[Verification of rotational conformation radiotherapy using megavoltage CT scanning]

Although portal imaging is a promising method of verification during static multi-port irradiation, it cannot be applied directly to dynamic irradiation such as rotational conformation with multi-leaf collimator movement. A real-time beam monitoring system based on megavoltage computed tomography scanning has been developed to establish a verification method for the rotational conformation technique. The exit beam through the patient is extracted by the same detector unit as that used for megavoltage scanning during the actual treatment. The beam edge is defined as the 50% level of the maximum dose of the detector array. Megavoltage computed tomography is done after patient setup and just prior to the actual irradiation. Detected beam pathways are overlaid on this image approximately every 1 sec. Therapists can monitor the correlation between the target and actual beam pathways on a real-time computer display. The accuracy of field edge detection has been proven to be less than 2 mm from various measurements. Field errors were identified in two of 54 sessions using this method. Although several limitations remain to be solved, the method presented is an useful tool for treatment verification of high accuracy radiation therapy, particularly rotational conformation irradiation.