A study into the relationship between the measured penumbra and effective source size in the modeling of the Pinnacle RTPS

Abstract

The effect of detector size in the broadening of the penumbra on the model in the Pinnacle RTPS is investigated. A second order polynomial was devised to correlate the source size parameter with the RTPS-calculated penumbra. The optimal source size parameter was calculated for penumbra measurements based on the diamond detector and a standard ionization chamber (IC). This work was done for Jaw fields, MLC fields with a leaf end radius of 8cm, and MLC fields with a leaf end radius of 12cm. The optimum source size of the 8cm MLC fields matched the jaw fields, and an average (based on field sizes studied) of 1.1mm for the diamond detector data and 2.4mm for the ionization chamber was established. The effect of this overestimation of the source size parameter based on detector-induced penumbra broadening was considered for a clinical IMRT prostate plan by using two models (diamond and IC). There were differences in the DVH of the PTV and of OARs but these effects were of negligible clinical significance. Dose difference distributions showed dose difference areas to be in penumbra regions of the segments, with larger dose differences where penumbras intersected and/or there was a significant weighting on the segment. Gamma analysis was also performed between the two plans, and was found to increase the amount of fail rates significantly for both 2%/2mm and 3%/3mm criteria. This decreases the sensitivity of IMRT QA in the detection of systematic errors.