Characterization of a Novel Revolving Radiation Collimator

Introduction The ZAP-X is a novel self-contained and first-of-its-kind self-shielded therapeutic radiation device dedicated to brain and head and neck radiosurgery. By utilizing a 2.7-MV linear accelerator and incorporating a design in which a beam stop and major mechanical elements serve a radiation shielding function, the Zap-X does not typically require a radiation bunker. The unique collimator design of the Zap-X is especially critical to the performance of the overall system. The collimator consists of a shielded tungsten wheel oriented with its rotational axis perpendicular to the beam's central axis; the goal of this design is to minimize radiation leakage. Beam selection is accomplished by rotating the wheel within its tungsten-shielded housing. We investigated radiation leakage from the Zap-X collimator to determine its compliance with internationally accepted standards using direct radiation measurements. Materials and methods To measure collimator leakage in the plane of the patient, equidistant measurement stations were defined in a plane perpendicular to the central beam axis (cax) 1 m from this axis (1 m from the radiation focal spot). To measure leakage alongside and adjacent to the accelerator, equidistant measurement stations were located 1 m from the cax along a line parallel to the cax in the plane of the collimator wheel and along a line parallel to the cax 90 degrees offset from the first line of stations. Results Radiation leakage emanating from the collimating head of the linear accelerator in the patient plane ranged between 4.0 and 10.4 mR. Radiation along the linear accelerator (1000 R delivered in the primary beam) varied between 1.7 and 6.8 mR and constituted between 0.00017% to 0.00068% of the primary beam. The former radiation originated from X-ray target leakage, while the latter is produced directly by the linear accelerator and both contributed to the overall leakage radiation that would reach a patient. Discussion Due to the large diameter of the Zap-X tungsten collimator wheel and the massive Zap-X tungsten cylindrical collimator shield, the overall patient leakage is 0.00104% of the primary beam at a 1-m distance from the beam central axis in the patient plane. Leakage radiation in the patient plane is limited by the International Electrotechnical Commission (IEC) to 0.1% of the total primary radiation. Radiation leakage along the linear accelerator and the collimator housing was determined to be 0.00068% of primary radiation intensity. This leakage value is lower than the 0.1% leakage limit stipulated by IEC by more than a factor of 100. Conclusions Typically, an MV radiation therapy system minimizes exposure by utilizing a combination of device and structural shielding. However, the Zap-X has been uniquely designed to minimize the need for structural shielding. Our results indicate radiation leakage from the collimator meets internationally accepted standards as defined by the IEC.