SU-E-J-31: Direct Point Dose Verification Using IGRT for Actual Beam Delivery

In radiation therapy, treatment planning for patients is performed using pre-acquired CT images. However, many patients with head-and-neck (H&N) cancer have tumor shrinkage and/or weight loss during their treatment course. Daily positional error of patients also causes unexpected deviations from the planning. Thus, it is essential to evaluate actual delivered dose for accurate clinical dosimetric consequence. In this study, actual delivered dose for an H&N site was determined by direct point dose measurement with metal-oxide-semiconductor field-effect transistor (MOSFET) detectors using IGRT procedure. We experimentally evaluated usefulness of the IGRT procedure for accurate irradiations.

Treatment processes from planning to beam delivery were performed for an H&N site of an anthropomorphic phantom. The MOSFET detectors were fixed inside the phantom in advance. Then, the anthropomorphic phantom was immobilized with a mould and mask and scanned by simulation-CT. Beam irradiation condition was field size of 12 cm × 12 cm, gantry angle of 0°, 90° and 330°, and 6 MV X-ray. Dose distribution was calculated with superposition algorithm with 2 mm calculation grid. Before the dose measurement, the anthropomorphic phantom was positioned using a localization system of mega-voltage cone-beam CT (MVCBCT). The MOSFET detectors were exposed five times according to a treatment plan. Measured doses with the MOSFET detectors were compared with calculated doses.

Using the MVCBCT, the set-up of the anthropomorphic phantom was achieved within 1 mm in all directions of anterior/posterior, left/right, and superior/inferior. The calculated doses agreed well to the measured doses within ±3% even in evaluated region with high dose gradient.

The actual delivered dose for an H&N site of an anthropomorphic phantom was evaluated experimentally with the MOSFET detectors. The IGRT procedure was useful for accurate irradiations.