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A nondivergent specification of the mean treatment dose in interstitial brachytherapy.

The harmonic mean formalism is investigated as a nondivergent specification of the mean treatment dose in three distinct applications of interstitial brachytherapy: (a) permanent 125I prostate implant; (b) double plane 192Ir linear source array; and (c) multiple catheter optimized high dose rate implant. In each case the harmonic treatment dose (HTD) can be calculated in regions that contain radioactive sources or otherwise high dose gradient, to yield objective and consistent results with little computational effort. The need to identify local dose minima, geometric centers, or seed planes is largely avoided, which makes the formalism suitable for automated computation. Dose distribution uniformity is analyzed in terms of the fractional spread for the HTD and, where the periphery of the target volume is delineated, the uniformity number and the peripheral uniformity number. It is shown that the harmonic mean dose formalism is a practical alternative to the ICRU dose specification parameters in certain areas of brachytherapy.