Updating ⁹⁰Y Voxel S-Values including internal Bremsstrahlung: Monte Carlo study and development of an analytical...

Internal Bremsstrahlung (IB) is a process accompanying β-decay but neglected in Voxel S-Values (VSVs) calculation. Aims of this work were to calculate, through Monte Carlo (MC) simulation, updated ⁹⁰Y-VSVs including IB, and to develop an analytical model to evaluate ⁹⁰Y-VSVs for any voxel size of practical interest.

GATE (Geant4 Application for Tomographic Emission) was employed for simulating voxelized geometries of soft tissue, with voxels sides l ranging from 2 to 6 mm, in steps of 0.5 mm. The central voxel was set as a homogeneous source of ⁹⁰Y when IB photons are not modelled. For each l, the VSVs were computed for ⁹⁰Y decays alone and for ⁹⁰Y + IB. The analytical model was then built through fitting procedures of the VSVs including IB contribution.

Comparing GATE-VSVs with and without IB, differences between + 25% and + 30% were found for distances from the central voxel larger than the maximum β-range. The analytical model showed an agreement with MC simulations within ± 5% in the central voxel and in the Bremsstrahlung tails, for any l value examined, and relative differences lower than ± 40%, for other distances from the source.

The presented ⁹⁰Y-VSVs include for the first time the contribution due to IB, thus providing a more accurate set of dosimetric factors for three-dimensional internal dosimetry of ⁹⁰Y-labelled radiopharmaceuticals and medical devices. Furthermore, the analytical model constitutes an easy and fast alternative approach for ⁹⁰Y-VSVs estimation for non-standard voxel dimensions.