Quantification of residual dose estimation error on log file-based patient dose calculation

Purpose: The log file-based patient dose estimation includes a residual dose estimation error caused by leaf miscalibration, which cannot be reflected on the estimated dose. The purpose of this study is to determine this residual dose estimation error. Methods and materials: Modified log files for seven head-and-neck and prostate volumetric modulated arc therapy (VMAT) plans simulating leaf miscalibration were generated by shifting both leaf banks (systematic leaf gap errors: +/- 2.0, +/- 1.0, and +/- 0.5 mm in opposite directions and systematic leaf shifts: +/- 1.0 mm in the same direction) using MATLAB-based (MathWorks, Natick, MA) in-house software. The generated modified and non-modified log files were imported back into the treatment planning system and recalculated. Subsequently, the generalized equivalent uniform dose (gEUD) was quantified for the definition of the planning target volume (PTV) and organs at risks. Results: For MLC leaves calibrated within +/- 0.5 mm, the quantified residual dose estimation errors that obtained from the slope of the linear regression of gEUD changes between non- and modified log file doses per leaf gap are in head-and-neck plans 1.32 +/- 0.27% and 0.82 +/- 0.17 Gy for PTV and spinal cord, respectively, and in prostate plans 1.22 +/- 0.36%, 0.95 +/- 0.14 Gy, and 0.45 +/- 0.08 Gy for PTV, rectum, and bladder, respectively. Conclusions: In this work, we determine the residual dose estimation errors for VMAT delivery using the log file-based patient dose calculation according to the MLC calibration accuracy. (C) 2016 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.