Analysis of dose distributions for 4 MV X rays for symmetrical and offset fields using tapered multi-leaf collimators

Dose distributions for 4 MV X rays for symmetrical and offset fields using tapered multi-leaf collimators were measured with a computer controlled dosimeter system and analysis of the obtained data was carried out. Off-center axis ratios (OCR) for square and rectangular fields were different in the region of low dose. The equation for the correction was shown. OCR's for offset fields were almost the same as those for symmetrical fields when the field widths were less than about 10 cm and the depths were greater than about 6 cm. (author)