A "natural" volume-dose histogram for brachytherapy.

The most useful formulation of the volume-dose histogram for brachytherapy is one which suppresses inverse square law effects while preserving proportionality between volume and the area under the curve. These objectives are met by plotting the distribution of volume per unit-3/2 power of dose rate versus the-3/2 power of dose rate; such a distribution is constant for a single point source. Adding sources results in the formation of a peak, which provides a graphical indication of dose rate uniformity. Evaluation of other peak parameters such as width, position, and contained volume, in relation to treatment dose rate, permits a quantitative and clinically relevant volume-dose assessment of interstitial implants.