step‐size along the MLC travel direction. The aim of this paper was to analyse the step‐size influence during prostate treatments.Materials and methods Ten patients treated for prostate cancer with 6MV‐IMRT fields were selected. Each treatment was calculated with the Konrad v2.2, Siemens, TPS using leaves step sizes of 3, 5 and 10mm, and delivered with a Primus, Siemens, linac associated to an Optifocus MLC (10mm leaf width). For each step‐size, the plans were calculated so as to equal the dose received by 95% of the PTV. Plans were compared in terms of PTV dose uniformity, organ‐at‐risk doses, treatment time and MU number. For each plan, the beams were imported in a water‐equivalent phantom and the doses calculated were compared with Farmer (0.6cc) and Pinpoint (0.015cc) ion chambers measurements. Results Reducing the step‐size from 10 to 5 or 3mm leads to a better PTV dose uniformity (31.2% and 35.8% dose standard deviation increase) and higher prostate minimum dose (+0.2% and +0.6%). Rectum V40 (V65) decreased of 3.2% (3.6%) and 6.9% (12%) for 5mm and 3mm. Bladder V65 and femoral heads V50 decreased of 9.5% and 26% for 5mm and 18% and 42% for 3mm. The treatment time and MU number increased of about 10% and 27% for 5mm and 20% and 48% for 3mm. The dose measurement verification showed that both chambers can be used for all step‐sizes with errors less than 1.3%. However, the PinPoint chamber use for 3mm beamlet sizes decreases the error from 1.3% to 0.3%. Conclusion Using 5mm step‐size seems to be a good compromise by improving the IMRT plan quality without treatment time increase.

Purpose: To evaluate the effect of the leaf width during RapidArc optimization Materials and Methods: Twenty patients were randomly selected from out department's database that were previously planned using the Eclipse treatment planning system with the RapidArc optimization capabilities. Prostate, brain,lung,liver and head and neck were the sites that were evaluated in this study. For each patient three plans were generated using linear accelerators with 120 Millennium, 120 High definition and 80 Millennium multileaf collimators. Each plan consisted of 2 arcs of the same span. The collimators of each arc were rotated by 90 degrees with respect to each other. The isocenter location and optimization parameters were kept the same for each plan within each patient. The DVH and isodose distributions were chosen for the evaluation and comparison of the plans. Results: The comparison of the DVHs showed that the high definition MLC produced superior plans with respect to PTV coverage. Also, lower critical organ doses were calculated for the high definition MLC optimized. The millennium 80 MLC produced the most inferior plans for all cases. The plan quality was similar between the 120 and 80 millennium MLC for targets that were lateral with the respect to the central axis of the beam where the MLC leaf width was the same. Conclusions: The MLC leaf width plays an important role during the plan optimization for centrally located targets. A superior plan is achieved with smaller MLC leaf width.

Purpose: To determine the feasibility of using a multileaf collimator(MLC) with leaf widths of 1.0 cm, to obtain dosimetrically similar results to an MLC with 0.5 cm leaf widths, for RapidArc treatments. Method and Materials: The CT data of 10 patients, who had recently completed treatment at our clinic, were loaded into the Eclipse treatment planning system. Two plans were generated for each patient: a single‐arc plan using a 0.5 cm MLC leaf width, and a two‐arc plan using a 1.0 cm MLC leaf width, with the isocenter of one field shifted by 0.5 cm in the direction perpendicular to leaf travel. The optimization parameters used were the same as those employed during the original patient treatment plan. Both plans were optimized by Eclipse's arc optimization algorithm, without any manipulation of constraints or other outside intervention once the process was underway. All plans were normalized such that 95% of the target volume received 100% of the prescribed dose. Results: For all 10 patients, the two‐arc 1.0 cm MLC plans were able to return at least equivalent results for PTV coverage; in all but one instance the two arcs had superior PTV coverage and homogeniety as compared to the single‐arc 0.5 cm MLC plan. Dose to organs at risk (OARs) were not identical, but no trend for higher OAR doses for a particular leaf width can be established. On average, the number of monitor units used for the two‐arc 1.0 cm MLC plans increased by 15%, when compared to the single‐arc 0.5 cm MLC plans. Conclusion: By using two arcs, with an isocenter shift of half a leaf‐width, it is possible for centers using a 1.0 cm MLC leaf‐width to plan RapidArc cases that are dosimetrically similar to those planned with a 0.5 cm leaf width.

Purpose: To develop a comprehensive evaluation of IMRT plans from different treatment planning software (TPS) and different MLC leaf width using dose volume histogram (DVH) parameters and biological equivalent dose (BED). Method and Materials: Four pelvis IMRT treatment plans were studied on a Siemens Linac with 1cm leaf MLC and 2.5 mm mount‐on moduleaf MLC. The plans were originally done in Eclipse TPS (v8.1) with 1cm MLC (named plan E‐1). Then CT slices and contoured anatomy structures were transferred to XKnife RT TPS (v4), where two new plans were carried using 1cm and 2.5 mm MLC, respectively (named plan X‐1, X‐0.25), following the same constraints as in Eclipse TPS for target and organ at risk (OAR). To evaluate the 3 plans (E‐1, X‐1, X‐0.25) from different TPS and using different width of MLC, three parameters were used to develop a comprehensive evaluation method: target volume ratio (TVR), PTV homogeneity H, and fraction biological equivalent dose (BED). Then each IMRT plan is represented by a point in 3‐dim space {TVR, H, BED}. The pattern differences between those points reflect the differences between IMRT algorithms, e.g. planning algorithm difference between plan points E‐1 and X‐1, MLC width difference between plan points X‐1 and X‐0.25. Results: Our results showed that the IMRT plans using 2.5 mm MLC (plan X‐0.25) always had better tumordose coverage and conformity than plans using 1cm MLC in the same TPS (plan X‐1), with smaller values of TVR and H in the 3‐dim plot (Fig.1). But BED for X‐1 plans was higher than X‐ 0.25 plans. Conclusion: A pattern comparison method was developed to evaluate IMRT plans. This method may help to define the clinical benefit of using small width MLC in radiation therapy. The comparison results may be further verified by dosimetry measurements.

Purpose: The multileaf collimator(MLC) has been a standard device for shaping radiation fields. However, the shaped field has stepwise boundary, and can not exactly conform to a treatment target. That means there is always under‐blocked and (or) over‐blocked areas. The total area discrepancy depends on MLC leaf width. The purpose of this study is to minimize the total area discrepancy through optimizing the width of each individual leaf under the assumption that the total number of leaves and the total width of all leaves remain the same. Method and Materials: A program was developed to realize the optimization algorithm. Its flow is like this: a). read data of target shapes; b). initialize leaf widths; c). calculate the total area discrepancy; d). randomly change leaf widths and judge whether to accept the change with simulating annealing technique; e). repeat step d till the iteration process is converged. The algorithm performance was preliminarily evaluated. 113 targets from different treatment sites were randomly selected from the patient data base of our TPS. The target width ranged from 6.8cm to 31.2cm, and had the average of 16.6cm. Results: We obtained the data for a MLC when the total number of leaf pairs was 5, 7, and 13, respectively. The data show that the optimal leaf width super‐linearly increases with the distance away from the central axis. Compared against the MLC with equal leaf width, the MLC with optimal leaf width reduced the total area discrepancy by 25.9%, 28.5%, and 31.4%, respectively. We are collecting data for MLC with more leaf pairs, and investigating the influence of number of targets on optimal leaf width. Conclusions: Optimizing leaf width significantly improves the conformity of MLC shape to target.

Purpose: To investigate the effect of variations in the field size, treatment depth, and treatment distance on penumbra at leaf‐end and leaf‐side of two micro‐leaf collimators (μMLCs). Method and Materials: The penumbra characteristics of the Elekta's Synergy‐S micro‐leaf beam modulator with 4mm leave width and BrainLab's Novalis μMLC with 3mm leaf width at isocenter were investigated. Kodak EDR2 film was exposed to approximately 200cGy for a variety of build‐up thicknesses (6MV photon beam, depth =1.5, 5, 10 and 15cm), field sizes (3cm2–10cm2), and treatment distances (90, 100, 105, 110, and 115cm SSD) for both square and circular fields. The penumbra information was expressed as 80% to 20% isodose distance. Results: Analysis of the measured penumbra revealed that BrainLab's μMLC had smaller effective penumbra (2.3mm–3.9mm at leaf‐side and 2.4mm–4.4mm at leaf‐end) as compared to that of Synergy‐S (3mm–6mm at leaf‐side and 4.3mm–7.8mm at leaf‐end) for square fields, at 100cm SAD. The average difference of effective penumbras at leaf‐side and at leaf‐end was about 50% for the Synergy‐S, while this difference was about 6% for BrainLab's μMLC. However, the effective penumbras for circular fields (3cm–9cm diameter) were comparable; ranging 4mm–7mm for the Synergy‐S μMLC and 3mm–6mm for the BrainLab's system. Discussion and Conclusions: Both the BrainLab's and the Elekta's μMLCs produce comparable effective penumbras for typical treatment conditions. The Synergy‐S does not use any adjustable jaw system. In this study, the adjustable jaws of BrainLab's μMLC were positioned at least 0.5cm behind the leaf‐ends to eliminate their effect on penumbra. The variation of penumbra on leaf‐end is significantly smaller for the BrainLab's μMLC. This difference could be due to several factors such as x‐ray target to leaf distance, target's shape and size, or the exact leaf tip design; these issues are being investigated.

Purpose: To evaluate the dose calculation difference between BrainSCAN (5.31) and Eclipse (6.5) for the same IMRT plan. Method and Materials: 9 IMRT plans (3 Head/Neck, 3 Brain, and 3 prostate) were developed using BrainSCAN and exported to Varis RV system. The same patient CT scan was imported into Eclipse and the same isocenter position was selected with the help of AP and lateral radio‐opaque markers. The field parameters were brought back into Eclipse from the treatment field in Varis, using the “convert to planning field” function. The MLC leaf motion information was brought back using the “convert to actual fluence” function for each field, with the same MU as the BrainSCAN plan. Dose distribution was calculated using Eclipse PBC algorithm. Isocenter dose was compared to that from BrainSCAN. The 2D isodoses of the 3 orthogonal planes across the isocenter were exported from both TPS and compared using RIT. The plan was mapped to the same phantom from each TPS separately, and a Kodak EDR2 film was exposed so that it can be used as a judge to the isodose comparison. A Varain 21EX with MLC80 was used for IMRT delivery. Both dynamic and step‐and‐shoot were included. Results: The isocenter dose shows a −0.1±1.1% difference. The isodose comparison also shows good general agreement. However, we do notice that the Eclipse calculated isodose tends to smooth out the step‐shapes created by the MLC leaves, while the BrainSCAN preserves those shapes, which are closer to the actual situation shown on the film. Conclusion: This comparison study showed that the IMRT dose calculations of BrainSCAN and Eclipse are generally in good agreement. Eclipse tends to smooth out the step‐shapes, which is very clear in our study because 1‐cm leaf width MLC was used. A comparison with Eclipse AAA algorithm is on progress.

Purpose: To compare IMRT planning between Elekta Synergy (ES) and Elekta Synergy‐S (ES‐S), which has a new “beam modulator” MLC design. Method and Materials: ES‐S is equipped with a “Beam Modulator” MLC (4 mm leaf width at isocenter), which provides a maximum field size of 21cm×16cm. IMRT plans for two patients (prostate cancer and parotid cancer disease sites) were generated for both ES and ES‐S. Planning CTs were obtained using Philips AcQSim and CT data were transferred into Pinnacle3 TPS 8.0. IMRT plans for each patient were calculated with two machine models, ES‐S and ES, with minimum segment areas of 1.6×1.6 cm2 and 2×2 cm2 respectively. All other calculation configurations are the same. Dose volume histograms (DVH) were generated and exported into Excel for comparison. Results: For prostate site, PTV DVH for ES‐S demonstrates more conformality than that for ES, possibly because ES‐S has smaller segment area. However, in the low dose region, bladder and rectum DVHs show higher doses from ES‐S versus ES. This may be due to more scatter and leakage doses from MLC of ES‐S. Same trend is observed for the parotid patient. While PTV DVH for ES‐S is slightly more conformal than that for ES, doses for cord expansion, left parotid, and brain stem expansion for ES‐S are slightly higher in low dose region compared to corresponding DVHs for ES. Conclusions: Preliminary results indicate IMRT plans with ES‐S provide slightly better conformality for target volume but slightly higher doses for critical structures in low dose region. For these two sites, there does not appear to be clinically significant differences for IMRT plans between Elekta Synergy‐S and Elekta Synergy. More case example studies are planned and findings will be discussed in depth including practical considerations such as transfer of treatment plans between the two types of machines.

Purpose: Evaluate treatment plan quality and quantify dosimetric accuracy of IMRT for different disease sites with MLC of 0.5 and 1.0 cm leaf width. Method and Materials: We tested a hypothesis, quality of IMRT treatment plans with smaller MLC leaf width is better, by developing treatment plans on Pinnacle treatment planning system (TPS) for multiple patients (total 60) with tumor sites located in Head and Neck (HN the other with 1.0 cm leaf width. The TPS beam modeling of the two virtual machines was exactly the same except for the MLC leaf width. The dose volume histogram (DVH) is used in the evaluations of plan quality. All target coverage is normalized to the criteria of 95% of the target volume receiving prescribed dose. A diode array (Mapcheck) was used to quantify the dosimetric accuracy of all plans created with two MLC leaf width resolutions. Results: DVHs of all the patient plans reveal very minor dosimetric differences between the IMRT plans of the two MLC systems for the same patient. There are almost identical target coverage and similar dose distributions among the critical structures in H&N patients and vertebra body patients. For prostate, plan of 1 cm MLC leaf width has slightly more hot spots in the target with the same dose coverage criteria. Planar doses comparisons with Mapcheck measurements indicate slightly larger uncertainty in 0.5 cm leaf plans. Conclusion: There are negligible dosimetric differences in IMRT plans created with 0.5 cm and 1.0 cm leaf width resolution. However, the dosimetric accuracy of plans generated with 1.0 cm leaf width is better than the plans generated with 0.5 cm leaf width.

Purpose: With the continuing rapid development of technology, more options are becoming available for the clinician. However, this is bringing forth technical challenges to the Physicist, especially in the integration of these technologies. The most commonly used Varian linear accelerator that has the 120 leaf MLC attached to its head prohibits the use of an additional Radionics' MMLC. Hence the 120 leaf MLC was integrated into the Radionics' XKnifeRT system to be used in both cranial and extracranial applications. This presentation provides technical information and clinical experience using this combination. Methods and Materials: The stereotactic technology utilizes the Radionics' XKnifeRT system consisting of a) treatment planningsoftware capable of both 3D conformal as well as step and shoot IMRT; b) stereotactic frames for cranial and extracranial applications. The Varian 120 leaf MLC with an inner leaf width of 5 mm was integrated into the XKnifeRT system so that either the 120MLC or the MMLC could be used clinically. The limitations in the use of the MMLC in this configuration include reduced clearance, especially for extracranial applications, and a reduced maximum field size of 9 × 11 cm. Thus far nearly 40 patients have been treated, mostly using the IMRT component, and include GTC/TLC for cranial sites, and, Head & Neck Localizer(HNL)/Body Localizer(BL) for extracranial sites. Results: A comparison of dose distributions/DVHs of the 120 MLC and the MMLC indicate only marginal improvement if the MMLC were to be used. Not only is the setup time at the linac reduced, the actual IMRT delivery is only a few seconds more than a corresponding dynamic delivery. A cranial and a C‐spine application of the technology validates this integration. Conclusions: We continue to utilize the 120 leaf MLC successfully for all stereotactic applications with XKnifeRT.

Purpose: To investigate the benefits of MLC rotation in Direct Aperture Optimization (DAO) inverse treatment planning.Method and Materials: An alternative to fluence based inverse planning is to optimize directly the leaf positions and field weights of MLC apertures. Here we introduce a new technique called Rotating Aperture Optimization (RAO) which is based on an extension of DAO. Our technique differs from existing aperture based IMRT techniques in that the MLC is rotated in between each aperture. Treatment plans are generated for 10 nasopharynx recurrence patients with and without MLC rotation for 5 mm and 1cm leaf width MLCs. A comparison study is performed between RAO and DAO in order to assess the benefits of RAO over and above those available with fixed collimator angle DAO. Film verification is also performed to evaluate the accuracy of fixed and rotated collimator aperture delivery. Results: An analysis of the final cost values and DVHs indicate that plans generated with RAO are as good as or better than DAO while maintaining a smaller number of apertures and MU than conventional IMRT. In particular, RAO with the 1cm leaf width MLC is able to produce better plans than DAO with the 1 cm leaf width MLC and plans that are equivalent to DAO with the higher resolution 5mm leaf width MLC. Film verification results show that RAO is less sensitive to tongue and groove effects than DAO. Although delivery time is increased due to the collimator rotation speed this is a mechanical limit that could be easily overcome. Conclusion: Our results indicate that RAO is able to provide superior dose distributions, particularly with larger (1 cm) leaf width MLCs, while maintaining the lower MU and number of apertures afforded by the direct aperture approach. Conflict of Interest: Supported in part by Varian Medical Systems.

Purpose: To determine the leaf design characteristics (separation between leaves, leaf width, step size, and divergent vs. non‐divergent leaves) and the optimal distance‐from‐patient of a multileaf collimator(MLC) used in a proton therapy facility. These parameters have a significant effect on the penumbra width and leakage dose. Method and Materials: The GEANT4.7 Monte‐Carlo code was used to simulate the various design parameters for the MLC. The geometry included a representative double‐scattered beam incident on the MLC spaced at variable distances from a water phantom. Modifications were made to the code to input the leaf geometry from an external file so the many variations of leaf design could be tested without recompiling the code. Results: The output of the Monte‐Carlo scored (1) the dose deposition in the water phantom, from which a measure of lateral penumbra as a function of depth and/or conformation to an pseudo‐PTV could be determined, and (2) the number of secondary particles, e.g. neutrons, that were incident on the phantom and whether these secondary particles came through the MLC or in the gaps between the leaves. Conclusion: These results, along with mechanical‐design, electrical‐design, and cost considerations, are being used to design the proton therapyMLC to provide good dose conformation to the target while minimizing the normal tissue dose caused by leakage.

Purpose: To investigate the effect of MLC leaf width and PTV margin on intensity modulated radiosurgery (IMSRS) and radiostherapy (IMSRT) dose distributions. Method and Materials: Twelve patients previously treated with IMSRS or IMSRT were retrospectively planned with a 5mm or 3mm MLC leaf width and a 3mm or 2mm PTV margin using the already contoured CTV, critical structures and organs at risk (OARs). The same beam arrangement, planning parameters and plan selection criteria were used in each four plans for a given patient. Same target coverage was achieved by renormalizing each plan so that the prescription dose covered at least 99% of the PTV. Plan indexes — D max, D min, and D mean, conformity index (CI), V70, V50, D 95 and V 95 were calculated from the dose‐volume histograms of PTV, normal tissue, or OARs. Ratios of plan indexes were computed and hypotheses tests were performed on the mean ratios to determine the significance of the relative changes. Results: The PTV was well covered for all plans. The PTV was 25% smaller when 2mm instead of 3mm PTV margin was used; CI of 3mm MLC was 7% lowered than that of 5mm MLC. The decrease of MLC leaf width had a similar effect as that of PTV margin in reducing V70 and V50 of the normal tissue and D mean of brainstem by ∼10%. However, D mean of optic nerve and chiasm was more sensitive to the change of PTV margin. Conclusion: For IMSRT, the combination of 5mm MLC and 2mm PTV margin is dosimetrically equal to that of 3mm MLC and 3mm PTV margin for both PTV coverage and normal tissue sparing. The use of 5mm MLC and 2mm PTV margin for IMSRS is problematic because V70 and V5 are ∼10% higher than that of 3mm MLC and 2mm PTV margin.