Monte Carlo Modelling of Well Chamber Response for Calibration of Diffusing Alpha-Emitter Radiation Therapy Brachytherapy Seeds
Abstract
Purpose
Diffusing Alpha-emitter Radiation Therapy (‘Alpha DaRT’) is a novel modality for treating solid tumours with alpha particles from the decay of 224Ra. This study aims to model well chamber response to derive activity calibration coefficients for simple and rapid source strength verification with readily available brachytherapy dosimetry equipment.
Methods
Monte Carlo (MC) simulations were performed using the Geant4-based GATE toolkit to model the Alpha DaRT source in a Standard Imaging HDR 1000 Plus well chamber. Energy deposited in the active volume of the well chamber was scored with statistical uncertainty < 0.5%. Preliminary validation of the well chamber model was conducted with a monoenergetic photon source with energies from 10 keV to 500 keV, the 125I IsoAid Advantage, and the 192Ir Flexisource with their corresponding source holders. Decay emissions from each radionuclide in the 224Ra decay chain were simulated individually and summed according to equilibrium activity conditions. MC simulation results for needle applicators loaded with one, three, or six Alpha DaRT seeds were compared to experimental measurements.
Results
Well chamber response to the monoenergetic photon source was consistent with previous literature. MC derived air kerma strength calibration coefficients for the 125I and 192Ir sources were within < 5% of calibration certificate values. Under decay equilibrium conditions, the single Alpha DaRT seed produced 0.0510 pC/kBq in a 30 s collection time. This activity calibration coefficient varied by < 1% for needles loaded with one, three, or six Alpha DaRT seeds. The MC derived calibration coefficients were consistent with experimental measurements within < 3% variation.
Conclusion
MC derived activity calibration coefficients were determined to correlate the charge collected by the well chamber to the total 224Ra activity. Our results demonstrate the utility of well chamber measurements for pre-treatment verification of Alpha DaRT source strength.