Shielding effects of Selectron applicator and pellets on isodose distributions.

The Selectron is a mobile remote afterloading device which was developed specifically to reduce the radiation exposure to hospital personnel during intracavitary brachytherapy. Applicators for these devices contain source trains of 48 spherical pellets per channel, composed of both active and inactive pellets which have an external diameter of 2.5 mm. The configuration of pellets is chosen to optimize the dose distribution in accordance with the patient's geometry. A new dose calculation model has been proposed based on the separation of radiation into primary and secondary components. This model takes into account the attenuation of the primary radiation by the pellets. Isodose distributions around a Selectron standard applicator containing a typical patient loading of active pellets were measured and compared with those calculated by program SEEDS of the Atomic Energy of Canada Ltd. (AECL) Theraplan computer planning system. The shielding effects of the stainless steel applicator tubes, the ovoids, screws, inactive and active pellets will be considered. The effects of scatter and attenuation by the pellets will be illustrated by measured isodose distributions around a single applicator tube.