Realization of the absorbed dose to water for 70 kV electronic brachytherapy sources for surface applications

Objective.The purpose of this work is to establish a primary standard for the absorbed dose to water for surface applications with 70 kV electronic brachytherapy sources and thereby to investigate several reference conditions with respect to applicability in metrology institutes, calibration labs and clinics.Approach.A primary standard for the absorbed dose to water for LDR-Seeds (ipFAC) was utilized. For this, the method of evaluation was modified to account for the different geometries in interstitial and surface brachytherapy and for the different energy distributions of the radiation fields, respectively. The correction factors required to determine the absorbed dose to water were evaluated using Monte Carlo (MC) simulations. MC-calculations were also used to estimate the uncertainties of this method.Main results.It could be shown that determining the absorbed dose to water in 1 cm depth below the surface of a water phantomD_w(1 cm) is feasible with the ipFAC. Thereby the method to determineD_w(1 cm) when the source is placed at 50 cm distance and the beam is collimated turns out to be more robust to variations in the directional emittance of the source than placing the source applicator assembly directly on the phantom.Significance.The first method features significantly lower uncertainties, 1.4% compared to 3.7% (both fork= 2), for the second one. However, a calibration based on a transfer chamber calibrated with the second method is more practical and easier to implement in clinical routine.