[A fast algorithm for homogeneous Ir-192--afterloading irradiation of cylindrical surfaces]

A fast algorithm was required to homogenise the dose on a cylinder mantle around a single straight applicator for Ir-192 afterloading. For linear applicators, an approximation formula for homogenisation of dose on one arbitrary cylinder mantle around the catheter is derived that predicts the required rise of dwell time tEnd at the end of the applicator. The other positions are loaded with a constant dwell time tMitte. The dwell time at the end tEnd appears to be independent of applicator length and can be described as a function of tMitte. In contrast to homogeneous loaded applicator, the dwell time in the central positions becomes also almost independent of catheter length, and is strictly proportional to the optimisation radius. TLD measurements and comparisons with a commercial planning system show the applicability of the procedure. This guarantees a homogeneous dose distribution even when modifications of the application length or optimisation distance become necessary. The algorithm can also be used to control the results of a planning system independently.