PENELOPE dose assessment comparison for ¹⁷⁷Lu and ¹⁶¹Tb using paediatric computational phantoms

Targeted Radiopharmaceutical Therapy (TRT) is one of the most intensively developing directions of nuclear medicine. TRT has a limited therapeutic use in the management of paediatric cancers, however, it is envisaged that could have a major role. The main aim of this work was to compare dosimetric performances among ¹⁷⁷Lu and ¹⁶¹Tb in paediatric phantoms with literature values and to study in which extent each radiation radionuclide emission contribute to the total absorbed dose. Dosimetry assessment was performed through Monte Carlo (MC) simulations using the PENELOPE-2018 program, with the paediatric ICRP reference computational phantoms, namely 10 and 15 years female phantoms. After a comparison step of Specific Absorbed Fractions (SAF) with literature data for model validation purposes, the dose assessment was performed by calculating organ S-values for ¹⁷⁷Lu and ¹⁶¹Tb. Our dosimetric performances are in agreement with literature results, showing, in general, greater absorbed doses in the target volume for ¹⁶¹Tb with respect to ¹⁷⁷Lu. The higher energy deposition is generally attributed to the higher yields of Auger and Internal Conversion (IC) electrons for ¹⁶¹Tb. However, the greater yield emission of ultrasoft X-rays by ¹⁶¹Tb with respect to ¹⁷⁷Lu, could be another mechanism that could explain the greater energy deposition performances when using ¹⁶¹Tb. Finally, performing dosimetric calculations with different MC tools, remark the importance to have some common parametrization guidelines for MC dose coefficient assessment.