MRIgRT head and neck anthropomorphic QA phantom: Design, development, reproducibility, and feasibility study.

PURPOSE: The purpose of this paper was to design, manufacture, and evaluate a tissue equivalent, dual magnetic resonance/computed tomography (MR/CT) visible anthropomorphic head and neck (H&N) phantom. This phantom was specially designed as an end-to-end quality assurance (QA) tool for MR imaging guided radiotherapy (MRIgRT) systems participating in NCI-sponsored clinical trials. METHOD: The MRIgRT H&N phantom was constructed using a water-fillable acrylic shell and a custom insert that mimics an organ at risk (OAR) and target structures. The insert consists of a primary and secondary planning target volume (PTV) manufactured of a synthetic Clear Ballistic gel, an acrylic OAR and surrounding tissue fabricated using melted Superflab. Radiochromic EBT3 film and thermoluminescent detectors (TLDs) were used to measure the dose distribution and absolute dose, respectively. The phantom was evaluated by conducting an end-to-end test that included: imaging on a GE Lightspeed CT simulator, planning on Monaco treatment planning software (TPS), verifying treatment setup with MR, and irradiating on Elekta's 1.5 T Unity MR linac system. The phantom was irradiated three times using the same plan to determine reproducibility. Three institutions, equipped with either ViewRay MRIdian 60 Co or ViewRay MRIdian Linac, were used to conduct a feasibility study by performing independent end-to-end studies. Thermoluminescent detectors were evaluated in both reproducibility and feasibility studies by comparing ratios of measured TLD to reported TPS calculated values. Radiochromic film was used to compare measured planar dose distributions to expected TPS distributions. Film was evaluated by using an in-house gamma analysis software to measure the discrepancies between film and TPS. RESULTS: The MRIgRT H&N phantom on the Unity system resulted in reproducible TLD doses (SD < 1.5%). The measured TLD to calculated dose ratios for the Unity system ranged from 0.94 to 0.98. The Viewray dose result comparisons had a larger range (0.95-1.03) but these depended on the TPS dose calculations from each site. Using a 7%/4 mm gamma analysis, Viewray institutions had average axial and sagittal passing rates of 97.3% and 96.2% and the Unity system had average passing rates of 97.8% and 89.7%, respectively. All of the results were within the Imaging and Radiation Oncology Core in Houston (IROC-Houston) standard credentialing criteria of 7% on TLDs, and >85% of pixels passing gamma analysis using 7%/4 mm on films. CONCLUSIONS: An MRIgRT H&N phantom that is tissue equivalent and visible on both CT and MR was developed. The results from initial reproducibility and feasibility testing of the MRIgRT H&N phantom using the tested MGIgRT systems suggests the phantom's potential utility as a credentialing tool for NCI-clinical trials.

MRIgRT dynamic lung motion thorax anthropomorphic QA phantom: Design, development, reproducibility, and feasibility study.

PURPOSE: To design, manufacture, and evaluate a dynamic magnetic resonance imaging/computed tomography (MRI/CT)-compatible anthropomorphic thorax phantom used to credential MR image-guided radiotherapy (MRIgRT) systems participating in NCI-sponsored clinical trials. METHOD: The dynamic anthropomorphic thorax phantom was constructed from a water-fillable acrylic shell that contained several internal structures representing radiation-sensitive organs within the thoracic region. A custom MR/CT visible cylindrical insert was designed to simulate the left lung with a centrally located tumor target. The surrounding lung tissue was constructed from a heterogeneous in-house mixture using petroleum jelly and miniature (2-4 mm diameter) styrofoam balls and the tumor structure was manufactured from liquid PVC plastic. An MR conditional pneumatic system was developed to allow the MRIgRT insert to move in similar inhale/exhale motions. TLDs and radiochromic EBT3 film were inserted into the phantom to measure absolute point doses and dose distributions, respectively. The dynamic MRIgRT thorax phantom was evaluated through a reproducibility study and a feasibility study. Comprehensive end-to-end examinations were done where the phantom was imaged on a CT, an IMRT treatment plan was created and an MR image was captured to verify treatment setup. Then, the phantom was treated on an MRIgRT system. The reproducibility study evaluated how well the phantom could be reproduced in an MRIgRT system by irradiating three times on an Elekta's 1.5 T Unity system. The phantom was shipped to three independent institutions and was irradiated on either an MRIdian cobalt-60 (60 Co) or an MRIdian linear accelerator system. Treatment evaluations used TLDs and radiochromic film to compare the planned treatment reported on the treatment planning software against the measured dose on the dosimeters. RESULTS: The phantom on the Unity system had reproducible TLD doses measurements (SD < 1.5%). The measured TLD to calculated dose ratios from the reproducibility and feasibility studies ranged from 0.93 to 1.01 and 0.96 to 1.03, respectively. Using a 7%/5 mm gamma analysis criteria, the reproducibility and feasibility studies resulted in an average passing rate of 93.3% and 96.8%, respectively. No difference was noted in the results between the MRIdian 60 Co and MRIdian 6 MV linac delivery to the phantom and all treatment evaluations were within IROC-Houston's acceptable criterion. CONCLUSIONS: A dosimetrically tissue equivalent, CT/MR visible, motion-enabled anthropomorphic MRIgRT thorax phantom was constructed to simulate a lung cancer patient and was evaluated as an appropriate NIH credentialing tool used for MRIgRT systems.

Investigation of TLD and EBT3 performance under the presence of 1.5T, 0.35T, and 0T magnetic field strengths in MR/CT visible materials.

PURPOSE: The aim of this study was to investigate thermoluminescent dosimeters (TLD) and radiochromic EBT3 film inside MR/CT visible geometric head and thorax phantoms in the presence of: 0, 0.35, and 1.5 T magnetic fields. METHODS: Thermoluminescent Dosimeters reproducibility studies were examined by irradiating IROC-Houston's TLD acrylic block five times under 0 and 1.5 T configurations of Elekta's Unity system and three times under 0 and 0.35 T configurations of ViewRay's MRIdian Cobalt-60 (60 Co) system. Both systems were irradiated with an equivalent 10 × 10 cm2 field size, and a prescribed dose of 3 Gy to the maximum depth deposition (dmax). EBT3 film and TLDs were investigated using two geometrical Magnetic Resonance (MR)-guided Radiation Therapy (MRgRT) head and thorax phantoms. Each geometrical phantom had eight quadrants that combined to create a centrally located rectangular tumor (3 × 3 × 5 cm3 ) surrounded by tissue to form a 15 × 15 × 15 cm3 cubic phantom. Liquid polyvinyl chloride plastic and Superflab were used to simulate the tumor and surrounding tissue in the head phantom, respectively. Synthetic ballistic gel and a heterogeneous in-house mixture were used to construct the tumor and surrounding tissue in the thorax phantom, respectively. EBT3 and double-loaded TLDs were used in the phantoms to compare beam profiles and point dose measurements with and without magnetic fields. GEANT4 Monte Carlo simulations were performed to validate the detectors for both Unity 0 T/1.5 T and MRIdian 0 T/0.35 T configurations. RESULTS: Average TLD block measurements which, compared the magnetic field effects (magnetic field vs 0 T) on the Unity and MRIdian systems, were 0.5% and 0.6%, respectively. The average ratios between magnetic field effects for the geometric thorax and head phantoms under the Unity system were -0.2% and 1.6% and for the MRIdian system were 0.2% and -0.3%, respectively. Beam profiles generated with both systems agreed with Monte Carlo measurements and previous literature findings. CONCLUSIONS: TLDs and EBT3 film dosimeters could potentially be used in MR/CT visible tissue equivalent phantoms that will experience a magnetic field environment.

TLD and OSLD dosimetry systems for remote audits of radiotherapy external beam calibration.

The Imaging and Radiation Oncology Core QA Center in Houston (IROC-H) performs remote dosimetry audits of more than 20,000 megavoltage photon and electron beams each year. Both a thermoluminescent dosimeter (TLD-100) and optically stimulated luminescent dosimeter (OSLD; nanoDot) system are commissioned for this task, with the OSLD system being predominant due to the more time-efficient read-out process. The measurement apparatus includes 3 TLD or 2 OSLD in an acrylic mini-phantom, which are irradiated by the institution under reference geometry. Dosimetry systems are calibrated based on the signal-to-dose conversion established with reference dosimeters irradiated in a Co-60 beam, using a reference dose of 300 cGy for TLD and 100 cGy for OSLD. The uncertainty in the dose determination is 1.3% for TLD and 1.6% for OSLD at the one sigma level. This accuracy allows for a tolerance of ±5% to be used.

Direct megavoltage photon calibration service in Australia.

The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) maintains the Australian primary standard of absorbed dose. Until recently, the standard was used to calibrate ionisation chambers only in (60)Co gamma rays. These chambers are then used by radiotherapy clinics to determine linac output, using a correction factor (k Q) to take into account the different spectra of (60)Co and the linac. Over the period 2010-2013, ARPANSA adapted the primary standard to work in megavoltage linac beams, and has developed a calibration service at three photon beams (6, 10 and 18 MV) from an Elekta Synergy linac. We describe the details of the new calibration service, the method validation and the use of the new calibration factors with the International Atomic Energy Agency's TRS-398 dosimetry Code of Practice. The expected changes in absorbed dose measurements in the clinic when shifting from (60)Co to the direct calibration are determined. For a Farmer chamber (model 2571), the measured chamber calibration coefficient is expected to be reduced by 0.4, 1.0 and 1.1 % respectively for these three beams when compared to the factor derived from (60)Co. These results are in overall agreement with international absorbed dose standards and calculations by Muir and Rogers in 2010 of k Q factors using Monte Carlo techniques. The reasons for and against moving to the new service are discussed in the light of the requirements of clinical dosimetry.

Independent dose per monitor unit review of eight U.S.A. proton treatment facilities.

PURPOSE: Compare the dose per monitor unit at different proton treatment facilities using three different dosimetry methods. METHODS: Measurements of dose per monitor unit were performed by a single group at eight facilities using 11 test beams and up to six different clinical portal treatment sites. These measurements were compared to the facility reported dose per monitor unit values. RESULTS: Agreement between the measured and reported doses was similar using any of the three dosimetry methods. Use of the ICRU 59 ND,w based method gave results approximately 3% higher than both the ICRU 59 NX and ICRU 78 (TRS-398) ND,w based methods. CONCLUSIONS: Any single dosimetry method could be used for multi-institution trials with similar conformity between facilities. A multi-institutional trial could support facilities using both the ICRU 59 NX based and ICRU 78 (TRS-398) ND,w based methods but use of the ICRU 59 ND,w based method should not be allowed simultaneously with the other two until the difference is resolved.

SU-E-T-519: Experimental Evaluation of Deterministic Acuros XB Radiation Transport Algorithm for Heterogeneity Dose Calculation Using the Radiological Physics Center's Lung Phantom.

PURPOSE: To evaluate the heterogeneity corrected dose calculations from the Acuros XB (AXB), a novel deterministic dose calculation algorithm based on grid-based Boltzmann transport equation solver (GBBS), for IMRT and VMAT plans. METHODS: The Radiological Physics Center's lung phantom was used to create clinically equivalent IMRT and VMAT plans (RapidArc) with the Eclipse planning system 10.0 that were delivered using a Varian 23 iX. Absolute doses and relative dose distributions were measured with thermoluminescent dosimeters (TLDs) and radiochromic film. The measured dose distributions were compared with calculated doses from both AXB (11.0.3) and AAA (10.0.24) dose calculation algorithms. The AXB calculated dose-to-water and dose-to-medium were both compared to measurements. Gamma analysis (±7%/4mm, ±5%/3mm, and ±3%/3mm) was used to quantify correspondence between AXB dose distributions and the film measurements. The computation time between AAA and AXB were also evaluated. RESULTS: For TLD point doses, both AAA and AXB heterogeneity corrected dose calculations are within 5% inside the PTV for both IMRT and VMAT plans. The agreements observed between the measured and calculated doses for both AXB dose reporting methods are better than those observed with the AAA algorithm. The gamma analysis showed that the differences between AAA, AXB and film measurement met the RPC ±7%/4 mm criteria. The percent of pixels passing rate for both the AXB dose to medium and AXB dose to water are higher than AAA. The computation time between AAA and AXB are comparable for IMRT plans but AXB is significantly faster (4 times) than AAA for VMAT plans. CONCLUSIONS: The AXB implemented in the Eclipse planning system calculates a more accurate heterogeneity corrected dose than the AAA algorithm as compared to measurement in lung and improve the calculation speed for VMAT radiotherapy. Work supported by grants CA10953, CA81647, 2R44CA105806-02, CA016672 (NCI, DHHS).

SU-E-T-86: Development and Implementation of the Use of Optically Stimulated Luminescent Detectors in the Radiological Physics Center Anthropomorphic Quality Assurance Phantoms.

PURPOSE: To study the angular dependence of optically stimulated luminescence dosimeters (OSLD) in the Radiological Physics Center anthropomorphic quality assurance pelvic phantom to provide accurate dosimetric measurements as a replacement for TLD. METHODS: A spherical phantom was constructed to investigate the angular response of the OSLD as oriented in the RPC pelvic phantom. Three OSLD per irradiation angle, placed at the center of the spherical phantom, were irradiated with 100 cGy from six different angles. The angular response at each angle was determined relative to the OSLD response when the beam was incident normally on the OSLD surface. A pelvic phantom dosimetry insert was modified to include both TLD and OSLD. Three treatment plans were developed in Pinnacle v9.0 and one in Accuray's Multiplan, each with increasing angular beam delivery (4 field, IMRT, SmartArc, CyberKnife) for the pelvic phantom using a common dose prescription and constraints. Each plan was delivered to the phantom three times, containing two TLD and two OSLD, oriented in the transverse plane, at the center of the PTV. The dose delivered to the TLD and OSLD was calculated for each treatment and then compared. RESULTS: The angular dependence correction factor for the spherical phantom was found to be uniformly 1.041 ± 0.003 from single beam edge-on irradiations. The angular dependence correction in the pelvic phantom from multiple beam orientation irradiations was 1.024 ± 0.002, such that the OSLD dose agreed with the TLD dose. Applying the OSLD pelvic phantom correction factor, the RPC measured dose to planning system calculated dose ratio was 0.995 ± 0.009. The established RPC phantom TLD dose to calculated dose ratio was 0.995 ± 0.010. CONCLUSIONS: An anthropomorphic phantom OSLD angular dependence correction factor was established such that the final OSLD dose measurements agreed with RPC's TLD dose measurements to within 1%. Work supported by grant CA 10953, awarded by NCI, DHHS.

SU-E-T-102: Evaluation of the Characteristics of TLD LiF:Mg.Ti-100 Powder: A Measure of Consistency between Multiple Batches of Powder.

PURPOSE: Analyze the characteristics of TLD LiF-100 powder between multiple LiF crystal batches. METHODS: The RPC used TLD LiF-100 encapsulated powder to verify the output for photon and electron beams for 4 to 23 MV X-ray beams and 6 to 23 MeV electron beams, respectively, from the past 15 years. During that time period, the RPC commissioned more than 15 batches of TLD powder. Commissioning of each batch of powder encompassed determining the system sensitivity (dose response), linearity, energy and fading characteristics of each batch of powder to determine the correction factors for the calculation of dose. The system sensitivity is the signal/mg per unit known dose of 60Co for each reading session. Other correction factors account for the loss of signal (fading) between the irradiation and read dates, supralinearity of the dose response and energy differences as compared to the 60Co irradiated standards. RESULTS: More than 15 batches of TLD were commissioned to determine correction factors for the calculation of dose. The correction for fading, a characteristic of the LiF crystal, varied by ±1% between the multiple batches. The linearity correction, between 25 and 600cGy, normalized to 300cGy, showed a maximum variation of ±3% between batches. The energy correction factors, as defined for the RPC beam output audit system varied within ±1.7% (one std dev.) for the 15 batches. The system sensitivity is highly dependent on the LiF crystal grown for each batch, specific TLD reader and reading session conditions. The system sensitivity, while keeping the readers and reading sessions constant, varied by as much as 20% between batches. CONCLUSIONS: Each batch of LiF-100 TLD powder showed variability in their powder characteristics such that calculation of dose accurately, with minimal uncertainty, requires a new commissioning. Work supported by PHS CA010953 awarded by NCI, DHHS.

SU-E-T-95: Investigation of 3D Dosimetry for an Anthropomorphic Spine Phantom.

PURPOSE: To evaluate 3D dosimetry for a spinal cord treatment plan delivery using the Radiological Physics Center's (RPC) anthropomorphic spine phantom. METHODS: The RPC's spine phantom currently uses radiochromic film and thermoluminescent dosimeters (TLD) to evaluate spinal metastases treatments. A second dosimetry insert for the phantom was created to hold a PRESAGE® 3D dosimeter which matched the location of the TLD and film in the original insert. The phantom was CT imaged with each insert and an IMRT treatment plan was developed. The IMRT plan was delivered to the phantom twice; once with each insert. The film and PRESAGE® were scanned on a CCD microdensitometer and optical-CT system, reconstructed to a 2 mm slice width, respectively. The measured dose distributions were compared to the treatment plan calculated dose distribution using RPC in-house developed software or the Computational Environment for Radiotherapy Research (CERR). Film and PRESAGE® dose profiles were taken across several planes and compared for agreement. The distance to agreement (DTA) between the measured data and treatment plan, within the high dose gradient region, was quantified. RESULTS: The PRESAGE® and plan dose profiles agreed to within 2and 1 mm in the AP and SI directions, respectively. The film and plan also agreed to within 2 mm across all profiles. CONCLUSIONS: The PRESAGE® 3D dosimeter, based on these preliminary data, shows potential as a dosimeter for the RPC's phantom irradiation studies. Future work will add markers to the PRESAGE® insert to allow for a reproducible registration in CERR and a an optical-CT system, reconstructed to a 2 mm slice width dose calibration protocol will be created. CA 100835.

SU-E-T-223: High-Energy Photon Standard Dosimetry Data: A Quality Assurance Tool.

PURPOSE: Describe the Radiological Physics Center's (RPC) extensive standard dosimetry data set determined from on-site audits measurements. METHOD AND MATERIALS: Measurements were made during on-site audits to institutions participating in NCI funded cooperative clinical trials for 44 years using a 0.6cc cylindrical ionization chamber placed within the RPC's water tank. Measurements were made on Varian, Siemens, and Elekta/Philips accelerators for 11 different energies from 68 models of accelerators. We have measured percent depth dose, output factors, and off-axis factors for 123 different accelerator model/energy combinations for which we have 5 or more sets of measurements. The RPC analyzed these data and determined the 'standard data' for each model/energy combination. The RPC defines 'standard data' as the mean value of 5 or more sets of dosimetry data or agreement with published depth dose data (within 2%). RESULTS: The analysis of these standard data indicates that for modern accelerator models, the dosimetry data for a particular model/energy are within ï,±2%. The RPC has always found accelerators of the same make/model/energy combination have the same dosimetric properties in terms of depth dose, field size dependence and off-axis factors. Because of this consistency, the RPC can assign standard data for percent depth dose, average output factors and off-axis factors for a given combination of energy and accelerator make and model. CONCLUSIONS: The RPC standard data can be used as a redundant quality assurance tool to assist Medical Physicists to have confidence in their clinical data to within 2%. The next step is for the RPC to provide a way for institutions to submit data to the RPC to determine if their data agrees with the standard data as a redundant check. This work was supported by PHS grants CA10953 awarded by NCI, DHHS.

SU-E-T-447: Evaluation of the Anisotropic Analytical Algorithm (AAA) Heterogeneity Correction Dose Calculation in Flattened and Flattening- Filter-Free (FFF) Beams for High Energy X-Ray Beams Using the Radiological Physics Center (RPC) Lung Phantom.

PURPOSE: Compare the accuracy of AAA heterogeneity corrected dose calculation algorithm for high energy x-ray beams (>10 MV) for flattened and FFF beams using RPC anthropomorphic thorax phantom. METHODS: Six static beam SBRT treatment plans were created using the Varian Eclipse treatment planning system (TPS) AAA v.8.9.08 heterogeneity correction algorithm. Two flattened beam plans (6 MV and 18 MV) and four other plans (6 MV, 6 MV FFF, 10 MV FFF and 15 MV) were delivered using a Clinac 21EX and TrueBeam STx, respectively. Prescription dose/coverage, 6 Gy to 95% PTV, and constraints were the same for all plans. The phantom contained radiochromic films in the 3 major planes and TLDs in the heart, spine, and tumor. Point doses and 2D dose distributions were exported from the Eclipse TPS and compared with the measured doses. The gamma index analysis evaluation criteria of ±5% dose to agreement and 3 mm distance to agreement was used. RESULTS: TLD to TPS tumor point dose ratios were 0.971±0.006(6MV) and 0.957±0.002(6MV), 0.995±0.005(15MV), 1.114±0.006(18MV), and 0.957±0.003(6MV FFF), 0.974±0.011(10MV FFF) for the six plans. Using ±5%/3mm gamma analysis criteria, the average passing rates for all three films were 96.3% and 95.5%, 97.4%, 66.1%, 93.7%, and 96.3% for the 6 MV, 6 MV, 15 MV, 18 MV, 6 MV FFF, and 10 MV FFF plans, respectively. Dose profiles were also evaluated. CONCLUSIONS: The current RPC credentialing criteria are: RPC/Inst. tumor dose ratio of 0.97±0.05 and 85% of the pixels in each film plane must pass a ±5%/5mm gamma index analysis. These data demonstrate that the AAA heterogeneity correction dose calculation algorithm is accurate for photon energies in 6-15 MV range for flattened and FFF beams. Heterogeneity corrected dose calculations for photon energies >15 MV were not accurate. Work supported by grants CA10953 and CA81647 (NCI, DHHS).

SU-E-T-180: The Radiological Physics Center's Anthropomorphic Quality Assurance Phantom Program.

PURPOSE: To describe the phantoms, program logistics and current results for the Radiological Physics Center's (RPC) anthropomorphic QA phantom program for credentialing institutions for participation in NCI-sponsored advanced technology clinical trials. METHODS: The RPC has developed an extensive phantom credentialing program consisting of four different phantoms designs: H&N, pelvis, lung and spine. These QA phantoms are water-filled plastic shells with imageable targets, avoidance structures, and heterogeneities that contain TLD and radiochromic film dosimeters. Institutions wishing to be credentialed request a phantom and are prioritized for delivery. At the institution, the phantom is imaged, a treatment plan is developed, the phantom is positioned on the treatment couch and the treatment is delivered. The phantom is returned and the measured dose distributions are compared to the institution's electronically submitted treatment plan dosimetry data. RESULTS: The RPC currently has an inventory of 31 H&N, 10 pelvis, 9 lung, and 8 spine phantoms that are mailed to institutions nationally and internationally. In 2011, 444 of these phantoms were mailed out for credentialing. Once the phantom is sent, it takes the institution an average of 26 days to return it to the RPC. On average the dosimeters are analyzed within 17 days and the report is sent 21 days after receipt of the phantom data. In 2011 the percent of phantoms meeting the acceptance criteria increased by 12, 13 and 6 percentage points for the H&N, spine and lung phantoms, respectively. It fell by 5 percentage points for the pelvis phantom. CONCLUSIONS: The RPC's QA phantom program has been an effective and responsive QA tool for assessing the use of advanced technologies in NCI sponsored clinical trials. The RPC has been efficient in its mailing of phantoms, and analyzing and reporting results. Work supported by PHS grant CA10953 and CA081647 (NCI, DHHS).

SU-E-T-190: Design, Development, and Evaluation of a Modified, Anthropomorphic, Head, Quality Assurance Phantom for Use in Stereotactic Radiosurgery.

PURPOSE: To develop and evaluate a modified anthropomorphic head phantom for evaluation of stereotactic radiosurgery (SRS) dose planning and delivery. METHODS: A phantom was constructed from a water equivalent, plastic, head-shaped shell. The original phantom design, with only a spherical target, was modified to include a nonspherical target (pituitary) and an adjacent organ at risk (OAR) (optic chiasm), within 2 mm, simulating the anatomy encountered when treating acromegaly. The target and OAR spatial proximity provided a more realistic treatment planning and dose delivery exercise. A separate dosimetry insert contained two TLD for absolute dosimetry and radiochromic film, in the sagittal and coronal planes, for relative dosimetry. The prescription was 25Gy to 90% of the GTV with >= 10% of the OAR volume receiving >= 8Gy. The modified phantom was used to test the rigor of the treatment planning process, dosimeter reproducibility, and measured dose delivery agreement with calculated doses using a Gamma Knife, CyberKnife, and linear accelerator based radiosurgery systems. RESULTS: TLD results from multiple irradiations using either a CyberKnife or Gamma Knife agreed with the calculated target dose to within 4.7% with a maximum coefficient of variation of+/-2.0%. Gamma analysis in the coronal and sagittal film planes showed an average passing rate of 99.3% and 99.5% using +/-5%/3mm criteria, respectively. A treatment plan for linac delivery was developed meeting the prescription guidelines. Dosimeter reproducibility and dose delivery agreement for the linac is expected to have results similar to the results observed with the CyberKnife and Gamma Knife. CONCLUSIONS: A modified anatomically realistic SRS phantom was developed that provided a realistic clinical planning and delivery challenge that can be used to credential institutions wanting to participate in NCI funded clinical trials. Work supported by PHS CA010953, CA081647, CA21661 awarded by NCI. DHHS.

SU-E-T-199: The Radiological Physics Center's Credentialing Dosimetry Reviews: Their Effect on Clinical Trial Deviation Rates.

PURPOSE: To describe the Radiological Physics Center's (RPC) methods to evaluate an institution's ability to meet protocol guidelines in order to decrease NCI clinical trial deviation rate. METHODS: The RPC's dosimetry group utilizes 3 methods of assessing an institutions ability to meet the protocol treatment specifications. These methods involve a clinical and dosimetric review of a treatment plan submitted by the institution prior to the first patient being treated on a protocol. The three evaluation methods include use of site/treatment modality specific benchmark cases, evaluation of a previous patient treated in a similar fashion and a rapid review of the first patient placed on a trial prior to start of treatment. The dosimetric review consists of an independent dose recalculation using RPC measured data or RPC standard dosimetry data. The clinical review assesses the patient's DVHs and contouring of the tumor volume and critical structures, typically in conjunction with a radiation oncologist. RESULTS: Over the past 5 years the RPC has performed these QA reviews for several of the clinical trial groups for several different disease sites and treatment modalities. We have reviewed 1366 treatment plans as a part of credentialing (97 gynecological, 223 prostate, 1046 breast) where 222 failed the first submission requiring the RPC to interact with the submitting institution to resolve the discrepancy. The review of the benchmarks has resulted in 18% of the institutions requiring intervention by the RPC. Performing these reviews has identified potential clinical and dosimetric problem areas that could possibly have resulted in 17% of the charts reviewed to receive a minor or major deviation. CONCLUSIONS: The RPC's clinical and dosimetry review of submitted treatment plans before or early in the treatment process has helped to reduce the deviation rates on protocols. Work supported by PHS grant CA 10953 awarded by NCI, DHHS.

MO-D-BRB-02: The Radiological Physics Center's Quality Audit Program: Where Can We Improve?

PURPOSE: To analyze the findings of the Radiological Physics Center's (RPC) QA audits of institutions participating in NCI sponsored clinical trials. METHODS: The RPC has developed an extensive Quality Assurance (QA) program over the past 44 years. This program includes on-site dosimetry reviews where measurements on therapy machines are made, records are reviewed and personnel are interviewed. The program's remote audit tools include mailed dosimeters (OSLD/TLD) to verify output calibration, comparison of dosimetry data with RPC 'standard' data, evaluation of benchmark and patient calculations to verify the treatment planning algorithms, review of institution's QA procedures and records, and use of anthropomorphic phantoms to verify tumor dose delivery. The RPC endeavors to assist institutions in finding the origins of any detected discrepancies, and to resolve them. RESULTS: Ninety percent of institutions receiving dosimetry recommendations has remained level for the past 5 years. The most frequent recommendations were for not performing TG-40 QA tests, wedge factors, small field size output factors and off-axis factors. Since TG-51 was published, the number of beam calibrations audited during visits with ion chambers, that met the RPC's ±3% criterion, decreased initially but has risen to pre-TG-51 levels. The OSLD/TLD program shows that only ∼3% of the beams are outside our ±5% criteria, but these discrepancies are distributed over 12-20% of the institutions. The percent of institutions with ï,3 l beam outside the RPC's criteria is approximately the same whether OSLD/TLD or ion chambers were used. The first time passing rate for the anthropomorphic phantoms is increasing with time. The prostate phantom has the highest pass rate while the spine phantom has the lowest. CONCLUSIONS: Numerous dosimetry errors continue to be discovered by the RPC's QA program and the RPC continues to play an important role in helping institutions resolve these errors. This work was supported by PHS grants CA10953 and CA081647 awarded by NCI.

MO-D-BRB-05: An Analysis of 13,000 Patient-Specific IMRT QA Results from 13 Different Clinical Treatment Services.

PURPOSE: To review an institution's patient-specific IMRT quality assurance (QA) results, including absolute dose and gamma analysis measurements. METHODS: Patient-specific IMRT QA records were reviewed to obtain information on absolute dose difference (ion chamber measurement; ±3% agreement criteria) and percentage of pixels passing gamma (film measurement; 5%/3 mm agreement criteria) from 2005 to 2011. The 13,002 plans reviewed, were classified by treatment service: breast (n=67), central nervous system (n =1383), gastrointestinal (n=803), genitourinary (n=1831), gynecology (n=935), hematology (n=380), head and neck (n=3697), intensity-modulated stereotactic spine radiation therapy (n=341), melanoma (n=54), mesothelioma (n=52), pediatric (n=307), sarcoma (n=201), and thoracic (n=2951). All records were analyzed for trends according to measurement date and treatment service. Plans failing to meet QA criteria were further evaluated for subsequent measured data. RESULTS: Mean difference (± one standard deviation) between the measured and calculated doses was -0.29% ± 1.64% (with the calculated values being slightly higher). The mean percentage of pixels passing gamma was 97.7% (lower 95th percentile, 92.2%). The plan pass rates were 97.7% and 99.3% for absolute dose and gamma, respectively. We observed statistically significant differences (p < 0.05) in both absolute dose and gamma measurements as a function of treatment service (particularly for stereotactic spine and mesothelioma services) and measurement date (average agreement improved with time). However, despite improved agreement between measured and calculated doses, the percentage of treatment plans failing to meet the passing criteria has remained largely constant at ∼2.3%. CONCLUSIONS: Our retrospective review of 13,002 patient-specific IMRT QA plans demonstrated that plans continue to fail IMRT QA criteria at a consistent rate. This rate serves as a clinical reference for expected rates of QA plan pass and failure for a variety of treatment services.

MO-D-BRB-04: The Approval Process for the Use of Proton Therapy in NCI-Sponsored Clinical Trials.

PURPOSE: To describe the approval process for the use of proton therapy in NCI- sponsored clinical trials. METHODS: The RPC has developed a comprehensive system for the approval of proton therapy centers for participation in clinical trials. The approval process includes: 1) completion of the proton facility questionnaire, 2) participation in the RPC's annual TLD remote audit program, 3) electronic submission of treatment planning data to the Image-Guided Therapy Center (ITC), and 4) successful completion of an on-site dosimetry review visit, including the irradiation of two of the RPC's anthropomorphic proton phantoms (prostate and spine). The on-site audits allow the RPC to review the institution's treatment planning process, from simulation to treatment delivery, as well as their quality assurance practices. The RPC performs a complete set of measurements that tests the CT simulator's CT# vs. RSP conversion curve, treatment planning data, on-board imaging, and treatment delivery. These measurements detect gross errors that might lead to inaccurate proton dose delivery. The review of the institutions' QA procedures allows the RPC to encourage all proton centers to maintain a consistent level of periodic monitoring of their proton therapy delivery. Upon completion of the visit, a full report is written detailing the results from the visit, phantom irradiation, and recommendations for improving their treatment delivery and QA. RESULTS: To date, the RPC has approved seven proton therapy centers for the use of scattered or uniform scanning proton treatment delivery in clinical trials. Results of the phantom irradiations have identified an error in the HU vs RLSP curve. The site visits have identified several lapses in QA procedures, inappropriate HU vs RLSP values, and weaknesses in treatment planning. CONCLUSIONS: The RPC's proton therapy approval process has been developed and has identified areas of improvement for proton centers to use proton therapy in clinical trials. Work supported by grants CA10953, CA059267, and CA81647 (NCI, DHHS).

MO-D-BRB-03: Comparison of Proton Therapy Institutional Data Collected by the RPC.

PURPOSE: To detail and compare data collected during RPC onsite dosimetry review visits at proton therapy centers. METHODS: The RPC has established a complete review process for proton therapy institutions wishing to participate in NCI-funded clinical trials that includes an on-site dosimetry review visit performed by the RPC. During the visit, the RPC takes measurements that include CT# vs. relative stopping power (RSP) conversion, beam output, depth dose, lateral profiles, QA procedure reviews and anthropomorphic phantom irradiations. The RPC reviewed beam output, depth dose and lateral profiles for 5 specific anatomic treatment sites, reference, prostate, lung, brain, and spine as compared to the institution's measured or treatment planning-derived values. In addition, the RPC has compared results from each institution's proton prostate phantom irradiation. RESULTS: All of the institutions visited had RPC/Institution output ratios that ranged between 0.95 - 1.02 where the acceptance criterion was ±5%. For the CT# to RSP comparison, there was a larger variability. Only two institutions agreed within five percent of the recommended values, while the other five institutions had disagreements of up to 20 percent in the high density (high CT number) region of the conversion curve that may have a clinical impact on dose delivery. For the prostate phantom irradiation, 3 institutions failed to meet the RPC's ±7%/4mm acceptance criteria on the initial attempt, but in the end all 7 sites met the criteria. CONCLUSIONS: The proton beam output for 7 proton centers, as measured by the RPC, is comparable (±5%), however, there are large discrepancies in the CT# vs RSP conversion curves used from institution to institution. As a result of the RPC onsite dosimetry review visits, several institutions have modified their procedures and dosimetry parameters to improve proton therapy delivery for NCI funded clinical trials. Work supported by grants CA10953, CA059267, and CA81647 (NCI, DHHS).

Predictability of electron cone ratios with respect to linac make and model.

In the past, the Radiological Physics Center (RPC) has developed standard sets of photon depth-dose and wedge-factor data, specific to the make, model, and wedge design of the linear accelerator (linac). In this paper, the RPC extends the same concept to electron-cone ratios. Since 1987, the RPC has measured and documented cone-ratio (CR) values during on-site dosimetry review visits to institutions participating in National Cancer Institute cooperative clinical trials. Data have been collected for approximately 500 electron beams from a wide spectrum of linac models. The analysis presented in this paper indicates that CR values are predictable to 2% to 3% (two standard deviations) for a given make and model of linac with a few exceptions. The analysis also revealed some other interesting systematics. For some models, such as the Varian Clinac 2500 and the Elekta/Philips SL18, SL20, and SL25, CR values were nearly identical for cone sizes 15 cm x 15 cm (or 14 cm x 14 cm) and 20 cm x 20 cm across the range of available energies. Certain models of the same make of linac, such as the Mevatron MD, KD, and 6700 series models or the Clinac 2100 and 2300 models, exhibited indistinguishable CRs. Irrespective of linac model, two consistent general trends were observed: namely, an increase in CR value with incident beam energy for cone sizes smaller than 10 cm x 10 cm and a decrease with energy for cone sizes larger than 10 cm x 10 cm. These data are valuable to the RPC as a quality assurance remote-monitoring tool to identify potential dosimetry errors. The physics community will also find the data useful in several ways: as a redundant check for clinical values in use, to validate the values measured during commissioning of new machines or to ensure consistency of values measured during annual quality assurance procedures.