REAL-TIME BEAM MONITORING IN DYNAMIC CONFORMATION THERAPY

Purpose: Although portal imaging is a promising method of verification during static multiport irradiation, it cannot be applied directly to dynamic irradiation such as rotational conformation with multileaf co llimator movement. A real-time beam monitoring system based on megavol tage computed tomography scanning has been developed to establish a ve rification method for the rotational conformation technique. Methods a nd Materials: Exit beam through the patient is extracted by the same d etector unit as used for megavoltage scanning during the actual treatm ent. 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 s. Therapists can mo nitor correlation between the target and actual beam pathways on a rea l-time computer display. Results: The accuracy of field edge detection has been proven to be less than 2 mm from various measurements. Real- time monitoring is more useful in rotational conformation than in stat ic multiport irradiation due to dynamic movement of the collimator. Fi eld errors were identified in two of 54 sessions using this method. Co nclusions: Although several limitations remain to be solved, the metho d presented is a useful tool for treatment verification of high accura cy radiation therapy, particularly rotational conformation irradiation .