Automatic verification of step-and-shoot IMRT field segments using portal imaging

In step-and-shoot IMRT, many individual beam segments are delivered. These segments are generated by the IMRT treatment planning system and subsequently transmitted electronically through computer hardware and software modules before they are finally delivered. Hence, an independent system that monitors the actual field shape during treatment delivery is an added level of quality assurance in this complicated process. In this paper we describe the development and testing of such a system. The system verifies the field shape by comparing the radiation field detected by the built-in portal imaging system on the linac to the actual field shape planned on the treatment planning system. The comparison is based on a software algorithm that detects the leaf edge positions of the radiation field on the portal image and compares that to the calculated positions. The process is fully automated and requires minimal intervention of the radiation therapists. The system has been tested with actual clinical plan sequences and was able to alert the operator of incorrect settings in real time.