Relevance of Internal Bremsstrahlung for estimating the exposure to pure beta emitters

Aim/Introduction: β-decay is accompanied by Internal Bremsstrahlung (IB) consisting of the emission of photons showing a continuous energy spectrum. Although theoretical approaches have been developed and several measurements have been carried out for modelling the IB spectral distribution, this process is disregarded in most estimates of exposure to pure βemitters. In our recent work [1], we demonstrated that the contribution of IB to the exposure to a 90Y source can ben worth of consideration. We set up two models for the IB spectral distribution and implemented them in Monte Carlo (MC) simulations to evaluate the absorbed dose in case of external irradiation from liquid sources in vials, both without and with the emission of IB photons. We found that, depending on the chosen model, IB photons can contribute to the total absorbed dose up to 30- 60%.The aim of this study was to compare our MC simulations with radiometric measurements for a 90Y source and to extend the analysis also to other high-energy beta emitters such as 32P. Materials and Methods: We compared radiometric measurements in a well ionization chamber with Monte Carlo simulations for 32P and 90Y. MC setup was preliminarily validated by comparing measurements and simulations for 57Co, 133Ba, 137Cs and 60Co γ-emitters. Measurements were carried out using a commercial radionuclide calibrator and, for each isotope, a vial was filled with a standardized solution. MC simulations were performed using GAMOS. Concerning 32P and 90Y, a first set of MC estimates was obtained by neglecting IB photons; then, IB spectral distribution was properly modelled for each beta emitter and a second set of simulations was run including IB photons. Results: We found that MC estimates deviate up to about -15% from measurements when IB process is disregarded in simulations, while the inclusion of IB photons allows achieving a reasonable agreement thus also giving indications on the appropriate modelling of the IB spectral distribution. The obtained results outline the need to include IB among the interaction processes to be considered when high-energy pure β-emitters are simulated. Conclusion: IB process is usually neglected when estimating the exposure to pure beta emitters. However, we demonstrated that for high-energy beta emitters such as 90Y and 32P, Internal Bremsstrahlung can play a relevant role for enhancing the absorbed dose values. For some β-emitters, the inclusion of IB in MC simulations deserves, in our opinion, consideration. References: [1] Italiano A., et al. Physica Medica 2020;76:159-65.