Effect of field size on the radiation penumbra and integral dose for telegamma therapy units

Because of the large source size in telegamma therapy units, the boundaries of the radiation beam beyond the collimator are not well defined and a zone of falling dose and resulting penumbra are produced. In general, the collimator design is such that the geometrical penumbra is nearly equal in magnitude to the source diameter. However, the magnitude of the physical or radiation penumbra is influenced by the radiation transmitted through the edge of the collimator, the geometrical penumbra and the scatter from the source and the collimator. In the present study, the variation of radiation penumbra with field size has been investigated for two different makes of telegamma units. It is seen that the radiation penumbra can be as large as 4.8 cm for a 20 X 20 cm field. The effect of such large penumbra on the integral dose is discussed.