A semi-empirical formula for approximate estimation of the fall-off of intensity in the penumbra region of 60co teletherapy beam.

The nature of the observed fall-off of intensity across the teletherapy beams for two different 60Co therapy units has been studied. The fall-offs for various field sizes have been compared and an empirical function derived to represent the fall-off. This function is of trigonometric tan inverse form and makes use of only one parameter. The value of this parameter is directly related to the geometrical penumbra width, which can be calculated from the source diameter, the source-to-diaphragm distance and the source-to-skin distance.