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, F_β(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, F_β+IB(R/X₉₀). The relative percent difference, δ, between the DPKs obtained by the two approaches, F_β+IB vs. F_β, 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, F_β+IB values are higher by 30-40% than F_β.

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.

Technical note: The contribution of internal bremsstrahlung to the 90 Y dose point kernel

BACKGROUND

Internal dosimetry has an increasing role in the planning and verification of nuclear medicine therapies with radiopharmaceuticals. Dose Point Kernels (DPKs), quantifying the energy deposition all around a point source, in a homogenous medium, are extensively used for 3D dosimetry and nowadays are mostly evaluated by Monte Carlo (MC) simulation. To our knowledge, DPK for beta emitters is estimated neglecting the continuous photon emission due to the Internal Bremsstrahlung (IB), whose contribution to the absorbed dose can be relevant beyond the maximum range of betas, as evidenced in recent works.

PURPOSE

Aim of this study was to investigate and quantify, by means of MC simulations, the contribution of IB photons to DPK calculated for ⁹⁰ Y and provide the updated ⁹⁰ Y DPK.

METHODS

The overall radiation due to the decay of a ⁹⁰ Y point source, placed at the centre of concentric water shells of increasing radii from 0.02 cm to 20 cm, was simulated with GAMOS, including the IB source term whose spectral distribution was described by an analytical model. Energy deposition was scored in the shells as a function of the distance from the source, R, and DPK was estimated in terms of the scaled absorbed dose fraction, F(R/X₉₀ ), where X₉₀ is the range within which the beta particles deposit 90% of their energy.

RESULTS

A comparison between the two simulated absorbed dose distributions, calculated with or without IB, clearly shows that the latter (incomplete) choice is consistent with the findings of other Authors and systematically underestimates the absorbed dose imparted to the tissue. ⁹⁰ Y DPK values currently used are underestimated by 20%-34% for R>2X₉₀ .

CONCLUSIONS

The revised values provided in this work suggest that the inclusion of IB emission in DPK evaluations is advisable for pure beta emitters.