Room: ePoster Forums
Purpose: To determine the calculated dosimetric effects caused by the variations of beam modeling parameters in Eclipse on IROC head and neck (H&N) phantom results to help identify specific causes of phantom failures.
Methods: A 6 MV beam for a Varian Clinac 2100iX with Millennium 120 MLC was commissioned in Eclipse (AAA 13.5.35) to match reference data collected from the IROC Houston site visit program and median values of beam modeling survey responses from over 600 institutions. The model was verified via 23 output measurements, including PDD values and 12 square test fields (2x2 cm to 30x30 cm) using both primary collimator- and MLC-defined apertures. Ten clinically acceptable H&N phantom plans (4 IMRT, 6 VMAT) were then recalculated with modifications of the dosimetric leaf gap (DLG), MLC transmission factor, and effective target spot size. Parameter values chosen represent the 2.5th, 25th, 50th, 75th, and 97.5th percentiles of IROC survey responses in order to encompass the extent of modeling variance in the radiotherapy community. Plan objectives examined for variations were the minimum, maximum, and mean doses to TLD structures, target volumes, and organ at risk.
Results: The 6 MV reference beam model represented IROC data very well, having an average deviation of 0.28%. For the parameters tested, dose calculations were most sensitive to changes in the DLG, which produced changes from -6% to +4% of the calculated dose to identified structures based on the most extreme values tested (0.048 cm and 0.235 cm). MLC transmission and effective target spot size contributed less, yielding Â±1% dose difference.
Conclusion: Careful consideration is necessary when commissioning clinical beam models, especially regarding the measurement of the DLG. The use of parameter values deemed clinically acceptable, but are far from typical, are shown to potentially contribute to failing phantom results and significant deviations in dose calculations.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by Public Health Service Grants CA180803 and CA214526 awarded by the National Cancer Institute. Mallory Glenn is a recipient of the Rosalie B Hite Fellowship and American Legion Auxiliary Fellowship, awarded by the MD Anderson Cancer Center UTHealth Graduate School of Biomedical Sciences.