(Saturday, 4/7/2018)
Room: Foyer
Purpose: To develop a novel approach for determining optimum values for MLC transmission, additional interleaf leakage, and tongue and groove width that will improve modulated beam modeling in Pinnacle TPS v9.10 for 6MV, 6MV-FFF, 10MV, and 10MV-FFF photon beams.
Methods: Open and MLC closed fields were measured using EDR2 film for Varian TrueBeam Linac with Millennium 120 MLC. Calibration curves were used to convert the films from intensity to absolute dose. The ratio of MLC matrix to open matrix was calculated. The average of every column of data was determined along the direction of the MLCs. This resulted in a single average dose profile of interleaf leakage and MLC transmission. The peaks (interleaf leakage) and valleys (MLC transmission) were averaged together to get the final average interleaf leakage and MLC transmission. Finally, the tongue and groove width was estimated dosimetrically by the FWHM for each period. A final correction for additional interleaf leakage was applied to the model.
Results: We compared gamma pass rates from measurements of 14 VMAT arcs and 75 IMRT fields covering a range of anatomical sites. The new model was compared to the current model by analyzing the mean pass rate in percentage points. The most dramatic improvement in pass rates was observed in 10MV VMAT arcs with an improvement of 23.5 percentage points using gamma criteria of 2%/2mm without Van Dyk and the lowest improvement of 0.5 percentage points for 6MV-FFF VMAT using gamma criteria of 3%/3mm with Van Dyk. IMRT fields showed little improvement with minimal degradation of pass rates.
Conclusion: This study has demonstrated a novel and thorough method for measuring and implementing improved values required by Pinnacle TPS. The results showed marked improvements in gamma pass rates for highly modulated fields with little change in pass rates for lightly modulated fields.
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