Internal Bremsstrahlung, the missing process in beta decay Monte Carlo simulation: The relevance in 32P Dose-Point-Kernel estimation

Purpose: In nuclear medicine, Dose Point Kernels (DPKs), representing the energy deposited all around a point isotropic source, are extensively used for dosimetry and are usually obtained by Monte Carlo (MC) simulations. For beta-decaying nuclides, DPK is usually estimated neglecting Internal Bremsstrahlung (IB) emission, a process always accompanying the beta decay and consisting in the emission of photons having a continuous spectral distribution. This work aims to study the significance of IB emission for DPK estimation in the case of 32P and provide DPK values corrected for the IB photon contribution. Methods: DPK, in terms of the scaled absorbed dose fraction, F(R/X90), was first estimated by GAMOS MC simulation using the standard beta decay spectrum of 32P, Fβ(R/X90). Subsequently, an additional source term accounting for IB photons and their spectral distribution was defined and used for a further MC simulation, thus evaluating the contribution of IB emission to DPK values, Fβ+IB(R/X90). The relative percent difference, δ, between the DPKs obtained by the two approaches, Fβ+IB vs. Fβ, was studied as a function of the radial distance, R. Results: As far as the energy deposition is mainly due to the beta particles, IB photons does not significantly contribute to DPK; conversely, for larger R, Fβ+IB values are higher by 30–40% than Fβ. Conclusions: The inclusion of IB emission in the MC simulations for DPK estimations is recommended, as well as the use of the DPK values corrected for IB photons, here provided.