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Objective <p>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 <sup>177</sup>Lu-DOTATATE, using single-photon emission computed tomography/computed tomography (SPECT/CT) imaging.</p> Methods <p>Quantitative SPECT/CT images were obtained from 17 patients across the abdominal region at four time points: approximately 4, 24, 72, and 120&#xa0;h following the administration of <sup>177</sup>Lu-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.</p> Results <p>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%.</p> Conclusion <p>RT-PHITS is a capable tool for calculating absorbed doses in <sup>177</sup>Lu-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.</p>

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Clinical implementation of voxel-based dosimetry using image-based RT-PHITS Monte Carlo simulations for 177Lu-DOTATATE radionuclide therapy

  • 期刊论文

  • 发表日期：

    2025年11月8日

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  • Khajonsak Tantiwetchayanon,
  • Kosuke Matsubara,
  • Chatnapa Nuntue,
  • Takayuki Shibutani,
  • Takahiro Konishi,
  • Hiroshi Wakabayashi

摘要

Objective

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.

Methods

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.

Results

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%.

Conclusion

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.