Feasibility of individual dosimetry using RT-PHITS for patients with SPECT/CT imaging after 177Lu-DOTATATE peptide receptor radionuclide therapy.

A radiotherapy package based on the Particle and Heavy Ion Transport code System (RT-PHITS) can calculate internal 3-dimensional dose distribution from SPECT/CT images of individual patients coupled with Monte Carlo radiation transport simulation. This study aims to determine the feasibility of individual dosimetry using RT-PHITS for patients after ¹⁷⁷Lu-DOTATATE peptide receptor radionuclide therapy (PRRT). We acquired SPECT/CT images from two patients from the ¹⁷⁷Lu SNMMI Dosimetry Challenge (patients A and B) and one from our institute (patient C) at various time points. The images were converted to source/geometry information in the PHITS input format using RT-PHITS. The 3D dose-rate distribution in each patient was calculated using Monte Carlo radiation transport simulation. The output data from the PHITS simulation were converted to DICOM RT-dose format and analyzed using 3D Slicer to identify dose rates in lesions and organs at risk. The time variations of the calculated dose rates were linearly interpolated, considering the physical decay constant. The absorbed dose was evaluated as the integration of the time variation of the dose rate. Agreements between the absorbed doses obtained from RT-PHITS and ¹⁷⁷Lu Dosimetry Challenge Task 4 were generally satisfactory (< 20%), although discrepancies were noted in some normal organs of patient A. This was likely due to difficulties in estimating the tail of the dose rate curve after the last imaging time point. The EQD2 was slightly, but not significantly increased compared with the absorbed dose in patient C. Individual dosimetry using RT-PHITS is feasible for assessing the effects of ¹⁷⁷Lu-DOTATATE PRRT.