The use of a-emitting radionuclides in radiopharmaceutical therapies has gained prominence due to their high linear energy transfer (LET) and localized energy deposition, making them particularly effective for treating micrometastases and small tumours. This study addresses the challenges of a-particle dosimetry by calculating Dose Point Kernels (DPKs) for monoenergetic sources (4-10 MeV) and clinically relevant radionuclides, specifically 225Ac and 223Ra, using the FLUKA and PHITS Monte Carlo codes. DPKs were derived for a comprehensive dataset of biological tissues, and scaled DPK models were developed to account for monoenergetic emissions as well as individual decay chain members and complete decay chains. In practical applications, voxel dose kernels (DVKs) were used to estimate the absorbed doses from 225Ac-PSMA in prostate cancer histological images. The results show consistent trends in dose deposition, with the main differences between DVKs derived from dose point kernels (DPKs) calculated using FLUKA and PHITS simulations occurring primarily in low-dose regions. These findings establish a robust framework for a-particle dosimetry at the small-scale, enhancing the precision of dose estimations in heterogeneous radiopharmaceutical distributions.