Dose calibrator response to brachytherapy sources: A Monte Carlo and analytic evaluation

It is well known that the dose calibrator response/unit exposure rate depends significantly upon source energy. However, investigation of ¹³⁷Cs, ¹⁹²Ir, and ²²⁶Ra brachytherapy sources by empirical, analytical, and Monte Carlo techniques shows that source filtration significantly affects the calibrator reading to exposure rate conversion factor. The results demonstrate that for each clinically used filtration thickness an exposure calibrated standard source is required to establish the response of the well chamber. An interesting consequence of this analysis is that the Sievert point dose algorithm for clinical sources overestimates the dose on the order of 3% at distances of approximately 3.5 cm from the source.