Characterization of a Novel Revolving Radiation Collimator
The ZAP-X is a novel self-contained and first-of-its-kind self-shielded therapeutic radiationdevice dedicated to brain and head and neck radiosurgery. By utilizing a 2.7-MV linearaccelerator and incorporating a design in which a beam stop and major mechanical elementsserve 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 theoverall system. The collimator consists of a shielded tungsten wheel oriented with its rotationalaxis perpendicular to the beam’s central axis; the goal of this design is to minimize radiationleakage. Beam selection is accomplished by rotating the wheel within its tungsten-shieldedhousing. We investigated radiation leakage from the Zap-X collimator to determine itscompliance with internationally accepted standards using direct radiation measurements.
Materials and methods
To measure collimator leakage in the plane of the patient, equidistant measurement stationswere defined in a plane perpendicular to the central beam axis (cax) 1 m from this axis (1 mfrom 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 caxin the plane of the collimator wheel and along a line parallel to the cax 90 degrees offset fromthe first line of stations.
Radiation leakage emanating from the collimating head of the linear accelerator in the patientplane ranged between 4.0 and 10.4 mR. Radiation along the linear accelerator (1000 R deliveredin the primary beam) varied between 1.7 and 6.8 mR and constituted between 0.00017% to0.00068% 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 overallleakage radiation that would reach a patient.
Due to the large diameter of the Zap-X tungsten collimator wheel and the massive Zap-Xtungsten cylindrical collimator shield, the overall patient leakage is 0.00104% of the primarybeam at a 1-m distance from the beam central axis in the patient plane. Leakage radiation in thepatient plane is limited by the International Electrotechnical Commission (IEC) to 0.1% of thetotal 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 islower than the 0.1% leakage limit stipulated by IEC by more than a factor of 100.
Typically, an MV radiation therapy system minimizes exposure by utilizing a combination ofdevice and structural shielding. However, the Zap-X has been uniquely designed to minimizethe need for structural shielding. Our results indicate radiation leakage from the collimatormeets internationally accepted standards as defined by the IEC