A fundamental accuracy limitation on measurements of brachytherapy sources.

The dose rate gradients in the first few centimeters away from brachytherapy sources are extremely high. Attempts at direct measurements of the dose rates in the near vicinity of such sources are made with the smallest detectors available. Nevertheless, it is not obvious that the agreement between reported measurements and theoretical calculations can be justified when the dose rate gradients across the detector volumes employed are so great. A figure of merit is derived here which indicates how well the dose averaged over the volume of the detector corresponds to the dose at the center of the detector. This figure of merit provides a means to assess the maximum accuracy one can expect to achieve in a measurement made at a given distance from a linear source based on the dimensions of the source and the dimensions and shape of the detector.