Improvement of small-field penumbra and dose distributions on a 4 MV accelerator

The penumbra and dose distributions for small X ray fields from a 4 MV accelerator, designed for 80 cm source-to-surface distance (SSD) but used at 100 cm SSD, were observed to be similar to those obtained from cobalt-60 units. Penumbra was reduced and flatter isodose curves were obtained by the addition of tungsten trimmers at a distance of 48 cm from the X ray source. This stratagem improves the definition of irradiation volume of small fields which are common in the treatment of lesions in the head and neck.