Sensitivity of Beam Quality Conversion Factors (kQ) In HDR Brachytherapy Dosimetry
Abstract
Purpose
This work applies the TG-51 kQ formalism to energies below Co-60 in determining beam quality conversion factors (kQ) for thimble ionization chambers in HDR brachytherapy. The sensitivity of calculations to source-to-detector distance and phantom size is investigated. This approach provides an independent framework for validating dose calculations based on the TG-43 formalism.
Methods
Monte Carlo simulations using EGSnrc were performed to calculate kQ values, with dose-to-water and dose-to-chamber scored for standard Co-60 reference conditions and also brachytherapy source geometries over source-to-detector distances from 1–5 cm. Detailed models of the brachytherapy source and ionization chambers were used, and variance-reduction techniques were employed to achieve high statistical precision. Sensitivity of kQ to phantom size and backscatter conditions was explicitly evaluated and the results were compared with published kQ values for several ionization chamber types.
Results
The kQ values obtained combine energy and geometry dependencies and showed a variation with source-to-detector distance, decreasing from approximately 1.02 at 1 cm and approaching unity with increasing distance. This behavior was consistent with reduced volume-averaging and spectral effects at larger distances from the source. Phantom size impacted the absolute dose delivered at the measurement point by up to 6%, but kQ values were largely insensitive, with differences below 0.2% between large and realistic phantom-sized geometries. Calculated kQ values agreed with published MC results within reported uncertainties of 0.2%, confirming the validity of the methodology.
Conclusion
Beam quality conversion factors have been determined for four commonly-used ionization types. A systematic kQ variation of approximately 2% is observed over source-to-detector distances from 1 cm to 5 cm, reflecting changes in volume averaging and photon spectrum, which must be taken into account in high-accuracy applications. The findings help close a long-standing QA gap between HDR-BT and EBRT, while also informing future protocol development.