A new geometric and mechanical verification device for medical LINACs

The goal of quality assurance (QA) for a radiation oncology medical LINAC is to maintain an acceptable level of equipment performance and reliability. The increasing complexity of Radiation Oncology equipment and treatment techniques have led to increased demands on the work load of the medical physicist. Regular testing needs to be as efficient as possible. Generally, the QA tests, as recommended by the AAPM Task Group 40 for medical LINACs, can be grouped into two categories: dosimetry and mechanical checks. A new QA device has been developed that facilitates many of the daily and monthly mechanical QA checks. Its efficiency and speed is achieved through a set of QA tools that are mounted on a single platform, which is designed to fit into the accessory mount of the medical LINAC. Named Mini-GARD (MG), it verifies the accuracy of the digital readouts for gantry angles, collimator angles, and field sizes. It also tests crosshair position, the optical distance indicator (ODI), and patient setup laser alignment. It uses two calibrated digital levels for the gantry and collimator angle verification, an electronic tape measure for ODI verification, and a calibrated transparent projection scale for the remaining tests. This paper evaluates the stability and accuracy of the device in clinical tests over a period of a year. Results show that the MG is reliable and capable of measuring gantry and collimator angle constancy to +/-0.3 degrees, ODI constancy to +/-0.05 cm, and field size accuracies to +/-0.05 cm.