Establishment of a practical methodology for evaluating equieffective dose of individual patients based on RT-PHITS

The RadioTherapy package based on PHITS (RT-PHITS) is an individual dosimetry system applicable to both targeted radionuclide therapy (TRT) and external radiotherapy. This study aims to establish a practical methodology for evaluating both absorbed doses and equieffective doses (EQDX) by improving RT-PHITS.

We developed an Excel-based program, ExPORT-PHITS, which simplifies the conversion of the dose rates of specific organs and tumors calculated by RT-PHITS into corresponding absorbed doses and EQDX. ExPORT-PHITS offers two options for evaluating EQDX, each adopting a different type of microdosimetric kinetic model, to assess its dependence. The performance of the improved RT-PHITS, including ExPORT-PHITS, was evaluated using SPECT/CT images of two castration-resistant prostate cancer patients with bone metastases after the injection of ²²³RaCl₂ and ^99mTc-MDP.

Reasonable agreement was observed between absorbed doses calculated by RT-PHITS, IDAC Dose 2.1, and MIRDcalc, although absorbed doses in normal organs following the injection of ²²³RaCl₂ were comparatively higher than those reported in other studies. Results for ²²³RaCl₂ also showed that EQD2 tended to exceed the corresponding absorbed doses and RBE-weighted doses, while the relation was reversed for the injection of ^99mTc-MDP.

These findings underscore RT-PHITS as a valuable tool for accurately modeling and optimizing individual TRT, especially for treatments involving α-ray emitters.