PURPOSE:We aim to investigate perioperative and subacute postoperative complications in patients undergoing LDR or HDR monotherapy for prostate cancer. We hypothesize a low rate of complications, and a favorable toxicity profile in patients treated with HDR compared to LDR. MATERIALS AND METHODS:A prospectively collected institutional database was queried for patients treated with HDR or LDR prostate monotherapy between 1998 and 2021. Toxicities were determined per CTCAE. Claims based billing codes were obtained to identify additional events. Events occurring within 4 months of treatment were defined as perioperative or subacute postoperative complications. RESULTS:759 patients were identified, 446 received LDR with 125I, and 313 received HDR with 192Ir. HDR patients had higher risk features: 75.7% with Gleason score 7+ versus 2.4% of LDR, and 16% with initial PSA 10+ ng/mL versus 2.7% of LDR. Toxicities were mild with the most common being grade 1 GU frequency and nocturia at ∼50%. HDR patients had significantly less grade 2 dysuria (2.6% vs. 9.0%), frequency (4.8% vs. 9.4%), hematuria (1.0% vs. 5.2%), nocturia (3.8% vs. 9.4%), and urinary obstructive symptoms (7.3% vs. 11.2%), all statistically significant. 11 (1.4%) patients had infection requiring antibiotics: 8 (1.8%) from the LDR group and 3 (1%) from the HDR group. Cardiopulmonary events were low at <2% overall, without difference between HDR and LDR. CONCLUSIONS:Overall toxicity rates support the safety of prostate brachytherapy. HDR monotherapy is associated with significantly less perioperative and subacute postoperative GU events when compared to LDR monotherapy. Cardiopulmonary events were equally rare in both groups.

Purpose The efficacy of low dose rate (LDR) and high dose rate (HDR) brachytherapy for prostate cancer have been well established. Fewer studies directly compare short-term perioperative complications between these modalities, and these often have small sample sizes and heterogeneous settings including brachytherapy boost after external beam radiotherapy (EBRT). The purpose of this study is to investigate the perioperative and subacute postoperative complications in patients undergoing exclusively LDR or HDR monotherapy at a single institution with a high volume brachytherapy program. We hypothesize a favorable acute and subacute toxicity profile in patients treated with HDR monotherapy compared to LDR. Materials and Methods Patients age >18 with biopsy proven prostate cancer and treated with HDR or LDR implants alone, between 1998-2021 were identified from a prospectively collected, IRB approved institutional database. Toxicities and grades were determined as per CTCAE v3, and those within 4 months of treatment were defined as perioperative or subacute postoperative complications associated with brachytherapy. We used Fisher's exact tests and Wilcoxon rank sum tests to determine the association between the complication groups and patient/treatment factors. Results There were a total of 759 patients included for analysis, 446 received LDR using 125iodine (125I) or 103palladium (103Pd), and 313 received HDR using 192iridium (192Ir). HDR patients were slightly older than LDR patients (median age 67 vs 65, p=0.001) and had higher risk features: HDR patients had higher Gleason scores (75.7% with GS 7+ versus 2.4% of LDR patients) and higher initial PSA values (16% with 10+ ng/mL versus 2.7% of LDR patients). Reported acute and subacute toxicity were mild with the most common toxicity among all patients being grade 1 GU frequency and nocturia, each at ∼50%. GI toxicity was rare, with highest incidence being grade 1 frequency in 12.6% of patients. Grade 3 toxicities were exceedingly rare, with only 1 grade 3 GI toxicity, and 18 (2.4%) cases of grade 3 urinary obstructive symptoms. There is a significant difference in toxicity profile between LDR and HDR. Patients receiving HDR had significantly less grade 2+ urinary symptoms, including dysuria (2.6% vs 9.0%, p<0.001), frequency (4.8% vs 9.9%, p=0.01), hematuria (1.0% vs 6.1%, p<0.001), nocturia (3.8% vs 9.9%, p=0.002), and urinary obstructive symptoms (8.9% vs 14.1%, p=0.03). Infection rates were low, with 12 (1.6%) patients total experiencing GU related infections requiring antibiotics: 8 (1.8%) from the LDR group and 4 (1.3%) from the HDR group. Lastly, there is a clear pattern of change in institutional practice, evolving from LDR based to now an HDR based program. Conclusions In this large single institution analysis, HDR monotherapy is associated with significantly less perioperative and subacute post-operative complications when compared to LDR monotherapy. Toxicity rates were low in both groups, although GU toxicities were significantly more common than GI in both modalities. These data provide evidence supporting a favorable toxicity profile of HDR monotherapy compared to LDR monotherapy for prostate cancer. Interestingly, data from this high volume brachytherapy program also revealed a greater proportion of HDR patients having more advanced disease prior to therapy, and the evolution of an institutional trend in treatment from only performing LDR to HDR only over time. The efficacy of low dose rate (LDR) and high dose rate (HDR) brachytherapy for prostate cancer have been well established. Fewer studies directly compare short-term perioperative complications between these modalities, and these often have small sample sizes and heterogeneous settings including brachytherapy boost after external beam radiotherapy (EBRT). The purpose of this study is to investigate the perioperative and subacute postoperative complications in patients undergoing exclusively LDR or HDR monotherapy at a single institution with a high volume brachytherapy program. We hypothesize a favorable acute and subacute toxicity profile in patients treated with HDR monotherapy compared to LDR. Patients age >18 with biopsy proven prostate cancer and treated with HDR or LDR implants alone, between 1998-2021 were identified from a prospectively collected, IRB approved institutional database. Toxicities and grades were determined as per CTCAE v3, and those within 4 months of treatment were defined as perioperative or subacute postoperative complications associated with brachytherapy. We used Fisher's exact tests and Wilcoxon rank sum tests to determine the association between the complication groups and patient/treatment factors. There were a total of 759 patients included for analysis, 446 received LDR using 125iodine (125I) or 103palladium (103Pd), and 313 received HDR using 192iridium (192Ir). HDR patients were slightly older than LDR patients (median age 67 vs 65, p=0.001) and had higher risk features: HDR patients had higher Gleason scores (75.7% with GS 7+ versus 2.4% of LDR patients) and higher initial PSA values (16% with 10+ ng/mL versus 2.7% of LDR patients). Reported acute and subacute toxicity were mild with the most common toxicity among all patients being grade 1 GU frequency and nocturia, each at ∼50%. GI toxicity was rare, with highest incidence being grade 1 frequency in 12.6% of patients. Grade 3 toxicities were exceedingly rare, with only 1 grade 3 GI toxicity, and 18 (2.4%) cases of grade 3 urinary obstructive symptoms. There is a significant difference in toxicity profile between LDR and HDR. Patients receiving HDR had significantly less grade 2+ urinary symptoms, including dysuria (2.6% vs 9.0%, p<0.001), frequency (4.8% vs 9.9%, p=0.01), hematuria (1.0% vs 6.1%, p<0.001), nocturia (3.8% vs 9.9%, p=0.002), and urinary obstructive symptoms (8.9% vs 14.1%, p=0.03). Infection rates were low, with 12 (1.6%) patients total experiencing GU related infections requiring antibiotics: 8 (1.8%) from the LDR group and 4 (1.3%) from the HDR group. Lastly, there is a clear pattern of change in institutional practice, evolving from LDR based to now an HDR based program. In this large single institution analysis, HDR monotherapy is associated with significantly less perioperative and subacute post-operative complications when compared to LDR monotherapy. Toxicity rates were low in both groups, although GU toxicities were significantly more common than GI in both modalities. These data provide evidence supporting a favorable toxicity profile of HDR monotherapy compared to LDR monotherapy for prostate cancer. Interestingly, data from this high volume brachytherapy program also revealed a greater proportion of HDR patients having more advanced disease prior to therapy, and the evolution of an institutional trend in treatment from only performing LDR to HDR only over time.

Modern fluoroscopes used for image guidance have become quite complex. Adding to this complexity are the many regulatory and accreditation requirements that must be fulfilled during acceptance testing of a new unit. Further, some of these acceptance tests have pass/fail criteria, whereas others do not, making acceptance testing a subjective and time-consuming task. The AAPM Task Group 272 Report spells out the details of tests that are required and gives visibility to some of the tests that while not yet required are recommended as good practice. The organization of the report begins with the most complicated fluoroscopes used in interventional radiology or cardiology and continues with general fluoroscopy and mobile C-arms. Finally, the appendices of the report provide useful information, an example report form and topics that needed their own section due to the level of detail.

Purpose: To evaluate AAPM TG-218 recommended tolerances for IMRT QA for conventional and SBRT delivery. Methods: QA analysis was repeated for 150 IMRT/VMAT patients with varying gamma criteria. True composite delivery was utilized, corrected for detector and output variation. Universal tolerance (TLuniv) and action limits (AL(univ)) were compared with statistical process control (SPC) TLSPC and AL(SPC) values. Analysis was repeated as a function of plan complexity for 250 non-stereotactic body radiotherapy (SBRT) VMAT patients at 3%/2mm and a threshold of 10% and for 75 SBRT VMAT patients at 2%/2 mm and a threshold of 50% with results plotted as a function of PTV volume. Regions of failure were dose-scaled on the planning CT data sets based on delivery results. Results: The IMRT/VMAT TLSPC and AL(SPC) for gamma criteria of 3%/3 mm were 96.5% and 95.6% and for 3%/2 mm were 91.2% and 89.2%, respectively. Correlation with plan complexity for conventional fractionation VMAT was "low" for all sites with pelvis having the highest r value at -0.35. The equivalent SBRT PTV diameter ranged from 2.0 cm to 5.6 cm. Negative low correlation was found for 38 of 75 VMAT cases below AL(univ). Conclusions: The AL(univ) and AL(SPC) are similar for 3%/2 mm. However, our 5% failure rate for AL(univ), may result in treatment start delays approximately 2 times/month, given 40 new cases/month. VMAT QA failure at stricter criteria did not correlate strongly with plan complexity. Site-specific action limits vary less than 3% from the average. SBRT QA results do not strongly correlate with target size over the range studied.

To provide accurate and fast 3-D dose verification for hypofractionated stereotactic radiotherapy (SRT/SBRT) of small and multi targets calculated with a Varian Eclipse treatment planning system (TPS) delivered on a Varian accelerator. Ten brain and lung hypofractionated SRT/SBRT linac-based and CyberKnife plans were generated by the Eclipse system for delivery on the accelerator with the Millenium-120 leaf multileaf collimator (MLC) and Multiplan for the CyberKnife machine. These clinical SRT/SBRT plans required accurate quality assurance measurements to obtain absolute point dose and 3-D dose distributions due to the low number of fractions and high fraction doses. For small-field and multi-target plans, the EGS4/MCSIM code was used to calculate the dose distribution. A 0.125 cc ion chamber, a 0.016 cc pin-point chamber and Kodak EDR2 film were used for the measurements and the results were compared with Monte Carlo (MC) calculations. The dosimetry for small-field and multi-target treatment plans is challenging due to the comparable range of secondary electrons and the field sizes defined by SRT/SBRT MLC segments. Our MC simulations can accurately reproduce the linac dose distributions (within 1%/1 mm) three dimensionally. For the clinical SRT/SBRT plans investigated in this work, the MC doses agreed within 3% with ion chamber measurements and within 2%/2 mm with film measurements. The doses calculated by the Eclipse AAA algorithm and Multiplan differed by no more than 5% from MC calculations for small (4–40 cc) Planning Target Volumes (PTVs). MC dose calculation provides accurate and fast 3-D dose verification for hypofractionated SRT for small and multi-target treatment plans generated by a Varian Eclipse TPS on a Varian accelerator and Multiplan treatment planning on the CyberKnife System.

Purpose: The purpose of this work is to present a practical, structured process allowing for consistent, safe radiation therapy delivery in the re-treatment environment. Methods and materials: A process for reirradiation is described with documentation in the form of a special physics consultation. Data acquisition associated with previous treatment is described from highest to lowest quality. Methods are presented for conversion to equieffective dose, as well as our departmental assumptions for tissue repair. The generation of organ-at-risk available physical dose for use in treatment planning is discussed. Results using our methods are compared with published values after conversion to biologically effective dose. Utilization of pulsed-low-dose-rate delivery is described, and data for reirradiation using these methods over the previous 5 years are presented. Results: Between 2015 and 2019, the number of patients in our department requiring equieffective dose calculation has doubled. We have developed guidelines for estimation of sublethal damage repair as a function of time between treatment courses ranging from 0% for <6 months to 50% for >1 year. These guidelines were developed based on available spinal cord data because we found that 84% of organs at risk involved nerve-like tissues. The average percent repair used increased from 32% to 37% over this time period. When comparing the results obtained using our methods with published values, 99% of patients had a cumulative biologically effective dose below the limits established for acceptable myelopathy rates. Pulsed-low-dose-rate use over this period tripled with an average prescription dose of 49 Gy. Conclusions: The methods described result in safe, effective treatment in the reirradiation setting. Further correlation with patient outcomes and side effects is warranted.

ObjectiveThis work investigates the time and frequency to observe fiducial markers in MLC-modulated fields during intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) beam delivery for real-time prostate localization.MethodsThirty seven prostate patients treated with IMRT or VMAT were included in this retrospective study. DRR images were generated for all MLC segments/control points using the TPS. The MLC leaf pattern of each control point was overlaid on the DRR, and the number of fiducials within the MLC opening was analyzed. EPID images of fiducials in a pelvic phantom were obtained to demonstrate the fiducial visibility during modulated beam delivery.ResultsGold fiducials were visible on EPID images. The probability of seeing a number of fiducials within the MLC opening was analyzed. At least one fiducial was visible during 42 ± 2% and 52 ± 2% beam-on time for IMRT of the prostate with and without lymph nodes, and during 81 ± 4% and 80 ± 5% beam-on time for VMAT of the prostate with and without lymph nodes, respectively. The mean time interval to observe at least one fiducial was 8.4 ± 0.7 and 5.9 ± 0.5 s for IMRT of the prostate with and without the lymph nodes, respectively, and 1.6 ± 0.1 s for VMAT prostate patients. The estimated potential dosimetric uncertainty was 7% and 2% for IMRT and VMAT, respectively.ConclusionsOur results demonstrated that the time and frequency to observe fiducial markers in MLC-modulated fields during IMRT/VMAT beam delivery were adequate for real-time prostate localization. The beam’s eye view fiducial positions could be used for intrafractional target monitoring and motion correction in prostate radiotherapy.

OBJECTIVE:To investigate motion artifacts on kV CBCT and MV CBCT images with metal localization devices for image-guided radiation therapy.METHODS:The 8 μ pelvis CBCT template for the Siemens Artiste MVision and Pelvis template for the Varian IX on-board Exact Arms kV were used to acquire CBCT images in this study. Images from both CBCT modalities were compared in CNRs, metal landmark absolute positions, and image volume distortion on three different planes of view. The images were taken on a breathing-simulated thoracic phantom in which several typical metal localization devices were implanted, including clips and wires for breast patients, gold seeds for prostate patients, and BBs as skin markers. To magnify the artifacts, a 4 cm diameter metal ball was also implanted into the thoracic phantom to mimic the metal artifacts.RESULTS:For MV CBCT, the CNR at a 4 sec breathing cycle with 1 cm breathing amplitude was 5.0, 3.4 and 4.6 for clips, gold seeds and BBs, respectively while it was 1.5, 2.0 and 1.6 for the kV CBCT. On the images, the kV CBCT showed symmetric streaking artifacts both in the transverse and longitudinal directions relative to the motion direction. The kV CBCT images predicted 89 % of the expected volume, while the MV CBCT images predicted 95 % of the expected volume. The simulated soft tissue observed in the MVCT could not be detected in the kV CBCT.CONCLUSION:The MV CBCT images showed better volume prediction, less streaking effects and better CNRs of a moving metal target, i.e. clips, BBs, gold seeds and metal balls than on the kV CBCT images. The MV CBCT was more advantageous compared to the kV CBCT with less motion artifacts for metal localization devices.ADVANCES IN KNOWLEDGE:This study would benefit clinicians to prescribe MV CBCT as localization modality for radiation treatment with moving target when metal markers are implanted.

Abstract Purpose Two different respiratory monitoring systems (Varian's Real‐Time Position Management (RPM) System and Siemens’ ANZAI belt Respiratory Gating System) are compared in the context of respiratory signals and 4D CT images that are accordingly reconstructed. This study aims to evaluate the feasibility of combined use of RPM and ANZAI systems for 4DCT simulation and gated radiotherapy treatment, respectively. Methods The RPM infrared reflecting marker and the ANZAI belt pressure sensor were both placed on the patient's abdomen during 4DCT scans. The respiratory signal collected by the two systems was synchronized. Fifteen patients were enrolled for respiratory signal collection and analysis. The discrepancies between the RPM and ANZAI traces can be characterized by phase shift and shape distortion. To reveal the impact of the changes in respiratory signals on 4D images, two sets of 4D images based on the same patient's raw data were reconstructed using the RPM and ANZAI data for phase sorting, respectively. The volume of whole lung and the position of diaphragm apex were measured and compared for each respiratory phase. Results The mean phase shift was measured as 0.2 ± 0.1 s averaged over 15 patients. The shape of the breathing trace was found to be in disagreement. For all the patients, the ANZAI trace had a steeper falloff in exhalation than RPM. The inhalation curve, however, was matched for nine patients, steeper in ANZAI for five patients and steeper in RPM for one patient. For 4D image comparison, the difference in whole‐lung volume was about −4% to +4% and the difference in diaphragm position was about −5 mm to +4 mm, compared in each individual phase and averaged over seven patients. Conclusions Combined use of one system for 4D CT simulation and the other for gated treatment should be avoided as the resultant gating window would not fully match with each other due to the remarkable discrepancy in breathing traces acquired by the two different surrogate systems.

When a linear accelerator is unavailable for treatment, a clinical decision is imminent regarding whether a patient should be treated on a linear accelerator other than the machine the patient was scheduled on, or whether treatment should be postponed until the original Linac becomes available. This work investigates the feasibility of switching patients to different accelerators for intensity-modulated radiation therapy (IMRT). We have performed Monte Carlo simulations of photon beams from different Linac models and vendors. Prostate and head and neck (H&N) treatment plans for Siemens Primus, Primart, and Varian 21EX accelerators are studied in this work. Dose distributions for given plans are recalculated using different beam data with the same nominal energy from different Linacs. We have compared dose-volume histograms (DVHs) and the maximum, the minimum, and the mean doses to the target and critical structures because of switching accelerators. In the process of switching a treatment plan to a different accelerator, issues exist, including optimum penumbra compensation, dose distribution at the boundary of target and critical structures, and multileaf collimator (MLC) leaf-width effects, which need to be considered and verified with measurements. Our Monte Carlo simulation results confirm that, for the cases we tested, the dose received by 95% of the planning target volume differs by 0.2% to 1.5% between Siemens Primus and Varian 21EX Linacs. The discrepancy is within our clinical acceptance criteria of 3% for IMRT treatments. In making the final decision on whether to switch machines or not, the tumor control probabilities (TCPs) based on a linear-quadratic model are compared. Based on the analyses performed in this work, it is therapeutically more beneficial to switch a patient to a different machine than to postpone a treatment until the original machine is available, especially for fast-growing tumors such as H&N cancers.

Statistics of Medical Imaging, Lei Tianhu, CRC Press, Boca Raton, FL, 2011. 438 pp. Price: $99.95. ISBN 9781420088427 (hardcover).

PURPOSE:To investigate whether the three-dimensional cone-beam CT (CBCT) is clinically equivalent to the four-dimensional computed tomography (4DCT) maximum intensity projection (MIP) reconstructed images for internal target volume (ITV) localization in image-guided lung stereotactic radiotherapy.METHODS:A ball-shaped polystyrene phantom with built-in cube, sphere, and cone of known volumes was attached to a motor-driven platform, which simulates a sinusoidal movement with changeable motion amplitude and frequency. Target motion was simulated in the patient in a superior-inferior (S-I) direction with three motion periods and 2 cm peak-to-peak amplitudes. The Varian onboard Exact-Arms kV CBCT system and the GE LightSpeed four-slice CT integrated with the respiratory-position-management 4DCT scanner were used to scan the moving phantom. MIP images were generated from the 4DCT images. The clinical equivalence of the two sets of images was evaluated by comparing the extreme locations of the moving objects along the motion direction, the centroid position of the ITV, and the ITV volumes that were contoured automatically by Velocity or calculated with an imaging gradient method. The authors compared the ITV volumes determined by the above methods with those theoretically predicted by taking into account the physical object dimensions and the motion amplitudes. The extreme locations were determined by the gradient method along the S-I axis through the center of the object. The centroid positions were determined by autocenter functions. The effect of motion period on the volume sizes was also studied.RESULTS:It was found that the extreme locations of the objects determined from the two image modalities agreed with each other satisfactorily. They were not affected by the motion period. The average difference between the two modalities in the extreme locations was 0.68% for the cube, 1.35% for the sphere, and 0.5% for the cone, respectively. The maximum difference in the centroid position of the cylinder, sphere, and cone was less than 1.4 mm between the two modalities for all motion periods studied. For the ITV volume evaluation, the authors found that both MIP-based and CBCT-based ITVs increased with increases of motion period. Furthermore, the MIP-based ITV volumes were generally larger than those determined from the CBCT images, with the difference in autocontoured volumes being 2.57%, 1.66%, and 1.82% for the sphere, cylinder, and cone, respectively, while these differences increased to 9.57%, 3.52%, 8.71% for the above objects when the gradient method was used. The authors found that the autocontour method was accurate enough to predict the actual ITV values with the absolute differences less than 2.4% comparing to the theoretically predicted values.CONCLUSIONS:The extreme location and the centroid position of the objects agree with each other between the two image modalities when the breathing motion is sinusoidal. Although the ITV volumes delineated from both image modalities changed with the motion period, the differences in ITV between the two modalities were minimal when an optimized window level was used. The authors' results suggest that CBCT and MIP images are equivalent in determining an ITV's position in the conditions studied. The CBCT is adequate in providing imaging-guidance for lung cancer treatment.

PURPOSE:The purpose of this study is to investigate some characteristics of the beam delivery system and their effects on the dose distribution of intensity-modulated radiation therapy (IMRT) and the results of patient-specific IMRT quality assurance (QA). These characteristics include the accelerator source distribution and multileaf collimator (MLC) geometry.METHODS:Monte Carlo dose calculations based on intensity maps that were built from the actual deliverable IMRT leaf sequences with and without considering the characteristics of the beam delivery system were performed in this study using in-house Monte Carlo software. The effect of the resolution of the intensity maps on the dose distribution was investigated first. The mean dose of the treatment target and the voxel doses in the high dose region of seven IMRT plans generated by different treatment planning systems for Varian 21EX and Siemens Primus linear accelerators were used for comparison and evaluation.RESULTS:The results show that a 0.2 x 0.2 mm2 or smaller pixel size is needed for the intensity maps for accurate IMRT dose calculation. The extrafocal source, MLC leaf thickness, leakage, tongue-and-groove structure, and the effective leaf offset can affect the mean dose by up to 1.5%, 4.5%, 5.6%, 5.3%, and 7.8%, respectively, when these factors are considered separately. They also cause significant uncertainties to the voxel dose with standard deviations up to 2.5%, 0.7%, 2.1%, 1.3%, and 5%, respectively. The overall effect on the mean dose is up to 8% and the standard deviation of the voxel dose uncertainty is up to 6.4% when all the effects are included. The maximum standard deviation is reduced to 4.6% if the voxel size of the dose calculation matrix is increased from 0.04 to 0.3 cm3 to make it comparable with the sensitive volume of the ionization chamber used for IMRT QA measurements.CONCLUSIONS:It can be concluded that the characteristics of the beam delivery system are the major contributors to the uncertainty of measurement-based IMRT QA because most of them are not fully considered in the currently available treatment planning systems.

Increasing the dose rate offers time saving for IMRT delivery but the dosimetric accuracy is a concern, especially in the case of treating a moving target. The objective of this work is to determine the effect of dose rate associated with organ motion and gated treatment using step-and-shoot IMRT delivery. Both measurements and analytical simulation on clinical plans are performed to study the dosimetric differences between high dose rate and low dose rate gated IMRT step-and-shoot delivery. Various sites of IMRT plans for liver, lung, pancreas, and breast cancers were delivered to a custom-made motorized phantom, which simulated sinusoidal movement. Repeated measurements were taken for gated and nongated delivery with different gating settings and three dose rates, 100, 500, and 1000 MU/min using ion chambers and extended dose range films. For the study of the residual motion effect for individual segment dose and composite dose of IMRT plans, our measurements with 30%-60% phase gating and without gating for various dose rates were compared. A small but clinically acceptable difference in delivered dose was observed between 1000, 500, and 100 MU/min at 30%-60% phase gating. A simulation is presented, which can be used for predicting dose profiles for patient cases in the presence of motion and gating to confirm that IMRT step-and-shoot delivery with gating for 1000 MU/min are not much different from 500 MU/min. Based on the authors sample plan analyses, our preliminary results suggest that using 1000 MU/Min dose rate is dosimetrically accurate and efficient for IMRT treatment delivery with gating. Nonetheless, for the concern of patient care and safety, a patient specific QA should be performed as usual for IMRT plans for high dose rate deliveries.