A formalism for traceable dosimetry in superficial electronic brachytherapy (eBT).

Abstract

Despite the number of treatments performed with electronic brachytherapy there is no uniform methodology for reference dosimetry for international traceability to primary dosimetry standards in different eBT systems. The objective of this study is to propose a formalism for traceability reference dosimetry in superficial eBT, that is easy to apply in the clinic. This method was investigated for an Elekta Esteya with one applicator. 
The calibration x-ray spectrum at the primary standards dosimetry laboratory was matched to the measured eBT photon spectrum. Two ionization chambers were calibrated at the PSDL in terms of air kerma. The chambers were used to measure reading ratios in-air at different distances from the applicator. Monte Carlo based air kerma ratios were calculated at different positions from the eBT applicator as well as backscatter factors in water and average mass energy absorption ratios in water and in air. Relative measurements with radiochromic films were performed in a water phantom to determine the ratio of absorbed dose to water, Dw, at the surface to Dw at 1 cm depth in water. These were compared with Monte Carlo calculations. Main results. Calculations and measurements were combined to estimate the Dw at the surface and at 1 cm depth in water. Ionization chamber agreement of the surface dose was 1.7%, within an uncertainty of 6.8% (k = 2). They agreed with the manufacturer dosimetry within 1.8%, with an uncertainty of 5.0% (k = 2).
The feasibility of the formalism and methodology for the Esteya system was demonstrated. Significance. This study proposes a method for harmonization of traceable reference dosimetry for eBT contact treatments which does not involve a detailed simulation of the ionization chamber. The method demonstrated feasibility for one eBT system using one surface applicator. In the future the method could be applied for different eBT systems.