Room: Exhibit Hall
Purpose: Portal Dosimetry is widely used to verify IMRT/VMAT type dose delivery in radiation therapy. However, it is known that the response of some portal imagers changes with dose rate, which may lead to errors in evaluation for some plans. Transmission through the MLC is delivered at a much lower dose rate than the rest of the beam. This may affect the results of QA for highly modulated IMRT plans.
Methods: The Eclipse Portal Dosimetry Algorithm was commissioned for several linacs in our department. Values of MLC transmission were first obtained for the portal dose models using chamber measurements and then from electronic portal imaging device (EPID) images from standard MLC patterns. 10 IMRS patient plans that failed our usual portal dose evaluation criteria (95% gamma pass rate at 3%/2mm agreement) with chamber transmission values were then recalculated using EPID-based transmission values to see whether agreement would be improved.
Results: When taking EPID images for a field completely blocked by the MLC, the measured portal dose values were between 60% and 70% of those predicted by chamber measurements, depending on the model of the EPID and the photon energy. Using the EPID-based transmission value, gamma pass rate for 10 patient plans was raised from 90.85 Â± 5.78% to 95.96 Â± 2.82%. CAX dose agreement was improved from 96.09 Â± 2.32% to 97.32 Â± 1.83%.
Conclusion: Portal dosimetry systematically underestimates transmission dose through the MLC in highly modulated IMRT plans. Agreement between predicted and measured doses in IMRT QA may be improved by creating a revised portal dose model with an artificially low value of MLC transmission, in order to better match the response of the imager.