Clinical implementation of voxel-based dosimetry using image-based RT-PHITS Monte Carlo simulations for...

This study aimed to employ RadioTherapy extension of the Particle and Heavy Ion Transport code System (RT-PHITS) Monte Carlo (MC) simulation for estimating absorbed doses in target organs and tumors in patients administered with ¹⁷⁷Lu-DOTATATE, using single-photon emission computed tomography/computed tomography (SPECT/CT) imaging.

Quantitative SPECT/CT images were obtained from 17 patients across the abdominal region at four time points: approximately 4, 24, 72, and 120 h following the administration of ¹⁷⁷Lu-DOTATATE. The liver, spleen, left and right kidneys, and total kidneys were automatically segmented on the CT images using the TotalSegmentator tool. Tumors were manually delineated based on SPECT/CT images. Image registration was performed using an Elastix-based method, with the first SPECT/CT time point serving as the reference. Voxel-level time-integrated activity (TIA) maps were created by fitting mono-exponential functions. These TIA maps, together with the reference CT images, were input into RT-PHITS to calculate dose distributions. The absorbed doses calculated by RT-PHITS were compared with those from IDAC-Dose 2.1 through two approaches: first, by using independently derived time-integrated activity coefficients (TIACs) from each method to assess the combined effects of kinetic modeling and dose calculation technique; second, by applying the same TIACs-obtained from time-integrated activity data-to both methods to isolate the influence of the dose calculation approach.

RT-PHITS yielded higher mean absorbed doses per unit of administered activity compared to IDAC-Dose. The relative differences ranged between 0.63% and 15.35%, with the right kidney showing the largest discrepancy. When the same time-integrated data were used for both RT-PHITS and IDAC-Dose, relative differences remained below 10.40%.

RT-PHITS is a capable tool for calculating absorbed doses in ¹⁷⁷Lu-DOTATATE therapy. It consistently produced higher dose estimates than the organ-based method, emphasizing the benefits of patient-specific dosimetry, especially in organs that contain or are near tumors.