Abstract
Modern linear accelerators have four independent jaws and multileaf collimators (MLC) of 1 cm windth at the isocenter. Asymmetric fields defined by such independent jaws and irregular multileaf collimated fields can be used to match adjacent fields or to spare the spinal cord in external photon beam radiotherapy. We have developed a new approximate algorithm for depth dose calculations at the collimator retation axis. The program is based on Clarkson's pronciple, and uses a more accurate modification of Day's method for asymmetric fields. Using this method, tissue-maximum ratios (TMR) and field factors of ten kinds of asymmetric fields. and ten different irregular multileaf collimated fields were calculated and compared with the measured data for 6 MV and 15 MV photon beams. The dose accuracy with the general A/Pe method was about 3%, however, with the new modified Day's method, accuracy was within 1.7% for TMR and 1.2% for field factors. The calculated TMR and field factors were found to be in good agreement with measurements for both the 6 MV and 15 MV photon beams.
