SU‐E‐T‐205: Physical Model for in Vivo Dose Image Prediction

Abstract

Purpose: To create a physical fluence model for portal dose image prediction that will be used to verify patient radiation treatment delivery. Methods and Materials: A physical fluence model was created to predict portal dose images. This model utilized Monte Carlo simulation and linac‐specific engineering schematics of the MLCs to create as accurate a model as possible. The fluence model consists of a focal and extra‐focal source, determined to be a Gaussian and Gaussian‐like function, respectively. The MLC transmission is calculated by attenuating a pre‐MLC BEAMnrc spectrum through the leaves; the MLC leaf‐tip and tongue‐and‐groove are modeled using the schematics. Incident energy fluence profiles from BEAMnrc are used to account for the field shape and beam horns. The asymmetric backscatter from the EPID arm is also modeled. The energy fluence is converted to dose using a superposition of EPID‐specific dose kernels. Scatter from the patient or phantom is approximated using Monte Carlo calculated scatter fluence kernels. The model is tested on simple slab phantoms for a variety of field sizes, thicknesses and air gaps. It was also tested on one field acquired during a patientˈs prostate IMRT treatment. Results: Predicted images with phantoms in the beam agreed within 2%, 2 mm in relative comparison with the measured images. After calibration of measured and predicted images to absolute units, the images agreed within 3% and 3 mm. The patient IMRT field predicted image was not ideal when compared to the measured image, mainly because the patient is heterogeneous. Conclusions: The physical fluence model, in conjunction with the patient scatter model, is accurate and could be used to verify patient treatment. The algorithm is versatile and can be applied to a variety of treatment scenarios.