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Purpose/Objective(s) Randomized trials have shown improved local control (LC) but no overall survival (OS) benefit with the addition of non-ablative radiation therapy (RT) dose compared to chemotherapy (CT) alone for pancreas cancer (PCa). Emerging data suggest that dose-escalated RT may improve LC and OS. A few studies suggest that stereotactic magnetic resonance-guided adaptive RT (SMART) can facilitate the safe delivery of ablative dose for inoperable PCa, although long-term outcomes are not well understood. Materials/Methods Inoperable PCa patients who received SMART were identified from the RSSearch Registry. Patients with Results A total of 148 PCa patients were treated on a 0.35T MR LINAC across 3 institutions between 2018-2020. Median age was 68 years (range 47-91), and 73.6% had ECOG 0-1 performance status. Patients had locally advanced (57.4%), borderline resectable (29.1%), or medically inoperable (13.5%) disease. Median CA19-9 at diagnosis was 202.1 U/mL (range 0.9-21,281). Induction CT was delivered to 89.2% for a median 3.9 mo (range 0.2-11.3); FOLFIRINOX (52.7%) or gemcitabine/nab-paclitaxel (23.4%) were common. Median prescribed RT dose was 50 Gy (range 40-50) in 5 fractions, mostly in consecutive days (96.6%) and in breath hold (95.3%). Median biologically effective dose (BED10) was 100 Gy10. All patients were treated with real-time tissue tracking and automated beam gating without fiducial markers. An elective target volume was rarely used (25%). Pancreaticoduodenectomy was performed in 23% at a median 46 days (range 34-304) after SMART. Median follow-up was 16 mo. from diagnosis for all patients (range, 4-39). Median, 1-year, and 2-year LC was not reached (NR), 94.6%, and 83%, respectively. Median, 1-year, and 2-year PFS was 18 mo., 72%, and 35.9%, respectively. Median, 1-year, and 2-year OS was 26 mo., 82%, and 52.7%, respectively. Acute and late grade 3 toxicity possibly related to SMART occurred in 4.1% and 12.8%, respectively. There was no reported grade 4+ toxicity. Conclusion To our knowledge, this is the largest reported analysis of 5-fraction SMART for inoperable PCa. These data add to the evidence that ablative radiation dose may improve long-term outcomes including OS. Prospective evaluation of this novel approach is warranted with attention directed at optimizing patient selection, understanding the clinical significance of cumulative dose delivered across all adapted fractions, and assessing treatment response after the delivery of ablative dose.

8513 Background: In advanced non-small cell lung cancer (NSCLC), high Programmed-Death-1 Ligand (PD-L1) (>50%) expression demonstrate superior response and survival with immune checkpoint inhibitors compared to chemotherapy. We hypothesize that it is safe and feasible to substitute durvalumab instead of chemotherapy concurrently with radiotherapy (RT) in patients with Locally Advanced-NSCLC (LA-NSCLC) and high PD-L1. Methods: NRG-LU004 (NCT03801902) is a Phase I study for patients with stage II-III unresectable or inoperable, LA-NSCLC with PD-L1> 50% (Dako 22C3 or Ventana SP263) expression. There were safety and expansion phases with a primary endpoint of safety. Patients started with 1500 mg durvalumab Q4 weeks and thoracic RT within 2 weeks from 1st infusion. Durvalumab continued once a month up to 1 year. In the safety cohort, 6 patients in cohort 1 were treated with accelerated fractionated RT (ACRT) to 60 Gy in 15 fractions, followed by a required safety hold for 90 days. During cohort 1 safety hold, cohort 2 patients were treated with conventional RT 60 Gy in 30 fractions (CONV) followed by a 60-day safety hold. A cohort advanced to the expansion phase to enroll 6 more patients if safety criteria (0-1 patients with a dose limiting toxicity [DLT]) were met. If both cohorts were deemed safe, patients would be randomized 1:1 to ACRT or CONV with safety defined as < 4 of 12 evaluable patients per arm experiencing a DLT. Feasibility was defined as at least 80% of patients in each arm receiving at least 80% of the planned dose of durvalumab during the first 8 weeks. Results: 24 evaluable patients enrolled between January 2019 and June 2021. No DLTs were reported in cohort 1, and 1 (unrelated bronchopulmonary hemorrhage leading to discontinuation of durvalumab) in cohort 2. Both safety cohorts advanced to the expansion phase. All but one patient (CONV) received RT per protocol/with an acceptable variation. At the time of analysis, 24% had received all 13 cycles of durvalumab. For the ACRT cohort, there were 4 grade 3, 1 grade 4 (lymphopenia), and 1 grade 5 AE (lung infection, assessed as unrelated to therapy). For CONV, there were 8 grade 3, 0 grade 4, and 1 grade 5 AE (respiratory failure, unrelated to therapy). For feasibility, 10 of 12 (85%) patients in the ACRT cohort received the second dose of durvalumab (2 not received due to shingles and unrelated death), while 9 of 12 (75%) of the CONV cohort received the second dose (reasons for not receiving: viral hepatitis, bronchopulmonary hemorrhage, and respiratory failure, all assessed as unrelated to therapy). Conclusions: Chemotherapy-free thoracic RT approaches (ACRT or CONV RT) are safe, when given with concurrent durvalumab in patients with PD-L1 high LA-NSCLC. A trial to compare immunoradiotherapy and consolidation durvalumab to standard chemoradiation and consolidation durvalumab is planned. Clinical trial information: NCT03801902.

Background The purpose of this study is to comprehensively evaluate the suitability of Gafchromic EBT3 and EBT-XD film for dosimetric quality assurance in 0.35 T MR-guided radiotherapy. Methods A 0.35 T magnetic field strength was utilized to evaluate magnetic field effects on EBT3 and EBT-XD Gafchromic films by studying the effect of film exposure time within the magnetic field using two timing sequences and film not exposed to MR, the effect of magnetic field exposure on the crystalline structure of the film, and the effect of orientation of the film with respect to the bore within the magnetic field. The orientation of the monomer crystal was qualitatively evaluated using scanning electron microscopy (SEM) compared to unirradiated film. Additionally, dosimetric impact was evaluated through measurements of a series of open field irradiations (0.83 × 0.83-cm2 to 19.92 × 19.92-cm2) and patient specific quality assurance measurements. Open fields were compared to planned dose and an independent dosimeter. Film dosimetry was applied to twenty conventional and twenty stereotactic body radiotherapy (SBRT) patient specific quality assurance cases. Results No visual changes in crystal orientation were observed in any evaluated SEM images nor were any optical density differences observed between films irradiated inside or outside the magnetic field for both EBT3 and EBT-XD film. At small field sizes, the average difference along dose profiles measured in film compared to the same points measured using an independent dosimeter and to predicted treatment planning system values was 1.23% and 1.56%, respectively. For large field sizes, the average differences were 1.91% and 1.21%, respectively. In open field tests, the average gamma pass rates were 99.8% and 97.2%, for 3%/3 mm and 3%/1 mm, respectively. The median (interquartile range) 3%/3 mm gamma pass rates in conventional QA cases were 98.4% (96.3 to 99.2%), and 3%/1 mm in SBRT QA cases were 95.8% (95.0 to 97.3%). Conclusions MR exposure at 0.35 T had negligible effects on EBT3 and EBT-XD Gafchromic film. Dosimetric film results were comparable to planned dose, ion chamber and diode measurements.

The integration of adaptive radiation therapy (ART), or modifying the treatment plan during the treatment course, is becoming more widely available in clinical practice. ART offers strong potential for minimizing treatment-related toxicity while escalating or de-escalating target doses based on the dose to organs at risk. Yet, ART workflows add complexity into the radiation therapy planning and delivery process that may introduce additional uncertainties. This work sought to review presently available ART workflows as well as technological considerations such as image quality, deformable image registration, and dose accumulation. Quality assurance considerations for ART components and minimum recommendations are described. Personnel and workflow efficiency recommendations are provided as well as a summary of currently available clinical evidence supporting the implementation of ART. Finally, to guide future clinical trial protocols, an example ART physician directive and a physics template following standard NRG Oncology protocol is provided.

Introduction The purpose of this study was to investigate the systematic localization accuracy, treatment planning capability, and delivery accuracy of an integrated magnetic resonance imaging guided Linear Accelerator (MR-Linac) platform for stereotactic radiosurgery. Materials and methods The phantom for the end-to-end test comprises three different compartments: a rectangular MR/CT target phantom, a Winston–Lutz cube, and a rectangular MR/CT isocenter phantom. Hidden target tests were performed at gantry angles of 0, 90, 180, and 270 degrees to quantify the systematic accuracy. Five patient plans with a total of eleven lesions were used to evaluate the dosimetric accuracy. Single-isocenter IMRT treatment plans using 10–15 coplanar beams were generated to treat the multiple metastases. Results The end-to-end localization accuracy of the system was 1.0 ± 0.1 mm. The conformity index, homogeneity index and gradient index of the plans were 1.26 ± 0.22, 1.22 ± 0.10, and 5.38 ± 1.44, respectively. The average absolute point dose difference between measured and calculated dose was 1.64 ± 1.90%, and the mean percentage of points passing the 3%/1 mm gamma criteria was 96.87%. Conclusions Our experience demonstrates that excellent plan quality and delivery accuracy was achievable on the MR-Linac for treating multiple brain metastases with a single isocenter.

Importance Non-small cell lung cancer (NSCLC) has relatively poor outcomes. Metformin has significant data supporting its use as an antineoplastic agent. Objective To compare chemoradiation alone vs chemoradiation and metformin in stage III NSCLC. Design, setting, and participants The NRG-LU001 randomized clinical trial was an open-label, phase 2 study conducted from August 24, 2014, to December 15, 2016. Patients without diabetes who were diagnosed with unresectable stage III NSCLC were stratified by performance status, histology, and stage. The setting was international and multi-institutional. This study examined prespecified endpoints, and data were analyzed on an intent-to-treat basis. Data were analyzed from February 25, 2019, to March 6, 2020. Interventions Chemoradiation and consolidation chemotherapy with or without metformin. Main outcomes and measures The primary outcome was 1-year progression-free survival (PFS), designed to detect 15% improvement in 1-year PFS from 50% to 65% (hazard ratio [HR], 0.622). Secondary end points included overall survival, time to local-regional recurrence, time to distant metastasis, and toxicity per Common Terminology Criteria for Adverse Events, version 4.03. Results A total of 170 patients were enrolled, with 167 eligible patients analyzed after exclusions (median age, 64 years [interquartile range, 58-72 years]; 97 men [58.1%]; 137 White patients [82.0%]), with 81 in the control group and 86 in the metformin group. Median follow-up was 27.7 months (range, 0.03-47.21 months) among living patients. One-year PFS rates were 60.4% (95% CI, 48.5%-70.4%) in the control group and 51.3% (95% CI, 39.8%-61.7%) in the metformin group (HR, 1.15; 95% CI, 0.77-1.73; P = .24). Clinical stage was the only factor significantly associated with PFS on multivariable analysis (HR, 1.79; 95% CI, 1.19-2.69; P = .005). One-year overall survival was 80.2% (95% CI, 69.3%-87.6%) in the control group and 80.8% (95% CI, 70.2%-87.9%) in the metformin group. There were no significant differences in local-regional recurrence or distant metastasis at 1 or 2 years. No significant difference in adverse events was observed between treatment groups. Conclusions and relevance In this randomized clinical trial, the addition of metformin to concurrent chemoradiation was well tolerated but did not improve survival among patients with unresectable stage III NSCLC. Trial registration ClinicalTrials.gov Identifier: NCT02186847.

Purpose To evaluate the performance of a commercial plastic scintillator detector (PSD) for small-field stereotactic patient-specific quality assurance (QA) measurements using flattening-filter-free beam. Methods A total of ten spherical targets (volume range: [0.03cc-2cc]) were planned with two techniques: (1) Dynamic Conformal Arc (DCA-10 plans), and (2) Volumetric Modulated Arc Therapy (VMAT-10 plans). All plans were generated using Varian Eclipse treatment planning system, and AcurosXB v.13 algorithm in 1.0 mm grid size. Additionally, 14 previously-treated cranial and spine SRS plans were evaluated (6 DCA, 8 VMAT, volume range: [0.04cc-119.02cc]). Plan modulation was quantified via two metrics: MU per prescription dose (MU/Rx) and Average Leaf Pair Opening (ALPO). QA was performed on the Varian Edge linear accelerator equipped with HDMLC. Three detectors were used: (1) PinPoint ion chamber (PTW; active volume 0.015cc), (2) Exradin W1 PSD (Standard Imaging; active volume 0.002cc), and (3) Gafchromic EBT3 film (Ashland). PinPoint chamber and PSD were positioned perpendicular to beam axis in a Lucy phantom (Standard Imaging); films were placed horizontally capturing the coronal plane. Results PSD, film and PinPoint chamber measured average differences of 1.00±1.54%, 1.30±1.69%, and -0.66±2.36% respectively, compared to AcurosXB dose calculation. As the target volume decreased, PinPoint chamber measured lower doses (maximum -5.07% at 0.07cc target), while PSD and film measured higher doses (2.87% and 2.54% at 0.03cc target) than AcurosXB. Film agreed with the benchmark detector PSD by an average difference of 0.31±1.20%, but suffered from larger uncertainty; PinPoint chamber underestimated dose by more than 4% for targets smaller than 0.2cc. Taking PSD as the measurement standard, DCA plans achieved good QA results across all volumes studied, with an average difference of -0.07±0.89%; for VMAT plans, PSD measured consistently higher dose (1.95±1.36%) than AcurosXB. Correlation study revealed that plan modulation quantified by both MU/Rx and ALPO correlated significantly with QA results. Conclusion Among all three detectors, PSD demonstrated superior performances in plans with small fields and heavy modulation. High consistency and low uncertainty made PSD a suitable detector for clinical routine SRS QA. PinPoint chamber should be avoided for targets smaller than 0.2cc; film dosimetry can be utilized with careful evaluation of its uncertainty bracket. Compared to PSD measurements, AcurosXB calculation demonstrated high accuracy for non-modulated small fields. The positive correlation between plan modulation and QA discrepancy calls for our attention for clinical SRS plans with high modulation. This article is protected by copyright. All rights reserved.

8502 Background: Metformin, a diabetes agent that inhibits mitochondria complex I, enhances radiotherapy and chemotherapy responses in pre-clinical models of NSCLC. NRG-LU001 examined whether metformin can improve outcomes of curative CRT in locally advanced (LA)-NSCLC. Methods: The primary endpoint of this trial was 1-year progression free survival (PFS). Unresected, non-diabetic, stage IIIA/B NSCLC patients were randomized (1:1) to either carboplatin-paclitaxel chemotherapy concurrent with chest RT (60Gy), followed by consolidation carboplatin-paclitaxel chemotherapy (Control Arm) or the same and oral metformin (2000mg daily) during cytotoxic therapy (Experimental Arm). PFS and overall survival (OS) were estimated with the Kaplan-Meier method; time to local-regional progression (TTLRP), time to distant metastasis (TTDM) were estimated using the cumulative incidence method. Adverse events (AEs) were graded with CTCAE v.4.0. Results: Between Aug.2014 and Dec.2016, 170 patients were accrued. Analysis was planned at 102 PFS events (Feb. 2019). There was no significant difference in rates or grade of toxicity between the two arms. 1- and 2-year PFS was 60.4% (95% CI: 48.5, 70.4) and 40.1% (95% CI: 29.0, 51.0) in Control vs 51.3% (95% CI: 39.8, 61.7) and 34.5% (95% CI: 24.2, 45.1) in the Metformin arm (multivariable Cox proportional HR=1.20 (95% CI: 0.81, 1.78), p=0.36). OS at 2 years was 65.4% (95% CI: 53.5, 75.0) for Control vs 64.9% (95% CI: 53.1, 74.5) for the Metformin arm (HR=1.03 (95% CI: 0.64, 1.68)), while deaths due to disease were 90% vs 71%, respectively. No significant differences were found for TTLRP or TTDM. Conclusions: NRG-LU001 center reported outcomes show that oral daily metformin was well-tolerated in combination with CRT treatment for LA-NSCLC. However, metformin did not improve PFS and OS and did not alter the rates of local-regional failure or distant metastasis. Acknowledgements: TT and HS are Co-Principal Investigators. This project was supported by National Cancer Institute (NCI) grants: U10CA180868 (NRG Oncology Operations), U10CA180822 (NRG SDMC), UG1CA189867 (NCORP), U24CA180803 (IROC). Clinical trial information: NCT02186847.

© 2016 The Authors. Purpose The American Society for Radiation Oncology (ASTRO) Physics Core Curriculum Subcommittee (PCCSC) has updated the recommended physics curriculum for radiation oncology resident education to improve consistency in teaching, intensity, and subject matter. Methods and Materials The ASTRO PCCSC is composed of physicists and physicians involved in radiation oncology residency education. The PCCSC updated existing sections within the curriculum, created new sections, and attempted to provide additional clinical context to the curricular material through creation of practical clinical experiences. Finally, we reviewed the American Board of Radiology (ABR) blueprint of examination topics for correlation with this curriculum. Results The new curriculum represents 56 hours of resident physics didactic education, including a 4-hour initial orientation. The committee recommends completion of this curriculum at least twice to assure both timely presentation of material and re-emphasis after clinical experience. In addition, practical clinical physics and treatment planning modules were created as a supplement to the didactic training. Major changes to the curriculum include addition of Fundamental Physics, Stereotactic Radiosurgery/Stereotactic Body Radiation Therapy, and Safety and Incidents sections, and elimination of the Radiopharmaceutical Physics and Dosimetry and Hyperthermia sections. Simulation and Treatment Verification and optional Research and Development in Radiation Oncology sections were also added. A feedback loop was established with the ABR to help assure that the physics component of the ABR radiation oncology initial certification examination remains consistent with this curriculum. Conclusions The ASTRO physics core curriculum for radiation oncology residents has been updated in an effort to identify the most important physics topics for preparing residents for careers in radiation oncology, to reflect changes in technology and practice since the publication of previous recommended curricula, and to provide practical training modules in clinical radiation oncology physics and treatment planning. The PCCSC is committed to keeping the curriculum current and consistent with the ABR examination blueprint.

Significantly reduced expression of immunoproteasome components and their regulators was observed to be associated with epithelial to mesenchymal transition in 42 NSCLC cell lines using proteomic profiling and microarray analysis. A highly variable immunoproteasome expression was also observed among NSCLC tissues. Immunohistochemistry data revealed loss of immunoproteasome subunit is significantly correlated with expression of CDH2 and concomitant loss of CDH1 in NSCLC tumors. Loss of immunoproteasome subunits was also significantly associated with advanced stage (p1⁄40.014), recurrence (p1⁄40.002) and metastasis (p

The purpose of this study is to characterize the dosimetric properties and accuracy of a novel treatment platform (Edge radiosurgery system) for localizing and treating patients with frameless, image‐guided stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Initial measurements of various components of the system, such as a comprehensive assessment of the dosimetric properties of the flattening filter‐free (FFF) beams for both high definition (HD120) MLC and conical cone‐based treatment, positioning accuracy and beam attenuation of a six degree of freedom (6DoF) couch, treatment head leakage test, and integrated end‐to‐end accuracy tests, have been performed. The end‐to‐end test of the system was performed by CT imaging a phantom and registering hidden targets on the treatment couch to determine the localization accuracy of the optical surface monitoring system (OSMS), cone‐beam CT (CBCT), and MV imaging systems, as well as the radiation isocenter targeting accuracy. The deviations between the percent depth‐dose curves acquired on the new linac‐based system (Edge), and the previously published machine with FFF beams (TrueBeam) beyond Dmax were within 1.0% for both energies. The maximum deviation of output factors between the Edge and TrueBeam was 1.6%. The optimized dosimetric leaf gap values, which were fitted using Eclipse dose calculations and measurements based on representative spine radiosurgery plans, were 0.700 mm and 1.000 mm, respectively. For the conical cones, 6X FFF has sharper penumbra ranging from 1.2−1.8 mm (80%‐20%) and 1.9−3.8 mm (90%‐10%) relative to 10X FFF, which has 1.2−2.2 mm and 2.3−5.1 mm, respectively. The relative attenuation measurements of the couch for PA, PA (rails‐in), oblique, oblique (rails‐out), oblique (rails‐in) were: −2.0%, −2.5%, −15.6%, −2.5%, −5.0% for 6X FFF and −1.4%, −1.5%, −12.2%, −2.5%, −5.0% for 10X FFF, respectively, with a slight decrease in attenuation versus field size. The systematic deviation between the OSMS and CBCT was −0.4±0.2 mm, 0.1±0.3 mm, and 0.0±0.1 mm in the vertical, longitudinal, and lateral directions. The mean values and standard deviations of the average deviation and maximum deviation of the daily Winston‐Lutz tests over three months are 0.20±0.03 mm and 0.66±0.18 mm, respectively. Initial testing of this novel system demonstrates the technology to be highly accurate and suitable for frameless, linac‐based SRS and SBRT treatment. PACS number: 87.56.J‐

Purpose To describe and evaluate a method for generating synthetic computed tomography (synCT) images from magnetic resonance simulation (MR-SIM) data for accurate digitally reconstructed radiograph (DRR) generation and dose calculations in prostate cancer radiation therapy. Methods and Materials A retrospective evaluation was performed in 9 prostate cancer patients who had undergone MR-SIM in addition to CT simulation (CT-SIM). MR-SIM data were used to generate synCT images by using a novel, voxel-based weighted summation approach. A subset of patients was used for weight optimization, and the number of patients to use during optimization was determined. Hounsfield unit (HU) differences between CT-SIM and synCT images were analyzed via mean absolute error (MAE). Original, CT-based treatment plans were mapped onto synCTs. DRRs were generated, and agreement between CT and synCT-generated DRRs was evaluated via Dice similarity coefficient (DSC). Dose was recalculated, and dose-volume metrics and gamma analysis were used to evaluate resulting treatment plans. Results Full field-of-view synCT MAE across all patients was 74.3 ± 10.9 HU with differences from CTs of 2.0 ± 8.1 HU and 11.9 ± 46.7 HU for soft tissue structures (prostate, bladder, and rectum) and femoral bones, respectively. Calculated DSCs for anterior-posterior and lateral DRRs were 0.90 ± 0.04 and 0.92 ± 0.05, respectively. Differences in D99%, mean dose, and maximum dose to the clinical target volume from CT-SIM dose calculations were 0.75% ± 0.35%, 0.63% ± 0.34%, and 0.54% ± 0.33%, respectively, for synCT-generated plans. Gamma analysis (2%/2 mm dose difference/distance to agreement) revealed pass rates of 99.9% ± 0.1% (range, 99.7%-100%). Conclusion Generated synCTs enabled accurate DRR generation and dose computation for prostate MR-only simulation. Dose recalculated on synCTs agreed well with original planning distributions. Further validation using a larger patient cohort is warranted.

Purpose: To use end-to-end dosimetric measurements with Gafchromic film to evaluate the effects of focal spot size parameter for small-field dose calculations using AAA for SRS lesions. Methods: A total of 13 plans, corresponding to 7 patients previously treated with cranial SRS, were analyzed in this study (target volume range:[0.67cc,13.9cc]). The plans included DCA delivery (4 plans total) and VMAT delivery (9 plans total). All plans were mapped to a solid water phantom (15 cm thickness; isocenter and film plane at 7.5 depth). Dose calculation was performed with AAA v.11 (1.0mm grid size); three focal spot size settings were tested: 0mm, 0.5mm, and 1.5mm. For each plan, three calculated doses (corresponding to each focal spot size setting) were compared to measured film dose using quantitative methods [Gamma Analysis(1%,1mm,10% threshold criteria)] and qualitative methods (visual dose profile comparison). Film calibration and analysis were performed using in-house calibration methods and software package. Results: Gamma(1%,1mm) analysis passing rate results [mean(st.dev){%}] were as follows. For DCA plans: 98.74(0.54)-[0mm Focal Spot Size]; 98.24(1.26)-[0.5mm Focal Spot Size]; 95.42(2.29)-[1.5mm Focal Spot Size]. For VMAT plans: 98.75(0.54)-[0mm Focal Spot Size]; 98.89(0.73)-[0.5mm Focal Spot Size]; 97.43(1.30)-[1.5mm Focal Spot Size]. The majority of failing points (Gamma value>1.0) were found to be within the high dose region for all Focal Spot Size calculation models. Visual inspection of the dose profile, showed that the 1.5mm Focal Spot size calculation exhibited blurring in the high dose region (defined as >85% of the peak dose), resulting in a more gradual shoulder of the dose profile relative to measurements. Conclusion: The dose calculation accuracy of DCA and VMAT plans is paramount for SRS treatment planning. Our results indicate similar behavior of the AAA model with focal spot sizes of 0mm and 0.5mm, while 1.5mm focal spot size tends to result in blurring of the high dose region. Henry Ford Health System has research agreements with Varian and Philips.

To present a clinical procedure that readjusts catheters to its planned positions based on pretreatment computed tomography (CT) for patients undergoing high-dose-rate (HDR) prostate brachytherapy, and evaluate the magnitude and dosimetric impact of the adjustments.Patients received a pretreatment verification CT (vCT) before each fraction. The vCT dataset was imported to the treatment-planning system and fused to the planning CT (pCT) by rigid-body registration based on the implanted fiducials within the prostate. Catheter positions in the vCT were then compared with catheter positions in the pCT in a reconstructed plane through each catheter. Any catheter with difference in penetration larger than 3 mm was manually adjusted by a radiation oncologist before treatment. To evaluate treatment quality, the patient's plan was applied to the vCT off-line and dose delivered to prostate and normal structures were compared with their planned value.Forty-four fractions of 13 consecutive patients were treated using this method. Thirty-nine fractions had at least one catheter adjusted before treatment. A total of 651 catheters were assessed, and 194 catheters (30%) were adjusted by an average amount of 5.8 ± 1.9 mm. In eight fractions the prostate D90 would have decreased by more than 10% from the planned value (with a maximum of 32%) if the catheter displacements were not rectified. After the adjustment, the maximum deviation of D90 was 10.6%. The improvement in D90 is 24% per 1 cm of time-averaged adjustment.Interfraction catheter motion occurs without any particular pattern. Using pretreatment CTs and restoring each catheter to its planned position ensures that the delivered treatment closely matches the treatment plan and therefore enhances the overall quality of the HDR treatment. The procedure can be readily implemented in any clinical setting.

Purpose To report on a prospective clinical trial of the use of daily kilovoltage cone-beam computed tomography (CBCT) to evaluate the interfraction and residual error motion of patients undergoing intensity-modulated radiotherapy for head-and-neck cancer. Methods and Materials Patients were treated with intensity-modulated radiotherapy with an Elekta linear accelerator using a mounted CBCT scanner. CBCT was performed before every treatment, and translational (but not rotational) corrections were performed. At least once per week, a CBCT scan was obtained after intensity-modulated radiotherapy. Variations were measured in the medial–lateral, superoinferior, and anteroposterior dimensions, as well as in the rotation around these axes. Results A total of 28 consecutive patients (1,013 CBCT scans) were studied. The average interfraction shift was 1.4 ± 1.4, 1.7 ± 1.9, and 1.8 ± 2.1 mm in the medial–lateral, superoinferior, and anteroposterior dimensions, respectively. The corresponding average residual error shifts were 0.7 ± 0.8, 0.9 ± 0.9, and 0.9 ± 0.9 mm. These data indicate that in the absence of daily CBCT image-guided radiotherapy, a clinical target volume to planning target volume margin of 3.9, 4.1, and 4.9 mm is needed in the medial–lateral, superoinferior, and anteroposterior dimensions, respectively. With daily CBCT, corresponding margins of 1.6, 2.5, and 1.9 mm should be acceptable. Subgroup analyses showed that larynx cancers and/or intratreatment weight loss indicate a need for slightly larger clinical target volume to planning target volume margins. Conclusion The results of our study have shown that image-guided radiotherapy using CBCT for head-and-neck cancer is effective. These data suggest it allows a reduction in the clinical target volume to planning target volume margins by about 50%, which could facilitate future studies of dose escalation and/or improved toxicity reduction. Caution is particularly warranted for cases in which the targets are mobile ( e.g., the tongue).

Purpose: Investigate use of standardized non-coplanar arcs to improve plan quality in lung Stereotactic Body Radiation Therapy(SBRT) VMAT planning. Methods: VMAT planning was performed for 9 patients previously treated with SBRT for peripheral lung tumors (tumor size:12.7cc to 32.5cc). For each patient, 7 VMAT plans (couch rotation values:0,5,10,15,20,25,and 30 deg) were generated; the coplanar plans were pushed to meet the RTOG0915 constraints and each non-coplanar plans utilized the same optimization constraints. The following plan dose metrics were used (taken from RTOG 0915): D-2cm: the maximum dose at 2 cm from the PTV, conformality index (CI), gradient index (GI), lung volume receiving 5 Gy (V5) and lung volume receiving 20 Gy (V20). The couch collision clearance was checked for each plan through a dry run using the couch position from the patient’s treatment. Results: Of the 9 cases, one coplanar plan failed to meet two protocol guidelines (both gradient index and D-2cm parameter), and an additional plan failed the D-2cm parameter. When introducing at least 5 degree couch rotation, all plans met the protocol guidelines. The largest feasible couch angle available was 15 to 20 degrees due to gantry collision issues. Non-coplanar plans resulted in the average (standard deviation) reduction of the following metrics: GI by 7.3% (3.7%); lung V20 by 11.1% (3.2%); D-2cm by 12.7% (3.9%). The CI was unchanged (−0.3%±0.6%), and lung V5 increased (3.8%±8.2%). Conclusion: The use of couch rotations as little as 5 degrees allows for plan quality that will meet RTOG0915 constraints while reducing D-2cm, GI, and lung V20. Using default couch rotations while planning SBRT cases will allow for more efficient planning with the stated goal of meeting RTOG0915 criteria for all clinical cases. Gantry clearance checks in the treatment room may be necessary to ensure safe treatments for larger couch rotation values.

Purpose: In image‐guidedradiotherapy(IGRT), cone‐beam CT(CBCT) scan parameters do not take into account patient size information, which can result in unnecessary dose. So‐called “smart scans” in radiology attempt to customize exposure parameters to reduce dose without sacrificing image quality. This work presents the feasibility of tube current modulation as a dose reduction technique in CBCT‐based IGRT. Methods: A CATPHAN® phantom with water‐equivalent annulus was used to evaluate noise. A CIRS phantom was used to evaluate dose characteristics. Gies et. al.[Med Phys, 26:1999], tells us that noise for the central pixel is minimal for tube current modulated by the square root of the given angular attenuation. This formulation is used to calculate modulation factors for each 10 degree imaging arc. Modulated image dataset was generated by compositing projection data from scans performed with each modulated current setting. Noise characterization was performed by analyzing the standard deviation of the uniformity section of the CATPHAN. Imagingdose was measured using an ionization chamber located in the phantom. For the modulated scan, each ten degree segment was measured separately and then accumulated to get a composite dose reading.Results: Central axis dose reduction of 31% was attained using this modulation scheme. Visual inspection of a modulated CBCT slices shows appreciable image quality relative to the conventional CBCT. The noise characteristics of the modulated scan were about 20 HU worse than a conventional scan, likely due to the presence of scatter in the enlarged CATPHAN phantom because theoretical predictions are based on primary beam attenuation alone Conclusions: Initial results demonstrate that the use of a modulated scanning technique, developed by taking into account patient anatomic variation, may be a feasible method to reduce dose while preserving image quality in CBCT‐based IGRT.

Purpose: To describe and evaluate a novel algorithm for generating synthetic CT images from MR-SIM data for dose calculations in MR-only treatment planning. Methods: A voxel-based weighted summation method was implemented to generate synthetic CT (synCT) images. MR data were acquired using Philips 1.0T Panorama high-field open MR-SIM. Retrospective patient data from seven prostate patients and one brain patient (three lesions) enrolled in an IRB-approved study were used. 3D T1-weighted fast field echo and 3D T2-weighted turbo spin echo sequences were utilized for all patients. A 3D balanced turbo field echo sequence using spectral presaturation with inversion recovery was acquired for prostate patients, but 3D ultra-short echo time (UTE)-DIXON was instead acquired for the brain patient to amplify bone signal for semi-automatic bone segmentation. Weight optimization was performed using a training subset of patients. HU value differences between planning CT and synCTs were analyzed using mean absolute error (MAE). Original patient CT-based treatment plans were mapped onto synCTs, dose was recalculated using original leaf motion and MU values, and DRRs were generated. Dosevolume metrics and gamma analysis were used for dosimetric evaluation. Results: Average whole-body MAE of synCTs across all patients was 75+12 HU. In prostate cancer patients, average HU difference between planning and synCTs was 0.9±1.0% for soft tissue structures and 4.3±2.5% for bony structures. DRRs were generated from synCTs and qualitatively showed good geometric agreement with planning CT-generated DRRs. D99, mean dose, and maximum dose to CTV calculated using the synCT remained within 1.2% of planning CT-based dose calculations. All gamma analysis evaluated at 2%/2mm dose difference/distance to agreement) pass rates were greater than 95% with an average of 99.9±0.1% for prostate patients and 98.4±2.2% for three brain lesions. Conclusion: SynCTs were generated with clinically acceptable accuracy comparable to planning CTs, enabling dose computations for MR-only simulation. Research supported in part by a grant from Philips HealthCare (Best, Netherlands).

Purpose: An important question rarely discussed in the CT vs. MRI simulation debate is whether MRI reference images provide an adequate image‐set to use with daily localization such as cone‐beam CT (CBCT). This study compares clinical couch shifts based on daily CBCT images to shifts measured from MR images as the reference dataset for prostate IMRT treatment. Methods: Eight patients undergoing a pilot study had MR imaging along with CT simulation with the intent of evaluating a MR‐simulation process. Patients had T1, T2 and bTFE (balanced Turbo Field Echo) sequences. The remainder of the treatment planning process continued using traditional procedures using CT. Therapists used only the CT scan as a reference for localization. Retrospectively, an observer measured shifts between daily CBCT images and MR reference images. Results: The differences in shift positions for the cohort between therapists and the observer are −0.16cm ± 0.25cm (AP), 0.04cm ± 0.19cm (SI), and — 0.01cm ± 0.14cm (LR). The mean group error for the therapists and the observer were less than 2 mm in all directions. Based on these shifts, the calculated margins for the therapists would be 0.87cm (AP), 0.65cm (SI), and 0.71cm (LR) and for the observer would be 1.1cm (AP), 0.66cm (SI), and 0.70cm (LR). For SI and LR directions both sets of margins are very close to one another. An outlier impacted the AP margin difference by 2.3mm and should be investigated further. This initial analysis suggests that each modality can be considered clinically sufficient for daily localization. Conclusion: The results of this study suggest that MR reference image‐sets can be used for daily image‐guided localization of prostate cancers with at least the same accuracy as current methods. MR simulation provides substantial soft‐tissue contrast and can improve tissue targeting in radiation oncology, as a Result further investigation is warranted.

Purpose: To establish a centralized database platform for radiotherapyquality assurance (QA). This can be an interactive platform where physicists can download digital benchmark cases from, perform radiotherapy planning and QA measurements with various planning and delivery systems, and upload the results. Data retrieval and analysis functions can then be developed for the quality assessment of the radiation plan and measurement in the context of prior data with similar planning, delivery and measurement technologies. Data mining of the system can in turn generate quantitative guidance for clinical trial QA requirements that have the optimum combination of stringent mandates and realistic accommodations. Methods: Built upon prior experience of Task Group(TG) 119 from IMRT workgroup of AAPM, and digital data phantoms from EORTC, these groups collaborate with clinical trial work group of AAPM and advanced technology consortium (ATC) to establish a virtual QA environment within National Biomedical Imaging Archive (NBIA). Digital phantoms from TG 119 and EORTC dosimetric evaluation with DICOM image, structures, plan and dose were uploaded to the virtual QA environment for the established community to download and perform evaluation of basic dosimetric parameters including, structure volumes, geometric location of structure center, designated points on dose volume histogram (DVH). Nine different treatment planning or related systems with various versions are included in these evaluations. Results: Volume variations among versions of these nine systems are 2.7%, 4.2%, 14.7%, 4.7% for prostate PTV, prostate, rectum and bladder respectively. DVH reading differences are 0.1%, 2.4% and 1.5% for prostate D95, rectum D30 and bladder D30. DVH reading differences for cube 10 are 1%, 86% and 100% for V0, V20 and V30 respectively. Conclusions: A virtual QA environment has been established within NBIA for radiation oncology inter‐system and inter‐institution QA. Initial testing in reading geometric and dosimetric parameters identified significant variations among systems.

Purpose: Needle applicators have shown displacement between fractions during HDR prostate brachytherapy, which will degrade plan quality. Typically, displacement is determined by analyzing catheters on a verification CT (vCT) or port films obtained before treatment relative to fiducials or bony landmarks. We present a procedure to verify applicator positions to achieve planned dose distributions by registering vCT directly to planning CTs (pCT). Method and Materials: Patients received a vCT prior to each fraction. This was imported to BrachyVision (Varian Medical Systems) and fused to the pCT by rigid‐body registration based on matching urethra and three prostate‐implanted gold fiducials. Then, applicators in the vCT were compared to applicators in the pCT in a reconstructed plane through each catheter and any difference larger than 3mm was manually adjusted by a radiation oncologist before treatment. To assess treatment quality, the prostate volume was copied from pCT to vCT and the planned dwell positions/time was applied to the vCT applicators. Results: Two consecutive patients were treated using our CT registration needle verification method. Both patients required applicator adjustments in the first two fractions. Patient one had an average 7mm cranial‐to‐caudal applicator displacement, corresponding to 23% and 12% fractional drop in V100prostate respectively; Patient two had displacement in both cranial‐to‐caudal and caudal‐to‐cranial directions up to 9mm, but due to minimum loading of affected applicators, no significant decrease in V100prostate was seen. Neither patient required applicator adjustment at the Srd/last fraction. Despite differences in applicator displacement, post‐adjustment, the achieved V100prostate at each fraction deviated from planned value by less than 4% for both patients. The distances between fiducials in vCTs were consistent with those in pCTs within 3mm, confirming that fiducials are appropriate landmarks for registration. Conclusion: We have successfully treated two patients using this method. This technique ensures the treatment quality closely matches plan quality.

Purpose: Patient specific IMRT QA has been an integral part of the IMRT process in most institutions since the advent of IMRT. There has been the question whether we need to continue with this practice for every patient now that we have data for multiple similar cases. However, there have always been singular cases that fail IMRT QA criteria. This study looks to correlate treatment planning parameters and other metrics with the IMRT QA passing rate hoping to predict cases which may still require IMRT QA measurements. Methods and Materials: 15 head and neck treatment plans (101 total treatment beams) were delivered and analyzed using a 2‐dimensional diode array (MapCheck®). Each treatment beam was analyzed using a gamma analysis with evaluation criteria of 10 percent threshold, 3 percent dose deviation and 3 millimeters distance‐to‐agreement as this is the standard for our institution. Parameters used to correlate with passing rate were: monitor units delivered, dose at isocenter, delivery efficiency (MU/cGy), number of segments, average leaf pair opening, and a 2‐dimensional modulation index developed from the original Webb equation. Results: Univariate analysis showed no factors that correlated with passing rate. The R‐squared values ranged from 0.0014 to 0.0565. MU number did have a p value of 0.016, but no other indicator of plan complexity predicted passing rate. Conclusions: This work demonstrates our attempt to show a relationship between IMRT QA passing rates and treatment plan parameters. We are unable to correlate any of these metrics with patient QA pass rate as specified. Treatment delivery uncertainties (e.g. flatness and symmetry) may play as pivot a role in QA passing rate as the treatment planning parameters used. Further investigation will involve combination of these uncertainties.

Purpose: To commission CMS Monaco IMRTtreatment planning system for clinical applications using a standard test suite. Methods and Materials: A system test including IMRT planning and plan delivery was performed for CMS Monaco IMRTtreatment planning system, version 1.0.2. Four cases from TG‐119 were studied: one head and neck case, one prostate case, and two C shape cases. IMRT plans were generated with the dose goals set by TG‐119 and were compared with the plans generated using CMS XiO treatment planning system. Analysis metrics were for dose coverage, number of segments, total MU, MU efficiency, number of segments per beam, and delivery time. The IMRT plans were delivered on an Elekta Synergy linear accelerator with step‐and‐shoot technique. This linac has 4‐mm MLC.IMRT QA was performed with field by field review using a diode array device Mapcheck. Results: Monaco plans showed similar target dose coverage as XiO plans and improved organ sparing in some cases. The ratios (Monaco/XiO) of number of total segments, total MU, number of segments per beam, and MU efficiency (Dose/MU) were 0.21–0.76, 0.56–0.77, 0.21–0.50, and 1.31–1.86, respectively. The delivery time of a Monaco plan was shorter than the XiO plan having the same number of beams by approximately 33%. IMRT QA pass rates of the Monaco plans were 93.8%–97.6% for the criteria of 3‐mm, 3%, and 10% threshold, which in three cases were higher than the XiO plans (93.1%–96.6%). Conclusion: The IMRT plans from the Monaco treatment planning system showed advantages over XiO plans. These advantages include smaller number of segments, smaller MU, smaller number of segments per beam, and higher MU efficiency. Additionally, the delivery time was shorter. The QA pass rates were similar to or higher than the XiO plans.

Purpose: With the increased presence of volumetric radiotherapy cone beam CT (RT‐CBCT) systems, the importance of high grade image quality is crucial to achieve daily image‐guided adaptive radiotherapy (IGART) capabilities. We subjected three commercially available RT‐CBCT systems to a battery of standard diagnostic image quality tests acquired under clinical conditions. This study reports on the evaluation of image qualities for RT‐CBCT's and a diagnostic CT.Methods and Materials: RT‐CBCT scans were performed on Elekta XVI, Nucletron Simulix, and Varian OBI Advanced Imaging using clinical pelvis scan settings on a CATPHAN Model 600. They were then compared to results from a GE Lightspeed CT scanner. The phantom contained modules allowing measurement of low contrast resolution, slice thickness, Hounsfield Unit (HU) sensitivity, spatial resolution, and image uniformity. Results: No CBCT system was able to detect any targets in the low contrast module. Six targets could be seen on diagnostic CT's. All RT‐CBCT systems had greater than 33% variations in slice thickness reconstruction. The HU sensitometry test showed absolute differences between accepted HU values and measured values for XVI and Simulix systems to be on average 6 times and 3 times the error seen in a diagnostic CT respectively. HU sensitivity for OBI is within 15% of a diagnostic CT. Both OBI and Simulix had spatial resolution twice that of XVI but were 3 lp/cm worse than a diagnostic CT. Measurements of image uniformity showed that all three RT‐CBCT systems have a standard deviation of HU values on the order of 10 times that of a diagnostic CT.Conclusions: The image qualities of three evaluated RT‐CBCT systems are relatively comparable, yet still inferior to those from helical CT. It is important to note that dosimetric settings can have a clear impact on image quality, and dose measurements were not done for this work.

Purpose: To evaluate the feasibility of respiratory gating with respect to Active Breathing Control (ABC) for motion reduction through quantitative comparison. Methods and Materials: 4D scans from a GE(Light speed)scanner with RPM were used for the feasibility study. Ten phase reconstructions were generated from the 4D projections and target volumes on all 10 phases were created by radiation oncologists. For free breathing treatments, target volumes from all 10 phases were added to generate the ITV used for clinical target. In order to simulate the gated treatment, the three phases of exhalation which best matched the gated delivery time period were combined. The target thus obtained was then compared with those obtained from the standard scan with the ABC device in place. Targets were contoured for the ABC scan as well. A volumetric comparison of the targets was performed for the above three methods. Results: We tested the feasibility of the method on one of our patient cases for whom we utilized the ABC device for breathing motion control. The target volume for the 10 phase target was 25.49cc. The target volume from the simulated gated phases was 12.09cc and the target volume with the ABC was 11.19cc. Both the gated technique and the ABC control are able to reduce the target volume significantly, with about the same amount of reduction (52.57% and 56.1%). Conclusions: This study established quantitatively that gated delivery technique can mimic the volumetric outcome of ABC delivery for motion control. Both are able to reduce the target volume significantly. We plan to further this study with more data collection.

Purpose:Treatment planning system/delivery system combinations affect IMRT optimization. Delivery method (step and shoot vs. dynamic), leakage radiation variations (lack of secondary collimators on the Elekta SL‐S MLC head), hardware constraints and leaf edge shape all factor in when a radiation treatment planning system (RTPS) optimizes MLC segments. Different optimization algorithms within treatment planning systems influence IMRT planning results as well. In our study we evaluated delivery efficiencies from Elekta, Varian and Siemens machines coupling with two different treatment planning systems. Method and Materials: 16 IMRT plans were created for prostate and headneck (HN) sites (5 and 6 co‐planar beams respectively) using CMS Xio and Philips ADAC Pinnacle. Plans were optimized using the same dose and DVH objectives and modulated using clinical thresholds. The monitor units needed to deliver the weight point dose for each field were tabulated for each site and machine source data. Results: All plans have similar quality meeting the objectives. HN plans have an average MU efficiency of 5.02 +/− 1.09 MU/cGy. The plans created on the Pinnacle RTPS had a lower mean efficiency but was not statistically significant due to the high standard deviation. Prostate plans were more efficient than the HN plans with an average value of 2.19 +/− 0.47 MU/cGy. Pinnacle prostate plans had a mean MU efficiency of 1.69 +/− 0.18 MU/cGy compared to 2.51 +/− 0.22 MU/cGy from Xio. Conclusion: There is no statistical difference in MU efficiency for HN plans from different planning/delivery system combinations. Prostate plans have higher delivery MU efficiencies than those from HN plans, most likely due to less modulation. For the prostate plans there exist slight differences in MU efficiencies with unknown clinical significance between different planning/delivery systems.

Purpose: As IGRT strategies evolve from localization to adaptive planning, it becomes more important to acquire guidance images of higher quality. We utilized a CATPHAN to subject radiotherapyCBCTimages acquired under various conditions to the same rigorous tests and evaluation that diagnostic CTs undergo. This study reports on the evaluation of image qualities of diagnostic CT and radiotherapyCBCT and identifies the area of quality differences between the images.Method and Materials: RT‐CBCT scans were performed on Elekta XVI using pelvis scan settings on a CATPHAN Model 600. They were then compared to results from a GE Lightspeed16 multidetector CT scanner. The phantom contained modules allowing measurement of slice thickness, spatial resolution, low contrast resolution, Hounsfield Unit (HU) sensitivity and image uniformity. HU uniformity in the axial direction and geometric distortion were also measured. Results: For low contrast resolution tests, no targets could be clearly defined in the medium resolution reconstruction on RT‐CBCT. Only three spheres could be seen with the high resolution reconstruction (not used clinically) compared to six on diagnostic CTs. Calculated thicknesses for a 1.0 mm slice varied from 1.33 to 2.45 mm. The HU sensitometry test showed large variation from accepted HU values of the targets. These variations were not linear or affected by the reconstruction method. Spatial resolution for clinical settings (medium resolution) was constant at three lp/cm and improved to ten lp/cm with the high resolution acceptance testing reconstruction. This is comparable to diagnostic CT resolution. Axial direction HU uniformity varied in a stepwise fashion as the uniformity module moved away from the CBCT isocenter. No measurable geometric distortion was found as a function of distance from isocenter. Conclusion: A consolidated CBCT QA procedure is feasible for monitoring changes in image acquisition systems over time for image quality evaluation in adaptive therapy.

Purpose: Commissioning treatment planning systems for IMRT require more tests and verification of the beam model than those required for 3D conformal techniques. This study presents the dependence of IMRT QA pass/fail rate on the modeling of MLC leaf transmission in a MiniMLC linear accelerator. This particular modeling parameter doesn't affect the tests for 3D conformal situations per commissioning guideline. The results of both film QA and Mapcheck analysis will be presented. Method and Materials: Elekta Synergy S linac equipped with Beam Modulator collimating head was used (4mm leaves, no movable backup diaphragms) Beams were modeled in CMS XIO following the guidelines of TG53. IMRT plans were calculated for a phantom target (mock prostate) and the accuracy of the dose delivery was measured using both film and Mapcheck Gamma analysis. Then, the beam model in XIO planning system was replaced by the model with substantially lower MLC transmission factors, the plans were re‐optimized, and re‐delivered. Results: Original beam fitting (following linac commissioning) was internally verified by at least two independent measurements and checked against standard datasets (average passing rate of 97.8%). However, in our test plans, these fits showed relatively poor agreement between planned and delivered treatment as verified by both film and Mapcheck‐based dosimetry. Average passing rate using film dosimetry was 88% and using Mapcheck it was 71%. After beam re‐fitting using the decreased MLC Transmission factor, the agreement between planned and delivered dose increased to 98% and 87%, respectively. Conclusion: Commissioning of treatment planning systems for IMRT should include measurement verifications for IMRT plans of relatively complex modulation, in addition to standard tests from guidelines for 3D conformal planning.

Background This study describes initial testing and evaluation of a vertical-field open Magnetic Resonance Imaging (MRI) scanner for the purpose of simulation in radiation therapy for prostate cancer. We have evaluated the clinical workflow of using open MRI as a sole modality for simulation and planning. Relevant results related to MRI alignment (vs. CT) reference dataset with Cone-Beam CT (CBCT) for daily localization are presented. Methods Ten patients participated in an IRB approved study utilizing MRI along with CT simulation with the intent of evaluating the MRI-simulation process. Differences in prostate gland volume, seminal vesicles, and penile bulb were assessed with MRI and compared to CT. To evaluate dose calculation accuracy, bulk-density-assignments were mapped onto respective MRI datasets and treated IMRT plans were re-calculated. For image localization purposes, 400 CBCTs were re-evaluated with MRI as the reference dataset and daily shifts compared against CBCT-to-CT registration. Planning margins based on MRI/CBCT shifts were computed using the van Herk formalism. Results Significant organ contour differences were noted between MRI and CT. Prostate volumes were on average 39.7% (p = 0.002) larger on CT than MRI. No significant difference was found in seminal vesicle volumes (p = 0.454). Penile bulb volumes were 61.1% higher on CT, without statistical significance (p = 0.074). MRI-based dose calculations with assigned bulk densities produced agreement within 1% with heterogeneity corrected CT calculations. The differences in shift positions for the cohort between CBCT-to-CT registration and CBCT-to-MRI registration are −0.15 ± 0.25 cm (anterior-posterior), 0.05 ± 0.19 cm (superior-inferior), and −0.01 ± 0.14 cm (left-right). Conclusions This study confirms the potential of using an open-field MRI scanner as primary imaging modality for prostate cancer treatment planning simulation, dose calculations and daily image localization.