Prediction of the portal image with an algorithm of overlap in the verification of IMRT treatments

Abstract

A patient quality assurance program should be performed in IMRT treatments in order to check if computed dose matches to delivered dose. Many different devices for verification are used, such as radiographic film, radiochromic film, detector arrays or electronical portal imaging devices (EPID).

The aim of this study was to introduce a new portal dose prediction method based on a superposition algorithm available in a treatment planning system (CMS XiO), considering the EPID as a phantom.

Two Varian 21EX linacs equipped with amorphous silicon EPID were available. EPID were calibrated according to manufacturer’s procedure, so that pixel number could be read as dose values.

The EPIDs were dismounted from the Exact-arm positioning device and CT images were taken. Using these CT slices, map doses were computed on the EPID imaging plane with superposition algorithm.

Some tests were made to compare computed dose maps versus portal dose images by gamma index evaluation, including prostate and head and neck IMRT cases, showing a good agreement.

In this method, the same energy model for the planning system was used both in the dose prediction and the treatment plan.