P Alvarez

Algorithms used in heterogeneous dose calculations show systematic differences as measured with the radiological physics center's anthropomorphic thorax phantom used for RTOG credentialing

PURPOSE To determine the impact of treatment planning algorithm on the accuracy of heterogeneous dose calculations in the Radiological Physics Center (RPC) thorax phantom. METHODS AND MATERIALS We retrospectively analyzed the results of 304 irradiations of the RPC thorax phantom at 221 different institutions as part of credentialing for Radiation Therapy Oncology Group clinical trials; the irradiations were all done using 6-MV beams. Treatment plans included those for intensity-modulated radiation therapy (IMRT) as well as 3-dimensional conformal therapy (3D-CRT). Heterogeneous plans were developed using Monte Carlo (MC), convolution/superposition (CS), and the anisotropic analytic algorithm (AAA), as well as pencil beam (PB) algorithms. For each plan and delivery, the absolute dose measured in the center of a lung target was compared to the calculated dose, as was the planar dose in 3 orthogonal planes. The difference between measured and calculated dose was examined as a function of planning algorithm as well as use of IMRT. RESULTS PB algorithms overestimated the dose delivered to the center of the target by 4.9% on average. Surprisingly, CS algorithms and AAA also showed a systematic overestimation of the dose to the center of the target, by 3.7% on average. In contrast, the MC algorithm dose calculations agreed with measurement within 0.6% on average. There was no difference observed between IMRT and 3D CRT calculation accuracy. CONCLUSION Unexpectedly, advanced treatment planning systems (those using CS and AAA algorithms) overestimated the dose that was delivered to the lung target. This issue requires attention in terms of heterogeneity calculations and potentially in terms of clinical practice.

Comparing the accuracy of four-dimensional photon dose calculations with three-dimensional calculations using moving and deforming phantoms.

PURPOSE Four-dimensional (4D) dose calculation algorithms, which explicitly incorporate respiratory motion in the calculation of doses, have the potential to improve the accuracy of dose calculations in thoracic treatment planning; however, they generally require greater computing power and resources than currently used for three-dimensional (3D) dose calculations. The purpose of this work was to quantify the increase in accuracy of 4D dose calculations versus 3D dose calculations. METHODS The accuracy of each dose calculation algorithm was assessed using measurements made with two phantoms. Specifically, the authors used a rigid moving anthropomorphic thoracic phantom and an anthropomorphic thoracic phantom with a deformable lung insert. To incorporate a clinically relevant range of scenarios, they programed the phantoms to move and deform with two motion patterns: A sinusoidal motion pattern and an irregular motion pattern that was extracted from an actual patients breathing profile. For each combination of phantom and motion pattern, three plans were created: A single-beam plan, a multiple-beam plan, and an intensity-modulated radiation therapy plan. Doses were calculated using 4D dose calculation methods as well as conventional 3D dose calculation methods. The rigid moving and deforming phantoms were irradiated according to the three treatment plans and doses were measured using thermoluminescent dosimeters (TLDs) and radiochromic film. The accuracy of each dose calculation algorithm was assessed using measured-to-calculated TLD doses and a gamma analysis. RESULTS No significant differences were observed between the measured-to-calculated TLD ratios among 4D and 3D dose calculations. The gamma results revealed that 4D dose calculations had significantly greater percentage of pixels passing the 5%/3 mm criteria than 3D dose calculations. CONCLUSIONS These results indicate no significant differences in the accuracy between the 4D and the 3D dose calculation methods inside the gross tumor volume. On the other hand, the film results demonstrated that the 4D dose calculations provided greater accuracy than 3D dose calculations in heterogeneous dose regions. The increase in accuracy of the 4D dose calculations was evident throughout the planning target volume.

Verification of four-dimensional photon dose calculations

Recent work in the area of thoracic treatment planning has been focused on trying to explicitly incorporate patient-specific organ motion in the calculation of dose. Four-dimensional (4D) dose calculation algorithms have been developed and incorporated in a research version of a commercial treatment planning system (Pinnacle3, Philips Medical Systems, Milpitas, CA). Before these 4D dose calculations can be used clinically, it is necessary to verify their accuracy with measurements. The primary purpose of this study therefore was to evaluate and validate the accuracy of a 4D dose calculation algorithm with phantom measurements. A secondary objective was to determine whether the performance of the 4D dose calculation algorithm varied between different motion patterns and treatment plans. Measurements were made using two phantoms: A rigid moving phantom and a deformable phantom. The rigid moving phantom consisted of an anthropomorphic thoracic phantom that rested on a programmable motion platform. The deformable phantom used the same anthropomorphic thoracic phantom with a deformable insert for one of the lungs. Two motion patterns were investigated for each phantom: A sinusoidal motion pattern and an irregular motion pattern extracted from a patient breathing profile. A single-beam plan, a multiple-beam plan, and an intensity-modulated radiation therapy plan were created. Doses were calculated in the treatment planning system using the 4D dose calculation algorithm. Then each plan was delivered to the phantoms and delivered doses were measured using thermoluminescent dosimeters (TLDs) and film. The measured doses were compared to the 4D-calculated doses using a measured-to-calculated TLD ratio and a gamma analysis. A relevant passing criteria (3% for the TLD and 5% /3 mm for the gamma metric) was applied to determine if the 4D dose calculations were accurate to within clinical standards. All the TLD measurements in both phantoms satisfied the passing criteria. Furthermore, 42 of the 48 evaluated films fulfilled the passing criteria. All films that did not pass the criteria were from the rigid phantom moving with irregular motion. The author concluded that if patient breathing is reproducible, the 4D dose calculations are accurate to within clinically acceptable standards. Furthermore, they found no statistically significant differences in the performance of the 4D dose calculation algorithm between treatment plans.

Development and implementation of a remote audit tool for high dose rate (HDR) Ir-192 brachytherapy using optically stimulated luminescence dosimetry

PURPOSE The aim of this work was to create a mailable phantom with measurement accuracy suitable for Radiological Physics Center (RPC) audits of high dose-rate (HDR) brachytherapy sources at institutions participating in National Cancer Institute-funded cooperative clinical trials. Optically stimulated luminescence dosimeters (OSLDs) were chosen as the dosimeter to be used with the phantom. METHODS The authors designed and built an 8 × 8 × 10 cm(3) prototype phantom that had two slots capable of holding Al2O3:C OSLDs (nanoDots; Landauer, Glenwood, IL) and a single channel capable of accepting all (192)Ir HDR brachytherapy sources in current clinical use in the United States. The authors irradiated the phantom with Nucletron and Varian (192)Ir HDR sources in order to determine correction factors for linearity with dose and the combined effects of irradiation energy and phantom characteristics. The phantom was then sent to eight institutions which volunteered to perform trial remote audits. RESULTS The linearity correction factor was kL = (-9.43 × 10(-5) × dose) + 1.009, where dose is in cGy, which differed from that determined by the RPC for the same batch of dosimeters using (60)Co irradiation. Separate block correction factors were determined for current versions of both Nucletron and Varian (192)Ir HDR sources and these vendor-specific correction factors differed by almost 2.6%. For the Nucletron source, the correction factor was 1.026 [95% confidence interval (CI) = 1.023-1.028], and for the Varian source, it was 1.000 (95% CI = 0.995-1.005). Variations in lateral source positioning up to 0.8 mm and distal∕proximal source positioning up to 10 mm had minimal effect on dose measurement accuracy. The overall dose measurement uncertainty of the system was estimated to be 2.4% and 2.5% for the Nucletron and Varian sources, respectively (95% CI). This uncertainty was sufficient to establish a ± 5% acceptance criterion for source strength audits under a formal RPC audit program. Trial audits of four Nucletron sources and four Varian sources revealed an average RPC-to-institution dose ratio of 1.000 (standard deviation = 0.011). CONCLUSIONS The authors have created an OSLD-based (192)Ir HDR brachytherapy source remote audit tool which offers sufficient dose measurement accuracy to allow the RPC to establish a remote audit program with a ± 5% acceptance criterion. The feasibility of the system has been demonstrated with eight trial audits to date.

Characterization of the nanoDot OSLD dosimeter in CT

PURPOSE The extensive use of computed tomography (CT) in diagnostic procedures is accompanied by a growing need for more accurate and patient-specific dosimetry techniques. Optically stimulated luminescent dosimeters (OSLDs) offer a potential solution for patient-specific CT point-based surface dosimetry by measuring air kerma. The purpose of this work was to characterize the OSLD nanoDot for CT dosimetry, quantifying necessary correction factors, and evaluating the uncertainty of these factors. METHODS A characterization of the Landauer OSL nanoDot (Landauer, Inc., Greenwood, IL) was conducted using both measurements and theoretical approaches in a CT environment. The effects of signal depletion, signal fading, dose linearity, and angular dependence were characterized through direct measurement for CT energies (80-140 kV) and delivered doses ranging from ∼5 to >1000 mGy. Energy dependence as a function of scan parameters was evaluated using two independent approaches: direct measurement and a theoretical approach based on Burlin cavity theory and Monte Carlo simulated spectra. This beam-quality dependence was evaluated for a range of CT scanning parameters. RESULTS Correction factors for the dosimeter response in terms of signal fading, dose linearity, and angular dependence were found to be small for most measurement conditions (<3%). The relative uncertainty was determined for each factor and reported at the two-sigma level. Differences in irradiation geometry (rotational versus static) resulted in a difference in dosimeter signal of 3% on average. Beam quality varied with scan parameters and necessitated the largest correction factor, ranging from 0.80 to 1.15 relative to a calibration performed in air using a 120 kV beam. Good agreement was found between the theoretical and measurement approaches. CONCLUSIONS Correction factors for the measurement of air kerma were generally small for CT dosimetry, although angular effects, and particularly effects due to changes in beam quality, could be more substantial. In particular, it would likely be necessary to account for variations in CT scan parameters and measurement location when performing CT dosimetry using OSLD.

Technical Report: Reference photon dosimetry data for Varian accelerators based on IROC‐Houston site visit data

PURPOSE Accurate data regarding linear accelerator (Linac) radiation characteristics are important for treatment planning system modeling as well as regular quality assurance of the machine. The Imaging and Radiation Oncology Core-Houston (IROC-H) has measured the dosimetric characteristics of numerous machines through their on-site dosimetry review protocols. Photon data are presented and can be used as a secondary check of acquired values, as a means to verify commissioning a new machine, or in preparation for an IROC-H site visit. METHODS Photon data from IROC-H on-site reviews from 2000 to 2014 were compiled and analyzed. Specifically, data from approximately 500 Varian machines were analyzed. Each dataset consisted of point measurements of several dosimetric parameters at various locations in a water phantom to assess the percentage depth dose, jaw output factors, multileaf collimator small field output factors, off-axis factors, and wedge factors. The data were analyzed by energy and parameter, with similarly performing machine models being assimilated into classes. Common statistical metrics are presented for each machine class. Measurement data were compared against other reference data where applicable. RESULTS Distributions of the parameter data were shown to be robust and derive from a students t distribution. Based on statistical and clinical criteria, all machine models were able to be classified into two or three classes for each energy, except for 6 MV for which there were eight classes. Quantitative analysis of the measurements for 6, 10, 15, and 18 MV photon beams is presented for each parameter; supplementary material has also been made available which contains further statistical information. CONCLUSIONS IROC-H has collected numerous data on Varian Linacs and the results of photon measurements from the past 15 years are presented. The data can be used as a comparison check of a physicists acquired values. Acquired values that are well outside the expected distribution should be verified by the physicist to identify whether the measurements are valid. Comparison of values to this reference data provides a redundant check to help prevent gross dosimetric treatment errors.

SU‐FF‐T‐306: Optically Stimulated Light Dosimetry: Commissioning of An Optically Stimulated Luminescence (OSL) System for Remote Dosimetry Audits, the Radiological Physics Center Experience

For the last 30 years the Radiological Physics Center (RPC) has used TLDdosimetry to conduct remote audits of beam output of photon and electron beams and energy checks for electron beams. Acrylic blocks containing capsules of TLDpowder were sent for each beam. Powder was used as a single‐use disposable dosimeter. The RPC has previously described the use of the remote audits to identify units with beam measurements exceeding 5% and 5 mm. The system uncertainty is 1.5% (1 standard deviation) indicating high confidence in the 5% threshold for acceptability. OSLdosimeters have been available since 1992. Several publications have described their use for dose measurements at therapeutic levels. After a promising initial evaluation, the RPC decided to purchase and commission OSLdosimeters and instrumentation with the goal of implementing an OSL‐based system into the remote audit program. As part of the commissioning procedure, the depletion characteristics of OSLdosimeters were studied as they relate to the number of readings, the reader and the changes after annealing. Dark current and standard signal characteristics of the reader were evaluated and compared against factory specifications. The mechanical properties of the reader positioning characteristics were also tested. Because OSLdosimeters have unique sensitivities, response factor was determined and analyzed in terms of its variability with each reader, annealing cycle and cumulative dose.Dose dependence was also tested against annealing and accumulated dose. Energy dependence factors and fading characteristics were determined. A group of dosimeters from a single production was purchased and commissioned for use. Dose response, energy and fading characteristics for the batch were defined together with the design of a reading session and a quality assurance program. This work was supported by PHS CA010953 awarded by NCI, DHHS

SU‐E‐T‐126: Analysis of Uncertainties for the RPC Remote Dosimetry Using Optically Stimulated Light Dosimetry (OSDL)

Purpose: Determine the uncertainty of the Radiological Physics Centers (RPC) remote audit dose measurement of photon and electron beam outputs using Optically Stimulated LuminescenceDosimeters (OSLD). Methods: In June 2010 the RPC implemented a switch from TLD to an OSLD mailed dosimetry audit using Landauers InLight nanoDot™ OSLdosimeters and microStar reader System™. The steps and factors for the calculation of dose with OSLD were designed to follow those already in use with TLD including the use of the same acrylic mini‐phantoms and irradiation instructions. The OSLD sensitivity was referenced to Co‐60, and corrections for linearity, individual dosimeter response, fading, use of mini‐phantoms and energy were determined for the OSLD. An analysis of the uncertainty of each of the correction factors was determined to give an overall uncertainty of the OSLD dose measurement under Co‐60 reference conditions and megavoltage beam audits. Results: The expectation was that with the physical processes for OSLD, being so closely resembling those of TLD, the uncertainties of OSLD would be comparable to those of TLD. A combined uncertainty of 0.8% for irradiations with Co‐60 under controlled reference conditions and 1.8% for irradiations with high‐energy X‐rays was determined as compared to 1% and 2% for the previously used TLD, respectively. The OSLD energy block correction factor (KE) is the primary source of the overall OSLD dose measurement uncertainty. To date, the OSLD audit results after a year are essentially identical to the previous 5 years of TLD results both showing a mean TLD/Inst. Ratio of 1.00 ± 0.018.Conclusion: The uncertainty of the RPCs new OSLD remote dosimetry audit programs dose measurement is equal to if not less than that for the historical TLD program. Work supported by PHS CA010953, awarded by NCI, DHHS.

SU-E-T-86: Evaluation of the OSLD System for Remote Dosimetry Audits Implemented by the RPC

Purpose: Analysis of the performance of the optically stimulated luminescencedosimetry (OSLD) system implemented by the Radiological Physics Center (RPC) for external audits compare to the termoluminescent dosimetry(TLD) system.Methods: The RPC translated the TLD system for external audits to the OSLD system on June 2010. This last system is based on nanoDo dosimeters. The logistic of the system was designed and is managed by the RPCs data base (RADS). The calculation of dose, the storage of data and evaluation of results is also done by the same data base. The verification of output is based on irradiation of dosimeters in acrylic phantoms. The geometry for irradiation is the reference geometry for output calibration. The dose level change from 300cGy for TLD to 100cGy for OSLD. Both system are designed for X‐ray beam from 4 to 23 MV and electrons beams from 6 to 23 MeV. Results: The average ratio between RPC dose and institution dose at the point of dose verification is 1.000 ± 1.9% for TLD system for the last 5 years. While this average is 0.999 ± 1.7% for OSLD system since it was implemented. This average was analyzed based on the whole population of results, The average was also analyzed per type of beam and per energy without showing differences. The OSLD system allows the RPC to evaluate more beams during reading sessions. Evaluation of accumulated dose and history of dosimeter are valuable Conclusions: The average ratio between RPC dose and institution dose at the point of dose verification did not change from TLD system to OSLD system. The identification of dosimeters is important to keep track of irradiations especially when reuse of the dosimeter is one of the advantages of this system. This work was supported by PHS CA010953 awarded by NCI, DHHS

WE‐D‐BRB‐08: Validation of the Commissioning of an Optically Stimulated Luminescence (OSL) System for Remote Dosimetry Audits

Purpose: Validate an Optically Stimulated Luminescence(OSL)dosimetry system based on the microStar® reader and a batch of nanoDotdosimeters from Landauer for use by the Radiological Physics Center (RPC). Materials and Methods: The factors involved in the calculation of dose from OSL, the calculation algorithm and the results of the commissioning were previously presented (AAPM 2009). The steps for determining dose from OSL were designed to resemble those used by the RPC for the TLD remote dosimetry audit program. As with TLD, the OSLdosimeters are irradiated in acrylic miniphantoms under standardized geometric conditions. The steps for a reading session, criteria for the number of dosimeters, readings per dosimeter, interspersing of “standard” and “control” dosimeters, and quality control steps have been determined. A method was developed and tested for the determination of the individual OSL correction factors used for the large number of OSLdosimeters (4,000) employed by the RPC. Validation of the process and assessment of the accuracy of the OSL system were performed through comparisons against TLD results. TLD and OSL were irradiated sequentially multiple times with photon and electron beams of varying energies at MD Anderson and at several other institutions and the results were compared. Results: Under reference conditions (60Co beam), the ratio of the OSLdose, read as described above, to ion chamberdose had a mean value of 0.997 ± 0.5% (1 SD). TLD versus OSL comparison based on irradiations at MD Anderson were within ±2%. The results from eleven institution agreed to within 1.1% for photon energies (60‐23MV) and electrons (5–23MeV). Conclusion: The OSL system has been commissioned and validated as an RPC remote audit tool for verification of beam output of photons and electron beams and electron energy. Work supported by PHS CA010953 (NCI, DHHS)