An automatic method of the isocentre position verification for micromultileaf collimator based radiosurgery system

An efficient procedure has been developed using an electronic portal imaging device (EPID) and in-house written software for verification of a target simulator alignment with the radiation isocentre. A 5 mm tungsten ball is aligned to a linac isocentre based on a lasers intersection point. The BrainLab(®) m3™ add-on multileaf collimator (MLC) forms a rectangular open field of 1.8 × 1.8 cm(2). At five different gantry and couch positions, EPID images are acquired. A computer search algorithm determines the centres of both a radiation field and a tungsten ball for each image. Based on the geometric differences between those centres, the optimum three-dimensional shift of a tungsten ball is calculated in order to minimise the misalignment error between a target simulator and a radiation isocentre. A decision can then be made whether or not the tungsten ball and lasers intersection point should be corrected. The accuracy and precision of the procedure has been tested and found to be 0.04 and 0.24 mm respectively at 95% confidence interval. The procedure is also quicker, easier and more reliable to perform compared to the previous method based on irradiating a radiographic film.