Purpose This study aimed to identify the current state of residency training in physics plan reviews. Methods and Materials A voluntary, anonymous survey was sent to all program directors of accredited therapeutic medical physics residency programs in North America. Survey questions were developed to determine whether and how residents are trained in physics plan reviews. Survey questions were developed using expert validation and cognitive pretesting. Results Using a prospectively approved study (COMIRB 18-1073), responses were collected from 70 program directors, representing a 70% response rate. All respondents (100%) designated patient safety to be the purpose of physics plan reviews. Of the respondents, 94% indicated that physicists should first receive training in physics plan reviews while in a residency program. The vast majority of respondents (99%) provide training to residents in physics plan reviews. Although 57 programs (81% of respondents) have residents perform physics plan reviews as part of clinical practice (with varying levels of independence), 13 programs (19% of respondents) do not. The majority of respondents use the following training methods: observe staff physicists (96%), perform supervised reviews on actual patients for training or clinical practice (93%), use a checklist (80%), and read reference materials (62%). Although simulation plans with embedded errors would be implemented by 71% of respondents, they are currently used in only 19% of programs. Conclusions The present study is the first to characterize chart-check teaching practices in medical physics residency programs. The vast majority of programs currently train residents in physics plan reviews. The most common teaching methods are observing and performing physics plan reviews, but there is variability in the level of resident involvement in clinical practice for physics plan reviews. There is room for the field to consider advancing current training methods, which is especially important given the critical roles that physics plan reviews have with regard to patient safety.

Purpose: An open‐source, convolution/superposition based kV‐treatment planning system(TPS) was developed for small animal radiotherapy from previously existed in‐house MV‐TPS. It is flexible and applicable to both step and shoot and helical tomotherapy treatment delivery. For initial commissioning process, the dose calculation from kV‐TPS was compared with measurements and Monte Carlo(MC) simulations. Methods: High resolution, low energy kernels were simulated using EGSnrc user code EDKnrc, which was used as an input in kV‐TPS together with MC‐simulated x‐ray beam spectrum. The Blue Water™ homogeneous phantom (with film inserts) and heterogeneous phantom (with film and TLD inserts) were fabricated. Phantom was placed at 100cm SSD, and was irradiated with 250 kVp beam for 10mins with 1.1cm × 1.1cm open field (at 100cm) created by newly designed binary micro‐MLC assembly positioned at 90cm SSD. Gafchromic™ EBT3 film was calibrated in‐phantom following AAPM TG‐61 guidelines, and were used for measurement at 5 different depths in phantom. Calibrated TLD‐100s were obtained from ADCL. EGS and MNCP5 simulation were used to model experimental irradiation set up calculation of dose in phantom. Results: Using the homogeneous phantom, dose difference between film and kV‐TPS was calculated: mean(x)=0.9%; maximum difference(MD)=3.1%; standard deviation(σ)=1.1%. Dose difference between MCNP5 and kV‐TPS was: x=1.5%; MD=4.6%; σ=1.9%. Dose difference between EGS and kV‐TPS was: x=0.8%; MD=1.9%; σ=0.8%. Using the heterogeneous phantom, dose difference between film and kV‐TPS was: x=2.6%; MD=3%; σ=1.1%; and dose difference between TLD and kV‐TPS was: x=2.9%; MD=6.4%; σ=2.5%. Conclusion: The inhouse, open‐source kV‐TPS dose calculation system was comparable within 5% of measurements and MC simulations in both homogeneous and heterogeneous phantoms. The dose calculation system of the kV‐TPS is validated as a part of initial commissioning process for small animal radiotherapy. The kV‐TPS has the potential for accurate dose calculation for any kV treatment or imaging modalities.

Experimental research in medical physics has expanded the limits of our knowledge and provided novel imaging and therapy technologies for patients around the world. However, experimental efforts are challenging due to constraints in funding, space, time and other forms of institutional support. In this joint ESTRO-AAPM symposium, four exciting experimental projects from four different countries are highlighted. Each project is focused on a different aspect of radiation therapy. From the USA, we will hear about a new linear accelerator concept for more compact and efficient therapy devices. From Canada, we will learn about novel linear accelerator target design and the implications for imaging and therapy. From France, we will discover a mature translational effort to incorporate theranostic nanoparticles in MR-guided radiation therapy. From Germany, we will find out about a novel in-treatment imaging modality for particle therapy. These examples of high impact, experimental medical physics research are representative of the diversity of such efforts that are on-going around the globe. J. Robar, Research is supported through collaboration with Varian Medical Systems and Brainlab AGD. Westerly, This work is supported by the Department of Radiation Oncology at the University of Colorado School of Medicine. COI: NONEK. Parodi, Part of the presented work is supported by the DFG (German Research Foundation) Cluster of Excellence MAP (Munich-Centre for Advanced Photonics) and has been carried out in collaboration with IBA.

Purpose:Collisions between treatment equipment and patients are potentially catastrophic. Modern technology now commonly involves automated remote motion during imaging and treatment, yet a systematic assessment to identify and mitigate collision risks has yet to be performed. Failure modes and effects analysis (FMEA) is a method of risk assessment that has been increasingly used in healthcare, yet can be resource intensive. This work presents an efficient approach to FMEA to identify collision risks and implement practical interventions within a modern radiation therapy department.Methods:Potential collisions (e.g. failure modes) were assessed for all treatment and simulation rooms by teams consisting of physicists, therapists, and radiation oncologists. Failure modes were grouped into classes according to similar characteristics. A single group meeting was held to identify implementable interventions for the highest priority classes of failure modes.Results:A total of 60 unique failure modes were identified by 6 different teams of physicists, therapists, and radiation oncologists. Failure modes were grouped into four main classes: specific patient setups, automated equipment motion, manual equipment motion, and actions in QA or service mode. Two of these classes, unusual patient setups and automated machine motion, were identified as being high priority in terms severity of consequence and addressability by interventions. The two highest risk classes consisted of 33 failure modes (55% of the total). In a single one hour group meeting, 6 interventions were identified. Those interventions addressed 100% of the high risk classes of failure modes (55% of all failure modes identified).Conclusion:A class‐based approach to FMEA was developed to efficiently identify collision risks and implement interventions in a modern radiation oncology department. Failure modes and interventions will be listed, and a comparison of this approach against traditional FMEA methods will be presented.

Targeted focal therapy strategies for treating single-lobe prostate cancer are under investigation. In this planning study, we investigate the feasibility of treating a portion of the prostate to full-dose external beam radiation with reduced dose to the opposite lobe, compared with full-dose radiation delivered to the entire gland using hypofractionated radiation. For 10 consecutive patients with low- to intermediate-risk prostate cancer, 2 hypofractionated, single-arc volumetric-modulated arc therapy (VMAT) plans were designed. The first plan (standard hypofractionation regimen [STD]) included the entire prostate gland, treated to 70 Gy delivered in 28 fractions. The second dose painting plan (DP) encompassed the involved lobe treated to 70 Gy delivered in 28 fractions, whereas the opposing, uninvolved lobe received 50.4 Gy in 28 fractions. Mean dose to the opposing neurovascular bundle (NVB) was considerably lower for DP vs STD, with a mean dose of 53.9 vs 72.3 Gy (p < 0.001). Mean penile bulb dose was 18.6 Gy for DP vs 19.2 Gy for STD (p = 0.880). Mean rectal dose was 21.0 Gy for DP vs 22.8 Gy for STD (p = 0.356). Rectum V70 (the volume receiving ≥70 Gy) was 2.01% for DP vs 2.74% for STD (p = 0.328). Bladder V70 was 1.69% for DP vs 2.78% for STD (p = 0.232). Planning target volume (PTV) maximum dose points were 76.5 and 76.3 Gy for DP and STD, respectively (p = 0.760). This study demonstrates the feasibility of using VMAT for partial-lobe prostate radiation in patients with prostate cancer involving 1 lobe. Partial-lobe prostate plans appeared to spare adjacent critical structures including the opposite NVB.

Radiation therapy (RT) for head and neck cancer (HNC) treatment is associated with significant lifelong morbidity. Xerostomia, in particular, is a common side effect of HNC RT and it has a significant long-term impact on patient quality of life. In addition to discomfort, xerostomia is associated with poor oral and dental health as well as speech and swallowing. Long-term HNC RT side effects are of more concern now than ever as the epidemiology of HNC is changing with more younger, HPV-positive patients, many of whom will become long-term survivors of HNC. For many years, parotid gland sparing has been practiced to mitigate xerostomia, yet it is well established that although the parotid glands play a major role in stimulated saliva production, it is the submandibular glands that are responsible for the majority of un-stimulated salivary flow. Based on established head and neck lymph drainage patterns, it has been proposed that the contralateral submandibular gland (cSMG) can be spared in many HNC patients, regardless of nodal status, but there is a substantial lack of outcome data for patients treated with cSMG-sparing plans. We retrospectively analyzed patients who were treated with cSMG-sparing techniques at the University of Colorado Cancer Center and at the Memorial Sloan-Kettering Cancer Center. cSMG doses less than or equal to 39Gy were considered spared glands. Only patients receiving treatment to the bilateral neck were included in the analysis. Patients were not excluded based on T or N stages or overall stage. We identified 82 patients who were treated with cSMG-sparing treatment plans, with a median age of 56 years. Fifty percent of the patients were current or former smokers. The majority of patients had primary tonsil cancers (48 of 82 patients), followed by base of tongue lesions (BOT) (30 of 82 patients). Eleven patients had T3 or T4 tumors, 65 patients had N2 or N3 disease, and 80 patients had overall stage III or IV disease. Eighty patients were treated with concurrent chemoradiation therapy using intensity modulated techniques. The mean dose to the cSMG was 32.54Gy and at a median follow-up of 27.4 months there were no patients who had recurrences in the contralateral level Ib lymph nodes. This is one of the largest series reporting on submandibular sparing in locally advanced HNC. These results are preliminary evidence that cSMG-sparing is feasible technically, and that cSMG-sparing is safe even in advanced stage, node positive cancers as well as in BOT lesions. These outcome data offer significant promise for decreasing morbidity in HNC patients, especially young, HPV-positive patients who will in many cases go on to live decades beyond their definitive cancer treatment.

PURPOSE:Latest generation linear accelerators (linacs), i.e., TrueBeam (Varian Medical Systems, Palo Alto, CA) and its stereotactic counterpart, TrueBeam STx, have several unique features, including high-dose-rate flattening-filter-free (FFF) photon modes, reengineered electron modes with new scattering foil geometries, updated imaging hardware/software, and a novel control system. An evaluation of five TrueBeam linacs at three different institutions has been performed and this work reports on the commissioning experience.METHODS:Acceptance and commissioning data were analyzed for five TrueBeam linacs equipped with 120 leaf (5 mm width) MLCs at three different institutions. Dosimetric data and mechanical parameters were compared. These included measurements of photon beam profiles (6X, 6XFFF, 10X, 10XFFF, 15X), photon and electron percent depth dose (PDD) curves (6, 9, 12 MeV), relative photon output factors (Scp), electron cone factors, mechanical isocenter accuracy, MLC transmission, and dosimetric leaf gap (DLG). End-to-end testing and IMRT commissioning were also conducted.RESULTS:Gantry/collimator isocentricity measurements were similar (0.27-0.28 mm), with overall couch/gantry/collimator values of 0.46-0.68 mm across the three institutions. Dosimetric data showed good agreement between machines. The average MLC DLGs for 6, 10, and 15 MV photons were 1.33 ± 0.23, 1.57 ± 0.24, and 1.61 ± 0.26 mm, respectively. 6XFFF and 10XFFF modes had average DLGs of 1.16 ± 0.22 and 1.44 ± 0.30 mm, respectively. MLC transmission showed minimal variation across the three institutions, with the standard deviation <0.2% for all linacs. Photon and electron PDDs were comparable for all energies. 6, 10, and 15 MV photon beam quality, %dd(10)x varied less than 0.3% for all linacs. Output factors (Scp) and electron cone factors agreed within 0.27%, on average; largest variations were observed for small field sizes (1.2% coefficient of variation, 10 MV, 2 × 2 cm(2)) and small cone sizes (<1% coefficient of variation, 6 × 6 cm(2) cone), respectively.CONCLUSIONS:Overall, excellent agreement was observed in TrueBeam commissioning data. This set of multi-institutional data can provide comparison data to others embarking on TrueBeam commissioning, ultimately improving the safety and quality of beam commissioning.

Proton therapy is potentially beneficial for treating pediatric cancer patients given the reduction in normal tissue doses afforded by the Bragg peak and the implications this has on secondary cancer risk. However, the cost of current proton therapy centers is prohibitive to the widespread implementation of this modality. Because facility costs typically increase with increasing proton energy, the goal of this work was to investigate the energy requirements for a dedicated pediatric treatment unit by examining the energy needed to treat deep-seated brain tumors. A CT scan of a 22 year old male brain was used as a surrogate for a large pediatric brain. The delineated brain volume was 1547 cc, with maximum cranial dimensions of 16.2 (lateral) x 21.4 (ant-post) x 14.9 (sup-inf) cm. Simulated 1 cc planning target volumes (PTVs) were contoured in the posterior fossa, pineal, and suprasellar regions. Each PTV was successively expanded in 1 cm increments to create 6 PTVs for each location, with each PTV clipped to remain inside the cranium. To determine the maximum proton range needed to reach the distal edge of each PTV from any achievable beam direction, a linear stopping power ratio grid was generated from the CT dataset and ray-traced from 3662 couch/gantry positions. Proton ranges were converted to kinetic energy using published range/energy tables. Vector analysis was used to determine all deliverable pairs of orthogonal beam angles and the minimum proton energy pairs were recorded as a function of target size and location. The minimum proton energies capable of treating the posterior fossa, pineal, and suprasellar targets with two orthogonal beams are 126, 131, and 131 MeV, respectively, for PTVs equal to 10% of the total brain volume. These numbers increase to 141, 142, and 145 MeV respectively for PTVs equaling 30% of the brain volume. Minimum energy beam angle pairs are posterior and lateral for posterior fossa PTVs, left and right anterior oblique for suprasellar PTVs, and left anterior and posterior oblique for pineal PTVs Minimum beam angles were similar for different PTV sizes, though a trend towards more superior beam angles was observed with increasing PTV volume. Our results show a proton energy of 145 MeV (corresponding to a water equivalent range of 14.9 cm) is sufficient to treat deep-seated brain tumors that range in size up to 30% of the total brain volume using two orthogonal beams. This represents a significant reduction in proton energy compared to the 230-250 MeV systems found in centers equipped to treat adult patients. This reduction in energy could translate into a meaningful cost savings for the design and construction of a proton treatment unit, which in turn makes the prospect of proton therapy for pediatric patients more economically viable.

Purpose/Objective(s)Learning from radiation incidents plays a vital role in improving the safety of radiation therapy treatments. In this work, we developed a procedure to improve the initial clinical chart review process by identifying safety-critical information to be highlighted in treatment plan report documentation through the analysis of an international error reporting database.Materials/MethodsThe Radiation Oncology Safety Information System (ROSIS) database is an international voluntary reporting database of incidents and near misses in radiation therapy. The database was queried for events involving external beam treatments that were reported from 2003 to present. Events that could occur prior to an initial chart review, such as at the time of planning or data transfer to the record and verify system, were identified. Errors originating at the treatment machine at the time of delivery would have occurred after an initial chart review and were therefore excluded from analysis. Safety-critical plan parameters that would appear in a treatment plan report were then identified from these events. Errors related to parameters that would not appear on a treatment plan report, for example erroneous marking of the patient's skin, were excluded.ResultsThree hundred sixty-four events were identified as originating prior to treatment delivery and relating to erroneous parameters that can be displayed in a treatment plan report. Seventy-six events were involved with beam setup and parameter selection during planning or simulation. Common events included erroneous manual fabrication of cerroband blocks, bolus, or compensators (n = 23); mistakes with field size or aperture shape selection (27); isocenter placement (10); and couch, collimator, or gantry angle selection (10). Furthermore, 77 events were involved with dose or monitor unit calculation. Common calculation errors included the incorrect use of transmission factors (20); calculation factors from incorrect beam energy, field size, distance, or treatment machine (21); treatment depth (14); and dose prescription or normalization (12). Last, erroneous planning parameters in the record and verify system or chart documents were identified in 184 events. Common parameters included field size or aperture shape (48); couch, collimator, or gantry settings (30); isocenter setup instructions (25); field monitor units (15), wedges (12); and verification DRR images (11).ConclusionsAn international incident and near miss reporting database was analyzed to identify erroneous planning parameters that can appear in treatment plan report documentation. These parameters can subsequently be highlighted in a customized treatment plan report for the purpose of improving the chart review process and enhancing patient safety. Purpose/Objective(s)Learning from radiation incidents plays a vital role in improving the safety of radiation therapy treatments. In this work, we developed a procedure to improve the initial clinical chart review process by identifying safety-critical information to be highlighted in treatment plan report documentation through the analysis of an international error reporting database. Learning from radiation incidents plays a vital role in improving the safety of radiation therapy treatments. In this work, we developed a procedure to improve the initial clinical chart review process by identifying safety-critical information to be highlighted in treatment plan report documentation through the analysis of an international error reporting database. Materials/MethodsThe Radiation Oncology Safety Information System (ROSIS) database is an international voluntary reporting database of incidents and near misses in radiation therapy. The database was queried for events involving external beam treatments that were reported from 2003 to present. Events that could occur prior to an initial chart review, such as at the time of planning or data transfer to the record and verify system, were identified. Errors originating at the treatment machine at the time of delivery would have occurred after an initial chart review and were therefore excluded from analysis. Safety-critical plan parameters that would appear in a treatment plan report were then identified from these events. Errors related to parameters that would not appear on a treatment plan report, for example erroneous marking of the patient's skin, were excluded. The Radiation Oncology Safety Information System (ROSIS) database is an international voluntary reporting database of incidents and near misses in radiation therapy. The database was queried for events involving external beam treatments that were reported from 2003 to present. Events that could occur prior to an initial chart review, such as at the time of planning or data transfer to the record and verify system, were identified. Errors originating at the treatment machine at the time of delivery would have occurred after an initial chart review and were therefore excluded from analysis. Safety-critical plan parameters that would appear in a treatment plan report were then identified from these events. Errors related to parameters that would not appear on a treatment plan report, for example erroneous marking of the patient's skin, were excluded. ResultsThree hundred sixty-four events were identified as originating prior to treatment delivery and relating to erroneous parameters that can be displayed in a treatment plan report. Seventy-six events were involved with beam setup and parameter selection during planning or simulation. Common events included erroneous manual fabrication of cerroband blocks, bolus, or compensators (n = 23); mistakes with field size or aperture shape selection (27); isocenter placement (10); and couch, collimator, or gantry angle selection (10). Furthermore, 77 events were involved with dose or monitor unit calculation. Common calculation errors included the incorrect use of transmission factors (20); calculation factors from incorrect beam energy, field size, distance, or treatment machine (21); treatment depth (14); and dose prescription or normalization (12). Last, erroneous planning parameters in the record and verify system or chart documents were identified in 184 events. Common parameters included field size or aperture shape (48); couch, collimator, or gantry settings (30); isocenter setup instructions (25); field monitor units (15), wedges (12); and verification DRR images (11). Three hundred sixty-four events were identified as originating prior to treatment delivery and relating to erroneous parameters that can be displayed in a treatment plan report. Seventy-six events were involved with beam setup and parameter selection during planning or simulation. Common events included erroneous manual fabrication of cerroband blocks, bolus, or compensators (n = 23); mistakes with field size or aperture shape selection (27); isocenter placement (10); and couch, collimator, or gantry angle selection (10). Furthermore, 77 events were involved with dose or monitor unit calculation. Common calculation errors included the incorrect use of transmission factors (20); calculation factors from incorrect beam energy, field size, distance, or treatment machine (21); treatment depth (14); and dose prescription or normalization (12). Last, erroneous planning parameters in the record and verify system or chart documents were identified in 184 events. Common parameters included field size or aperture shape (48); couch, collimator, or gantry settings (30); isocenter setup instructions (25); field monitor units (15), wedges (12); and verification DRR images (11). ConclusionsAn international incident and near miss reporting database was analyzed to identify erroneous planning parameters that can appear in treatment plan report documentation. These parameters can subsequently be highlighted in a customized treatment plan report for the purpose of improving the chart review process and enhancing patient safety. An international incident and near miss reporting database was analyzed to identify erroneous planning parameters that can appear in treatment plan report documentation. These parameters can subsequently be highlighted in a customized treatment plan report for the purpose of improving the chart review process and enhancing patient safety.

Purpose: Latest generation linear accelerators (linacs), TrueBeam (Varian Medical Systems, Palo Alto, CA) and its stereotactic counterpart, TrueBeam STx, have several unique features, including high-dose-rate flattening-filter-free (FFF) photon modes, reengineered electron modes with new scattering foil geometries, updated imaging hardware/software, and a novel control system. We have performed a comprehensive evaluation of three TrueBeam linacs at three different institutions and report on our commissioning experience. Methods: Acceptance and commissioning data were analyzed for three TrueBeam linacs equipped with 120 leaf (5 mm width) MLCs at three different institutions. Dosimetric data and mechanical parameters were compared. These included measurements of photon beam profiles (6X, 6XFFF, 10X, 10XFFF, 15X), photon and electron percent depth dose (PDD)curves (6MeV, 9MeV, 12MeV), relative photon output factors (Scp),electron cone factors, mechanical isocenter accuracy, MLC transmission, and dosimetric leaf gap (DLG). Results: Gantry/collimator isocentricity measurements were similar (0.27–0.28mm), with overall couch/gantry/collimator values of 0.46–0.68mm across the three institutions. Dosimetric data showed good agreement between machines. The largestdiscrepancy was observed with measured MLC DLG (for 6, 10 and 15 MV photons, average DLGs were 1.95 ± 1.15mm, 2.22 ± 1.30 mm, and 2.20 ± 1.24mm, respectively). Photon and electron PDDs were comparable for all energies. 6, 15 and 10 MV photon beam quality, %dd(10)x varied less than 0.3% for all machines. Electron beam quality specifier (R50) showed less than 1.7% variation for all energies. Output factors (Scp) and electron cone factors agreed within 0.27%, on average; largest variations were observed for small field sizes (0.77% variation, 2×2cm2) and small cone sizes (0.39% variation, 6×6 cm2 cone), respectively. Conclusions: Overall, strong agreement was observed in TrueBeam commissioning data. This comprehensive set of multi-institutional data may serve as a benchmark for other institutions embarking on TrueBeam commissioning, ultimately improving the safety/quality of beam commissioning

PURPOSE:Plan report documentation contains numerous details about the treatment plan, but critical information for patient safety is often presented without special emphasis. This can make it difficult to detect errors from treatment planning and data transfer during the initial chart review. The objective of this work is to improve safety measures in radiation therapy practice by customizing the treatment plan report to emphasize safety-critical information. METHODS:Commands within the template file from a commercial planning system (Eclipse, Varian Medical Systems) that automatically generates the treatment plan report were reviewed and modified. Safety-critical plan parameters were identified from published risks known to be inherent in the treatment planning process. Risks having medium to high potential impact on patient safety included incorrect patient identifiers, erroneous use of the treatment prescription, and incorrect transfer of beam parameters or consideration of accessories. Specific examples of critical information in the treatment plan report that can be overlooked during a chart review included prescribed dose per fraction and number of fractions, wedge and open field monitor units, presence of beam accessories, and table shifts for patient setup. RESULTS:Critical information was streamlined and concentrated. Patient and plan identification, dose prescription details, and patient positioning couch shift instructions were placed on the first page. Plan information to verify the correct data transfer to the record and verify system was re-organized in an easy to review tabular format and placed in the second page of the customized printout. Placeholders were introduced to indicate both the presence and absence of beam modifiers. Font sizes and spacing were adjusted for clarity, and departmental standards and terminology were introduced to streamline data communication among staff members. CONCLUSIONS:Plan reporting documentation has been customized to concentrate and emphasize safety-critical information, which should allow for a more efficient, robust chart review process.

To describe the day-to-day variation of rectal doses by evaluating daily cone beam CT (CBCT) scans in patients who had rectal balloon immobilization while receiving stereotactic body radiation therapy (SBRT) to the prostate. Retrospective review of patients who were enrolled on a prospective multi-institutional IRB approved trial of SBRT for low-to-intermediate risk prostate cancer. Patients who received SBRT at a single institution using a volumetric modulated arc therapy planning and delivery platform (Monaco TPS and Synergy, Elekta) were evaluated. A rectal balloon was inserted for simulation and on each day prior to treatment and inflated with 60 cc of air to stabilize the prostate and displace the posterior rectal wall. The PTV included the prostate plus a 3mm margin; the prescription dose was 50 Gy in 5 fractions, and the total anterior rectal wall (ARW) maximum point dose was constrained to < 52.5 Gy. Daily CBCT was used for target relocalization to ensure that the prostate was within the PTV during treatment. The ARW, bladder and rectal balloons were contoured on each CBCT scan. The CBCT image sets were uploaded into the TPS and fused with the planning images to allow calculation of the dose given to the region of interest during each fraction. Five consecutive patients were studied (5 CBCT scans per patient, total of 25 scans). All patients tolerated daily balloon placement and inflation without unacceptable discomfort. The ARW volume receiving >6 Gy/fraction (V6) and maximum point dose (ARWDmax) were analyzed. The V6 averages for each patient (mean cc ± SD) were 9.91 ± 0.63, 13.6 ± 0.63, 9.13 ± 1.31, 9.61 ± 0.78, and 8.1 ± 1. Estimation of the delivered ARWDmax is subject to a broader error range due to CBCT image characteristics in the steep dose gradient region posterior to the prostate. There was no correlation between delivered either V6 or ARWDmax to initial prostate volume, daily bladder volume, and daily balloon volume. Daily rectal balloon CBCT imaging can achieve reproducible rectal dose delivery for patients receiving prostate SBRT using a VMAT technique. Estimation of daily ARWDmax is limited by CBCT image resolution, but the intermediate daily dose volume (V6) was consistent between fractions for each patient. There was no observed correlation between prostate size, bladder size or balloon distention and anterior rectal wall V6.