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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).
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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.
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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.
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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).
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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).
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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.
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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 â¼3% 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.
