Search

PURPOSE To provide an assessment of safety regarding high-dose-rate after-loading brachytherapy (HDR-BT) based on adverse events reported to the OpenFDA, an open access database maintained by the United States Food and Drug Administration (FDA). METHODS OpenFDA was queried for HDR-BT events between 1993 and 2019. A brachytherapist categorized adverse events (AEs) based on disease site, applicator, manufacturer, event type, dosimetry impact, and outcomes. Important findings are summarized. RESULTS 372 AEs were reported between 1993 and 2019, with a downwards trend after 2014. Nearly half of AEs (48.9%) were caused by a device malfunction, and 27.4% resulted in patient injury. Breast (49.2%) and Gyn (23.7%) were the most common disease sites of AEs. Applicator breaks cause the majority of AEs (64.2%) and breast balloon implants were the most common applicator to malfunction (38.7%). User error contributed to only 16.7% of events. 11.0% of events required repair of the afterloader. There were no reported staff injuries or patient deaths from an AE, however 24.7% of patients received resultant incorrect radiation dose, 16.4% required additional procedures to rectify the AE, and 3.0% resulted in unintended radiation to staff. CONCLUSION The OpenFDA database has shown a decreasing trend in AEs since 2014 for HDR-BT. Most AEs are not caused by user error and do not cause patient injury or incorrect radiation dose. Investigation into methods to prevent failures and improve applicators such as the breast balloon could improve safety. These results support the continued use of HDR-BT as a safe treatment modality for cancer.

Purpose Brain metastases have a highly variable prognosis depending on the primary tumor and associated prognostic factors. Standard of care for patients with these tumors includes craniotomy, stereotactic radiosurgery (SRS), or whole brain radiotherapy (WBRT) for patients with brain metastases. Brachytherapy shows great promise as a therapy for brain metastases, but its role has not been sufficiently explored in the current literature. Material and methods The PubMed, Cochrane, and Scopus databases were searched using a combination of search terms and synonyms for brachytherapy, brain neoplasms, and brain metastases, for articles published between January 1st, 1990 and January 1st, 2018. Of the 596 articles initially identified, 37 met the inclusion criteria, of which 14 were review articles, while the remaining 23 papers with detailing individual studies were fully analyzed. Results Most data focused on 125I and suggested that it offers rates of local control and overall survival comparable to standard of care modalities such as SRS. However, radiation necrosis and regional recurrence were often high with this isotope. Studies using photon radiosurgery modality of brachytherapy have also been completed, resulting superior regional control as compared to SRS, but worse local control and higher rates of radiation necrosis than 125I. More recently, studies using the 131Cs for brachytherapy offered similar local control and survival benefits to 125I, with low rates of radiation necrosis. Conclusions For a variety of reasons including absence of physician expertise in brachytherapy, lack of published data on treatment outcomes, and rates of radiation necrosis, brachytherapy is not presently a part of standard paradigm for brain metastases. However, our review indicates brachytherapy as a modality that offers excellent local control and quality of life, and suggested that its use should be further studied.

Yttrium-90 microsphere brachytherapy of the liver exploits the distinctive features of the liver anatomy to treat liver malignancies with beta radiation and is gaining more wide spread clinical use. This report provides a general overview of microsphere liver brachytherapy and assists the treatment team in creating local treatment practices to provide safe and efficient patient treatment. Suggestions for future improvements are incorporated with the basic rationale for the therapy and currently used procedures. Imaging modalities utilized and their respective quality assurance are discussed. General as well as vendor specific delivery procedures are reviewed. The current dosimetry models are reviewed and suggestions for dosimetry advancement are made. Beta activity standards are reviewed and vendor implementation strategies are discussed. Radioactive material licensing and radiation safety are discussed given the unique requirements of microsphere brachytherapy. A general, team-based quality assurance program is reviewed to provide guidance for the creation of the local procedures. Finally, recommendations are given on how to deliver the current state of the art treatments and directions for future improvements in the therapy.

We have implemented a three-dimensional dose calculation technique accounting for dose inhomogeneity within the liver and tumor of a patient treated with 90 Y microspheres. Single-photon emission computed tomography(SPECT)images were used to derive the activity distribution within liver. A Monte Carlo calculation was performed to create a voxel dose kernel for the 90 Y source. The activity distribution was convolved with the voxel dose kernel to obtain the three-dimensional (3D) radiation absorbed dose distribution. An automated technique was developed to accurately register the computed tomography(CT) and SPECT scans in order to display the 3D dose distribution on the CT scans. In addition, dose-volume histograms were generated to fully analyze the tumor and liver doses. The calculated dose-volume histogram indicated that although the patient was treated to the nominal whole liver dose of 110 Gy, only 16% of the liver and 83% of the tumor received a dose higher than 110 Gy. The mean tumor and liver doses were 163 and 58 Gy, respectively.

A quality assurance method is developed for measuring, verifying and analyzing intensity modulated radiation fields. It is applicable for rotational and fixed-beam intensity modulated radiation therapy(IMRT)treatments. A gantry-mount device was constructed to measure the transmission dose of an IMRT field using radiographicfilms. A double-exposure technique with optimal kernel estimate method was developed to minimize the errors from measurements. A χ 2 confidence level test method was developed to detect the discrepancies between measured and prescribed IMRT fluence distributions. Our method was tested for rotational and fixed-beam IMRTtreatment verifications. The method was found insensitive to the hardware-related parameters for rotational and fixed-beam IMRTdeliveries. The χ 2 confidence level test was found to be more sensitive than linear correlation method in detecting relative small errors for cases with a few segments or narrow regions of interest. In conclusion, we demonstrated a quantitative method for verifying and analyzing IMRTtreatmentdeliveries.

Administration of yttrium-90 microspheres via the hepatic artery is an attractive approach to selectively delivertherapeuticdoses of radiation to livermalignancies. This procedure allows deliveringradiation absorbed doses in excess of 100 Gy to the tumors without significant liver toxicity. The microsphere therapy involves different specialties including medicaloncology,radiationoncology, nuclear medicine, interventional radiology, medical physics, and radiation safety. We have treated 80 patients with nonresectable hepatic tumors with yttrium-90 microspheres during the past two years on an institutional study protocol. The nominal radiation absorbed dose to the tumor in this study was 150 Gy. Required activity was calculated based on the nominal radiation absorbed dose and patient’s liver volume obtained from the CT scan, assuming a uniform distribution of the microspheres within the liver. Microspheres were administered via a catheter placed into the hepatic artery. The actual radiation absorbed doses to tumors and normal liver tissue were calculated retrospectively based on the patient’s 99 m Tc-MAA study and CT scans. As expected, the activity uptake within the liver was found to be highly nonuniform and multifold tumor to nontumor uptake was observed. A partition model was used to calculate the radiation absorbed dose within each region. For a typical patient the calculated radiation absorbed doses to the tumor and liver were 402 and 118 Gy, respectively. The radiation safety procedure involves confinement of the source and proper disposal of the contaminated materials. The average exposure rates at 1 m from the patients and on contact just anterior to the liver were 6 and 135 uSv/h, respectively. The special physics and dosimetry protocol developed for this procedure is presented.

Purpose: Intensity-modulated arc therapy (IMAT) is a method for delivering intensity-modulated radiation therapy (IMRT) using rotational beams. During delivery, the field shape, formed by a multileaf collimator (MLC), changes constantly. The objectives of this study were to ( 1 ) clinically implement the IMAT technique, and ( 2 ) evaluate the dosimetry in comparison with conventional three-dimensional (3D) conformal techniques. Methods and Materials: Forward planning with a commercial system (RenderPlan 3D, Precision Therapy International, Inc., Norcross, GA) was used for IMAT planning. Arcs were approximated as multiple shaped fields spaced every 5–10° around the patient. The number and ranges of the arcs were chosen manually. Multiple coplanar, superimposing arcs or noncoplanar arcs with or without a wedge were allowed. For comparison, conventional 3D conformal treatment plans were generated with the same commercial forward planning system as for IMAT. Intensity-modulated treatment plans were also created with a commercial inverse planning system (CORVUS, Nomos Corporation). A leaf-sequencing program was developed to generate the dynamic MLC prescriptions. IMAT treatment delivery was accomplished by programming the linear accelerator (linac) to deliver an arc and the MLC to step through a sequence of fields. Both gantry rotation and leaf motion were enslaved to the delivered MUs. Dosimetric accuracy of the entire process was verified with phantoms before IMAT was used clinically. For each IMAT treatment, a dry run was performed to assess the geometric and dosimetric accuracy. Both the central axis dose and dose distributions were measured and compared with predictions by the planning system. Results: By the end of May 2001, 50 patients had completed their treatments with the IMAT technique. Two to five arcs were needed to achieve highly conformal dose distributions. The IMAT plans provided better dose uniformity in the target and lower doses to normal structures than 3D conformal plans. The results varied when the comparison was made with fixed gantry IMRT. In general, IMAT plans provided more uniform dose distributions in the target, whereas the inverse-planned fixed gantry treatments had greater flexibility in controlling dose to the critical structures. Because the field sizes and shapes used in the IMAT were similar to those used in conventional treatments, the dosimetric uncertainty was very small. Of the first 32 patients treated, the average difference between the measured and predicted doses was −0.54 ± 1.72% at isocenter. The 80%–95% isodose contours measured with film dosimetry matched those predicted by the planning system to within 2 mm. The planning time for IMAT was slightly longer than for generating conventional 3D conformal plans. However, because of the need to create phantom plans for the dry run, the overall planning time was doubled. The average time a patient spent on the table for IMAT treatment was similar to conventional treatments. Conclusion: Initial results demonstrated the feasibility and accuracy of IMAT for achieving highly conformal dose distributions for different sites. If treatment plans can be optimized for IMAT cone beam delivery, we expect IMAT to achieve dose distributions that rival both slice-based and fixed-field IMRT techniques. The efficient delivery with existing linac and MLC makes IMAT a practical choice.

The authors have devised a computer controlled translational couch to administer total body irradiation accurately and safely. In this technique, patients comfortably rest on a couch in supine and prone positions and are transported slowly through a narrow beam with the gantry in upright position. Dose to the patient is determined by the couch velocity that is calculated based on physical parameters such as patient's dimensions, beam geometry and machine dose rate. In the authors' design, the couch velocity is continuously updated for possible machine dose rate fluctuations to eliminate dose non-uniformity within the patient. The translational couch technique provides better dose uniformity within the patient compared to fixed beam techniques, and allows a more precise shielding block placement for organs at risk. At the same time, it presents special challenges for dosimetry calculations. A dosimetry parameter is introduced that converts moving beam output to fixed beam output factor. Based on this factor, a simple dosimetry calculation method has been implemented that takes advantage of conventional dosimetry parameters, eliminating extensive dosimetry measurements. Comprehensive dose measurements within phantoms confirmed the validity of the calculations method. Further improvements in dose uniformity within the patient by modulating couch velocity with patient thickness and shielding the organs at risk are discussed.

Effective radioimmunotherapy may depend on a priori knowledge of the radiation absorbed dose distribution obtained by trace imaging activities administered to a patient before treatment. A new, fast, and effective treatment planning approach is developed to deal with a heterogeneous activity distribution. Calculation of the three-dimensional absorbed dose distribution requires convolution of a cumulated activity distribution matrix with a point-source kernel; both are represented by large matrices (64 x 64 x 64). To reduce the computation time required for these calculations, an implementation of convolution using three-dimensional (3-D) fast Hartley transform (FHT) is realized. Using the 3-D FHT convolution, absorbed dose calculation time was reduced over 1000 times. With this system, fast and accurate absorbed dose calculations are possible in radioimmunotherapy. This approach was validated in simple geometries and then was used to calculate the absorbed dose distribution for a patient's tumor and a bone marrow sample.

Purpose: Intensity Modulated Radiation Therapy (IMRT) dosimetry verification is routinely conducted via integrated or individual field dosimetry using film or a matrix of detectors. Composite dose evaluation (whole plan delivered intact to dosimetry device) may more closely present relevant 3D dosimetry but such measurements must still be performed in a surrogate phantom not in the patient’s anatomy at high sampling density enabling precise assessment of critical anatomy volumes. Techniques and software systems are commercially available which use individual field dosimetry measurements as input into algorithms that estimate 3D patient dose distributions on CT scan derived target volumes and OARs, thus allowing direct DVH analysis vs. TPS DVH. The purpose of this work is to present a systematic benchmarking technique to evaluate the latter claim, utilizing a flat phantom as a simple patient where measurements at different depths are used to compare measured and reconstructed dose planes in the phantom. Method and Materials: A MapCheck2 diode array and 3DVH™ software from Sun Nuclear were used for this study. Delivered planar dose was measured with the diode array as an input to 3DVH™ software that was used to estimate the 3D dose matrix. The 3DVH™ software uses the Planned Dose Perturbation (PDP) algorithm to reconstruct a 3D dose in the patient as defined by the boundaries of the patient “skin” contour. Conventional planar dose measurements at fixed depths are input to generate a 3D dose matrix for DVH evaluation. In this study, benchmarking consists of taking IMRT beams created in actual patient treatment plans; re-computing these on a flat phantom stored in the RTPS, and exporting these calculations to 3DVH™ as individual patient plans. Accuracy of the output of 3DVH™ is tested by comparing measured planar doses over a range of depths to the same planes reconstructed by 3DVH™. Four complex IMRT cases were selected and examined in this study. Planar measurements were conducted at three depths, yielding three sets of analyses per patient plan evaluated. The sensitivity to depth of measurement was evaluated. Additionally, patient cases computed on the patients’ CT scans are imported into 3DVH™ and DVH analyses are evaluated. Deviations in the DVH analyses were evaluated by comparing DVHs obtained by 3DVH™’s to the RTPS’ DVHs. Results: The Gamma Index analysis, comparing calculated 3D dose with measured 3D dose with 2% and 2mm DTA criteria returned a pass rate of > 90% for all patient cases calculated by the treatment planning system and it returned a pass rate of > 96% in 9 out of 10 cases calculated by 3DVH™. Extracted computed dose planes with 3DVH™ software at different depths in the flat phantom passed all gamma evaluation analyses when compared to measured planes at different depths using MapCheck2. Conclusion: Studying complex Head and Neck IMRT fields, it was shown that the 3D dose distribution predicted by the PDP algorithm is both accurate and consistent.

Yttrium-90 microsphere brachytherapy of the liver exploits the distinctive features of the liver anatomy to treat liver malignancies with beta radiation and is gaining more wide spread clinical use. This report provides a general overview of microsphere liver brachytherapy and assists the treatment team in creating local treatment practices to provide safe and efficient patient treatment. Suggestions for future improvements are incorporated with the basic rationale for the therapy and currently used procedures. Imaging modalities utilized and their respective quality assurance are discussed. General as well as vendor specific delivery procedures are reviewed. The current dosimetry models are reviewed and suggestions for dosimetry advancement are made. Beta activity standards are reviewed and vendor implementation strategies are discussed. Radioactive material licensing and radiation safety are discussed given the unique requirements of microsphere brachytherapy. A general, team-based quality assurance program is reviewed to provide guidance for the creation of the local procedures. Finally, recommendations are given on how to deliver the current state of the art treatments and directions for future improvements in the therapy.

A new treatment technique, intensity modulated arc therapy (IMAT), has been implemented in the authors' clinical practice. Instead of delivering intensity-modulated beams with fixed gentry angles, IMAT delivers optimized dose distributions using rotational beams. During delivery, the field shape, which is formed by a multileaf collimator (MLC), changes constantly. Treatment plans are developed with a combination of forward and inverse planning. Arcs are approximated as multiple shaped fields spaced every 10 degrees around the patient. Multiple coplanar or non-coplanar arcs are used in combination with wedges. The beam weights can be optimized outside of the commercial planning system. A leaf-sequencer is used to convert the fields from the final plan into MLC leaf sequences. Comparisons were made with conventional and inversely optimized treatment plans. Dosimetric accuracy of the entire process was verified with phantoms before IMAT was used clinically. Patient-specific verifications were performed using both the absolute and relative dose measurements in a humanoid phantom. The authors have demonstrated the clinical feasibility and accuracy of IMAT treatments on patients with head and neck cancers. Generally, no more than 3 arcs are needed to achieve a conformal dose distribution. The complexity of the IMAT is not significantly greater than existing are therapy from the point of view of the operator. The treatment time using IMAT is similar to conventional treatments. Clinical examples of IMAT treatments will be presented to illustrate the dosimetric advantage of rotational delivery.

We have developed an automated technique to accurately register the CT and SPECT scans of the liver of patients treated with radioactive microspheres for tumour targeting assessment. An anthropomorphic phantom was used to validate the accuracy of the registration algorithm. The phantom liver and three fiducial markers placed on its surface were filled with 99mTc. The phantom was scanned with CT and SPECT scanners in different positions. The liver volume was contoured on the CT scans from which a three-dimensional liver mask was created. By constraining the liver volume to the volume obtained from the CT scans, another liver mask was automatically created from the SPECT images. An adaptive simulated annealing algorithm was used to minimize the difference between the two volumes formed by the two sets of masks. The algorithm involved rigid transformation of the SPECT mask to reach the optimization goal. The accuracy of the algorithm was evaluated by the superposition of the fiducial markers on the CT and SPECT. The registered SPECT overlaid on the CT scan of the phantom showed congruence of the fiducial markers on CT and SPECT images within 1 mm. The technique was applied to a patient image set who received the microsphere infusion procedure. The registered CT and SPECT images of the patient showed that the majority of the activity was concentrated in the tumour, indicating a successful tumour targeting.

Treatment verification has been a weak link in external beam radiation therapy. As new and more complicated treatment techniques, such as intensity-modulated radiation therapy (IMRT), are implemented into clinical practice, verifying the accuracy of treatment delivery becomes increasingly important. Existing methods for treatment verification are highly labor intensive. We have developed a method for verifying the delivery of external beam radiotherapy and implemented the methodology into a system consisting of both hardware and software components. The system uses grayscale images acquired on the treatment machine from the planned treatment beams. From these images, the photon fluence distribution of each beam is derived. These measured photon fluence maps are then used as input to a separate dose calculation engine to compute the delivered absolute dose and the dose distribution in the same patient, assuming that the patient is set up as required by the treatment plan. The dose distribution generated from the measured fluence maps can then be compared to that of the treatment plan. Software tools, such as overlaying isodose curves generated with this method on those imported from the plan, dose difference maps, dose difference volume histograms, and three-dimensional perspective views of the dose differences, have also been developed. The system thus provides a means to verify the dose, the dose prescription, and the monitor units applied. The potential exists with a suitable electronic portal imaging system to reduce the quality assurance efforts, especially for IMRT.

Purpose The purpose of this work is to explore the possibility of using intensity-modulated radiation therapy (IMRT) to deliver the boost dose to the tumor bed simultaneously with the whole-breast IMRT to reduce the radiation treatment time by 1–2 weeks. Methods and materials The biologically effective dose (BED) for different treatments was calculated using the linear-quadratic (LQ) model with parameters previously derived for breast cancer from clinical data (α/β = 10Gy, α = 0.3Gy −1 ). A potential doubling time of 15 days (from in vitro measurements) for breast cancer and a generic α/β ratio of 3 Gy for normal tissues were used. A series of regimens that use IMRT as initial treatment and an IMRT simultaneous integrated boost (SIB) were derived using biologic equivalence to conventional schedules. Possible treatment plans with IMRT SIB to the tumor bed were generated for 2 selected breast patients, 1 with a shallow tumor and 1 with a deep-seated tumor. Plans with a simultaneous integrated electron boost were also generated for comparison. Dosimetric merits of these plans were evaluated based on dose volume histograms. Results A commonly used conventional treatment of 45 Gy (1.8 Gy × 25) to the whole breast and then a boost of 20 Gy (2 Gy × 10) is biologically equivalent to an alternative plan of 1.8 Gy × 25 to the whole breast with a 2.4 Gy × 25 SIB to the tumor bed. The new regime reduces treatment time from 7 to 5 weeks. For the patient with a deep-seated tumor, the IMRT plans reduce the volume of the breast that receives high doses (compared with the conventional photon boost plan) and provides good coverage of the target volumes. Conclusion It is biologically and dosimetrically feasible to reduce the overall treatment time for breast radiotherapy by using an IMRT simultaneous integrated boost. For selected patient groups, IMRT plans with a new regimen can be equal to or better than conventional plans.

We report in-vivo dosimetry and phantom study results for determining complication rate of the eye lens for trigeminal neuralgia (TN) patients treated using gamma knife radiosurgery. This study is driven by the observation that a large dose such as 70-80 Gy was generally prescribed for trigeminal neuralgia, and, therefore, a small percentage (2-3%) of scattering dose to the lens of the eye could reach or even exceed its tolerance dose. In-vivo dosimetry measurements on six patients and a head phantom were performed to investigate these effects. The average lens dose was measured to be 0.077/spl plusmn/0.006 Gy. This value is significantly lower than the 1-2% as predicated from the gamma knife treatment planning system. Using an empirical biological model, the probability of the lens complication was determined to be 0.1%. This low rate confirms our clinical findings that few TN patients develop cataracts.

We recently implemented intensity-modulated arc therapy (IMAT) at our institution. In this study, we evaluate the dosimetric merits of the application of this technique to the treatment of prostate cancer. Each IMAT treatment plan incorporated bilateral overlapping arcs. The dose from each beam segment was computed using the three-dimensional dose model of a clinical treatment planning system (Render Plan 3.5, Precision Therapy). The weights assigned to the individual arc segments were optimized using a gradient search method. For 12 patients, comparisons were made between the IMAT treatment plans and corresponding plans using fixed cone-beam intensity-modulated radiotherapy (IMRT) from a commercial inverse planning system (CORVUS, NOMOS Corp.). We found that the optimized IMAT treatments produced similar dose distributions to the IMRT deliveries. Compared with the IMRT treatments, the IMAT treatments produced slightly less target dose homogeneity with consistently greater sparing of the rectum in regions of lower dose. The trade-off between target dose conformity and rectum sparing can be adjusted in both optimization procedures. Because the total beam-on time for IMAT delivery is 1 to 2 minutes with approximately 5-6 minutes of patient setup time, the delivery efficiency of the IMAT treatment was significantly better than the multiple-beam IMRT treatment.

The frequent blocking of the irradiated volume in intensity modulated radiation therapy (IMRT) makes the head-scatter fraction of the incident photon fluence more significant than that in conventional therapy with open fields. On the other hand. certain collimator configurations block scatter photons directed to a given observation point while allowing primary photons to be transmitted. The 'anomalous blocking' makes the primary field a poor indicator of the scatter fluence. Since large MU-to-cGy ratios in IMRT can magnify head-scatter uncertainties, it becomes necessary to accurately model both the effective scatter source and the collimator structure that limits the scatter reaching the irradiated volume. First we obtain a dual-source model, using a Taylor series expansion to derive the effective scatter source distribution from the data measured for the Elekta SL20 linac equipped with a multi-leaf collimator (MLC). Then, using a raytracing algorithm, we calculate the transmission of scatter rays from the effective scatter source plane to points in the patient plane. The method can account for the anomalous blocking of scatter by the MLC leaves and the backup diaphragms. For a variety of collimator settings tested, the calculations agree with measurements to an accuracy of 0.002psi10 x 10, where psi10 x 10 is the total (primary + scatter) photon fluence of an open 10 x 10 cm2 field for the same MU delivered. Although the significance of collimator structure in IMRT depends strongly on fields shapes employed for the delivery, potential cumulative errors on the order of a few per cent can be avoided in fluence calculations if the proposed method is used.

Stereotactic Gamma Knife radiosurgery has been widely used for treating trigeminal neuralgia (TN). A single large fractional dose of 7000 to 9000 cGy is commonly prescribed as the maximum dose for these treatments. For this reason, if a small percentage of the prescribed dose such as 2–3 % scattered to the eye, it could reach or even exceed the tolerance dose of the lens. For several TN cases, we found that the Leksell Gamma Plan system calculates the lens dose about 0.5–2 % of the maximum dose independent of the use of eye shielding. These dose values are significantly high and it motivated us to investigate the lens dose for the TN patients treated with stereotactic Gamma Knife radiosurgery. Phantom studies and in vivo dosimetry measurements were carried out for six patients treated at our institution. The average dose to the lens ipsilateral to the treated nerve was measured to be 7.7±0.6 cGy. Based on the biological model of Lyman and Emami [Int. J. Radiat. Oncol. Biol. Phys. 21, 109–122 (1991)], the probability of the lens complication (cataract) was determined to be 0.1%. Our findings suggest that few TN patients would develop cataracts after receiving Gamma Knife radiosurgery. PACS number(s): 87.53.–j, 87.66.–a

Intensity-modulated arc therapy, or MAT, is one of the methods for delivering intensity-modulated radiation treatments to achieve high dose conformity. Instead of delivering intensity-modulated beams with fixed gantry angles [1], IMAT delivers optimized dose distributions by rotating the radiation beam around the patient. During delivery, the field shape, which is formed by a multileaf collimator (MLC), changes constantly as determined by the required intensity distributions at different beam angles. Intensity distributions at all angles around the patient are achieved with multiple overlapping arcs of different field apertures and gantry speed. Therefore, IMAT is also different from tomotherapy[2,3], which use intensity modulated fan beams rotating around the patient to delivery the treatment slice-by-slice. As with tomotherapy, IMAT combines intensity modulation and rotational delivery. Detailed description of the technique was reported by Yu in a previous article [4].

Purpose: Total body irradiation (TBI) with moving couch technique has been used in our clinic for years. Moving couch TMR for a field size has been acquired by multiplying TMR for a stationary beam and the ‘m factor’ defined as the ratio between moving phantom dose to stationary beam dose for same setup. The ‘m factor’ has been known to be field size dependent and not depth dependent. In order to replace lung and kidney blocks and achieve better dose uniformity using dynamic MLC, comprehensive understanding on the TMR for variable field sizes for an MLC leaf sequence is required. Method and Materials: A thorough measurement on the moving TMR has been performed. TMR's for field sizes of 5cm×40cm, 10cm×40cm, 15cm×40cm, 20cm×40cm and 30cm×40cm were measured both for the stationary and for the moving phantoms. Field sizes are defined at SAD 100cm and TMR's were measured at SCD 170cm for depths dmax to 34cm in 50cm×50cm×50cm water phantom with 6MV photon (21EX, Varian, Palo Alto, CA). Results: TMR's in moving phantom for different field sizes showed not more than 1% difference, while those of stationary phantom showed up to 34.8% difference for different sizes. The m factor which has been known to be a constant for a given field size rapidly increases after 15cm depth. Analysis of the data allowed us to understand the phenomena of measuring a point dose in a moving phantom. The key to the understanding was that both the phantom scatter and primary dose seen at the measurement position does not vary with field size when normalized to the reference condition. Conclusion: TMR's are independent to field size when couch is moving. This suggests that TMR variation does not need to be considered when designing the dose compensator with dynamic MLC for moving couch TBI.