Quality assurance of helical tomotherapy intensity modulated radiation therapy

Helical tomotherapy, like helical CT, has the fan beam continuously rotating around the patient as the couch is transporting the patient longitudinally through the ring gantry. The approach requires synchrony of gantry rotation, couch translation, accelerator pulsing and the opening and closing of the leaves of the binary multileaf collimator used to modulate the radiation beam. In the course of clinically implementing helical tomotherapy intensity modulated radiation therapy, we have developed a quality assurance (QA) system for patient treated with intensity modulated radiation therapy (IMRT) of helical tomotherapy. We describe the procedure developed in the Radiation Oncology Department of PLA General Hospital, and present the verification results of 84 patients. This procedure is composed of two kinds of measurements that are the absolute dose at the reference point of target volume with an ionization chamber of small sensitive volume, the relative dose distribution in a transverse plane with film. Here the method and results of IMRT are summarized and the QA system is set out along with the experimental details of its implementation. The systematic error is discussed during the phantom setup.