A. Faught

Development of a modified head and neck quality assurance phantom for use in stereotactic radiosurgery trials

An anthropomorphic head phantom, constructed from a water‐equivalent plastic shell 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 25 Gy to 90% of the GTV, with ≤10% of the OAR volume receiving ≥8Gy for the phantom trial. The modified phantom was used to test the rigor of the treatment planning process and phantom reproducibility using a Gamma Knife, CyberKnife, and linear accelerator (linac)‐based radiosurgery system. Delivery reproducibility was tested by repeating each irradiation three times. TLD results from three irradiations on a CyberKnife and Gamma Knife agreed with the calculated target dose to within ± 4% with a maximum coefficient of variation of ±2.1%. Gamma analysis in the coronal and sagittal film planes showed an average passing rate of 99.4% and 99.5% using ±5%/3mm criteria, respectively. Results from the linac irradiation were within ±6.2% for TLD with a coefficient of variation of ±0.1%. Distance to agreement was calculated to be 1.2 mm and 1.3 mm along the inferior and superior edges of the target in the sagittal film plane, and 1.2 mm for both superior and inferior edges in the coronal film plane. 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. PACS number: 87.55 ‐v

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‐F‐BRE‐06: Evaluation of Patient CT Dose Reconstruction From 3D Diode Array Measurements Using Anthropomorphic Phantoms

PURPOSE To compare 3D reconstructed dose of IMRT plans from 3D diode array measurements with measurements in anthropomorphic phantoms. METHODS Six IMRT plans were created for the IROC Houston (RPC) head and neck (H&N) and lung phantoms following IROC Houston planning protocols. The plans included flattened and unflattened beam energies ranging from 6 MV to 15 MV and both static and dynamic MLC tecH&Niques. Each plan was delivered three times to the respective anthropomorphic phantom, each of which contained thermoluminescent dosimeters (TLDs) and radiochromic films (RCFs). The plans were also delivered to a Delta4 diode array (Scandidos, Uppsala, Sweden). Irradiations were done using a TrueBeam STx (Varian Medical Systems, Palo Alto, CA). The dose in the patient was calculated by the Delta4 software, which used the diode measurements to estimate incident energy fluence and a kernel-based pencil beam algorithm to calculate dose. The 3D dose results were compared with the TLD and RCF measurements. RESULTS In the lung, the average difference between TLDs and Delta4 calculations was 5% (range 2%-7%). For the H&N, the average differences were 2.4% (range 0%-4.5%) and 1.1% (range 0%-2%) for the high- and low-dose targets, respectively, and 12% (range 10%-13%) for the organ-at-risk simulating the spinal cord. For the RCF and criteria of 7%/4mm, 5%/3mm, and 3%/3mm, the average gamma-index pass rates were 95.4%, 85.7%, and 76.1%, respectively for the H&N and 76.2%, 57.8%, and 49.5% for the lung. The pass-rate in the lung decreased with increasing beam energy, as expected for a pencil beam algorithm. CONCLUSION The H&N phantom dose reconstruction met the IROC Houston acceptance criteria for clinical trials; however, the lung phantom dose did not, most likely due to the inaccuracy of the pencil beam algorithm in the presence of low-density inhomogeneities. Work supported by PHS grant CA10953 and CA81647 (NCI, DHHS).

SU-E-T-87: Comparison Study of Dose Reconstruction From Cylindrical Diode Array Measurements, with TLD Measurements and Treatment Planning System Calculations in Anthropomorphic Head and Neck and Lung Phantoms

PURPOSE To assess dose calculated by the 3DVH software (Sun Nuclear Systems, Melbourne, FL) against TLD measurements and treatment planning system calculations in anthropomorphic phantoms. METHODS The IROC Houston (RPC) head and neck (HN) and lung phantoms were scanned and plans were generated using Eclipse (Varian Medical Systems, Milpitas, CA) following IROC Houston procedures. For the H&N phantom, 6 MV VMAT and 9-field dynamic MLC (DMLC) plans were created. For the lung phantom 6 MV VMAT and 15 MV 9-field dynamic MLC (DMLC) plans were created. The plans were delivered to the phantoms and to an ArcCHECK (Sun Nuclear Systems, Melbourne, FL). The head and neck phantom contained 8 TLDs located at PTV1 (4), PTV2 (2), and OAR Cord (2). The lung phantom contained 4 TLDs, 2 in the PTV, 1 in the cord, and 1 in the heart. Daily outputs were recorded before each measurement for correction. 3DVH dose reconstruction software was used to project the calculated dose to patient anatomy. RESULTS For the HN phantom, the maximum difference between 3DVH and TLDs was -3.4% and between 3DVH and Eclipse was 1.2%. For the lung plan the maximum difference between 3DVH and TLDs was 4.3%, except for the spinal cord for which 3DVH overestimated the TLD dose by 12%. The maximum difference between 3DVH and Eclipse was 0.3%. 3DVH agreed well with Eclipse because the dose reconstruction algorithm uses the diode measurements to perturb the dose calculated by the treatment planning system; therefore, if there is a problem in the modeling or heterogeneity correction, it will be carried through to 3DVH. CONCLUSION 3DVH agreed well with Eclipse and TLD measurements. Comparison of 3DVH with film measurements is ongoing. Work supported by PHS grant CA10953 and CA81647 (NCI, DHHS).

SU-E-T-167: Evaluation of Mobius Dose Calculation Engine Using Out of the Box Preconfigured Beam Data

PURPOSE Determine the dose calculation accuracy of a preconfigured Mobius server for use in secondary checks of a treatment planning system. METHODS 10 plans were created for irradiation on two of the IROC (formerly RPC) accreditation phantoms: 4 for the head and neck phantom and 6 for the lung phantom. The plans each were created using one of four different photon energies (6FFF, 10 FFF, 6X, and 15X) and were varied in treatment type including VMAT, step and shoot IMRT, dynamic MLC IMRT (DMLC), and conformal RT (CRT). The TLDs in the phantoms were contoured, and each plan was sent for calculation to Mobius software (Mobius Medical Systems, Houston, TX) with a default configuration. Each plan was then irradiated on the planned phantom 3 times to create an average reading across 56 TLDs. These readings were then compared against the corresponding Mobius calculation at each TLD location. RESULTS The mean difference (MD) normalized to the plan prescription dose between each TLD and Mobius calculation for all measurements was 0.5 ± 3.3%, with a maximum difference of 8.4%. The MD was 0.6 ± 3.8%, - 2.0 ± 1.9%, 1.7 ± 3.7%, and 1.9 ± 1.2% across the 6FFF, 10FFF, 6X and 15X energies respectively. The MD was -1.2 ± 2.3% for lung plans and 1.8 ± 3.5% for head/neck plans. Across treatment types, the MD ranged from - 1.8 ± 1.7% for CRT to 4.3 ± 2.4 % for DMLC. CONCLUSION Out of the box and preconfigured, Mobius provides accurate dose calculations with respect to beam energy, treatment type, and treatment site.

SU‐E‐T‐159: Development of An Independent, Monte Carlo, Dose Calculation, Quality Assurance Tool for Clinical Trials

PURPOSE To commission a multiple-source Monte Carlo model of Elekta linear accelerator beams of nominal energies 6MV and 10MV. METHODS A three source, Monte Carlo model of Elekta 6 and 10MV therapeutic x-ray beams was developed in a two-step process. Energy spectra of each of three sources, a primary source corresponding to photons created in the target, an extra-focal source corresponding to photons originating from scattered events in the linac head, and an electron contamination source, were determined. The two photon sources were determined by an optimization process that fit the relative fluence of 0.25 MeV energy bins to the product of Fatigue-Life and Fermi functions to match calculated percent depth dose (PDD) data with that measured in water for a 10×10cm2 field. Off-axis effects were modeled by fitting the off-axis fluence to a piece-wise linear function through optimization of relative fluence to match calculated dose profiles with measured dose profiles for a 40×40cm2 field. A 3rd degree polynomial was used to describe the off-axis half-value layer as a function of off-axis angle. The model was then commissioned by comparing calculated PDDs and dose profiles for field sizes ranging from 3×3cm2 to 30×30cm2 to those obtained from measurements. RESULTS Agreement between calculated and measured data was evaluated using 2%/2mm global gamma criterion for field sizes of 3×3, 5×5, 10×10, 15×15, 20×20, and 30×30cm2 . Along the central axis of the beam 99.5% and 99.6% of all data passed the criterion for 6 and 10MV models, respectively. Dose profiles at depths of dmax, 5, 10, 20, and 25cm agreed with measured data for 95.4% and 99.2% of data tested for 6 and 10MV models, respectively. CONCLUSION A Monte Carlo multiple-source model for Elekta 6 and 10MV therapeutic x-ray beams has been developed as a quality assurance tool for clinical trials. This work was supported by Public Health Service grants CA010953, CA081647, and CA21661 awarded by the National Cancer Institute, United States Department of Health and Human Services.

SU‐E‐T‐378: IMRT Severity Scoring for TG‐100: Do You Really Know?

PURPOSE Failure modes and effects analysis (FMEA) as defined in TG-100 has become a popular concept throughout the radiotherapy community. This risk mitigation technique involves detailed process mapping, analysis, and ranking of potential errors by means of a subjective, ordinal scoring system. This study aims to reduce the subjectivity of FMEA severity scoring for IMRT delivery by providing quantitative values. METHODS First we created an IMRT delivery process map for physics-applicable processes and identified 11 physical failure modes (FMs). To determine the magnitude of dose delivery errors for several of the physical FMs (i.e., the severity of the FM), FMs were induced and dosimetry measurements were performed on a Varian Clinac accelerator going out of clinical service. Treatment planning studies to simulate remaining FMs are to follow. The quantitative severity scores will be compared to recommended FMEA subjective scores. RESULTS We identified the following physical FMs to investigate: photon beam energy, symmetry, MLC position, MLC transmission and leakage, MLC rounded-end and leaf offset, MLC tongue-and-groove, CT look-up table, gantry angle, collimator angle, couch angle and displacement, and MU linearity. Within the PTV of a H&N IMRT phantom, the maximum dose delivery error of 3% absolute dose was seen for 2 mm systematic MLC offsets, 4% for 1.1% increased energy, and 4% for 3.5% symmetry error. Measurements will be compared to computational study results and then used for quantitative severity scoring determination. CONCLUSION Current FMEA practice for radiotherapy requires quantitative data in order to make accurate assessments associated with clinical QA programs. This study has shown examples of error magnitudes induced by IMRT physical FMs that can be used to quantitate and rank FMEA severity scoring. Work supported by grants CA10953 and CA81647 (NCI, DHHS).

SU‐E‐T‐731: Implementation of a One Dimensional Analytical Model for Calculating Three Dimensional Linear Energy Transfer in Proton Radiotherapy

Purpose: To develop and implement a three dimensional, analytical model for linear energy transfer (LET) calculations of a clinical proton beam based on a one dimensional model presented in the literature for use in calculating variable relative biological effectiveness (RBE) values within a patient treatment plan.Methods: Implementation of the one dimensional analytical model was performed in MATLAB and confirmed with Monte Carlo (MC) results for a proton beam matching the beam specifications of a clinical proton beam. Application to three dimensional geometries was achieved by referencing values to the generated one dimensional array to corresponding distances from a 4cm × 4cm planar source, for a water phantom. The three dimensional model was then applied to a prostate plan and compared to MC results.Results: One dimensional implementation of the model to a 70MeV and 160MeV proton beam showed good agreement with MC results along the proximal portion of the beam path and agreement within 8% and 13% along the distal 5mm of the beam path for each respective beam. Implementation in a three dimensional water phantom achieved an agreement within 0.5 keV/micron for 60.3% of all voxels and 2.5keV/micron for 98.3% of all voxels for a 160MeV plane source beam. Of the voxels that differed by more than 2.5keV/micron, 95.5% differed within the range of experimental uncertainty in the MC results. Conclusions: An analytical model for LET calculations in proton therapy may be used to quickly and efficiently estimate LET values within a three dimensional volume. This allows for the calculation of variable RBE values in a patient volume and the possibility of optimizing plans based on the biological effect of protonradiotherapy.