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Objective <p>Targeted radionuclide therapy (TRT) requires patient-specific dosimetry to optimize treatment efficacy while reducing toxicity. A RadioTherapy package based on PHITS (RT-PHITS) is a full Monte Carlo (MC) simulation-based dose calculation platform. This platform enables individualized 3D dose assessment using single photon emission computed tomography (SPECT)/computed tomography (CT) images of patients who undergo TRT with the peptide receptor DOTATATE labeled with <sup>177</sup>Lutetium (<sup>177</sup>Lu). The present study aimed to investigate the influence of uncertainties arising from the limited number of histories in MC simulation, the integration range of time-activity curves (TACs), and the numbers of imaging time points on the accuracy of dosimetry based on MC simulation.</p> Methods <p>SPECT/CT images acquired at multiple time points from two patients (Patients A and B) treated with <sup>177</sup>Lu-DOTATATE in the <sup>177</sup>Lu Dosimetry Challenge dataset were used for dosimetry analysis. Absorbed dose calculations were performed using the Monte Carlo–based dosimetry software RT-PHITS and an in-house dosimetry system. We then evaluated the dependence of statistical uncertainty on the total number of simulated particles and on a newly defined index–the volume-normalized histories for each lesion and organs at risk (OAR) in the MC simulation. Absorbed doses for lesions and OARs were calculated under a TAC integration range of 124–9,999&#xa0;h. Reduced datasets at three and two time points were also analyzed.</p> Results <p>Accuracy was reliable when 10<sup>7</sup> histories were achieved under our simulation conditions, although the optimal number of histories largely depends on total source volume and target size. In contrast, the coefficients of variation of calculated absorbed doses versus the volume-normalized histories were consistent across all lesions and OARs. Lesion dose estimates increased with longer TAC integration, whereas kidney doses reached a plateau &gt; 300&#xa0;h. Absorbed doses estimated based on three imaging time points were generally within approximately ± 10% of those obtained using four time points for most evaluated lesions and OARs, whereas the use of two time points resulted in larger deviations.</p> Conclusion <p>Accurate <sup>177</sup>Lu-DOTATATE dosimetry with RT-PHITS requires careful selection of TAC range and number of histories, particularly for lesion dose estimation.</p>

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Impact of simulation settings, time-activity curve integration range, and imaging time points on full Monte Carlo-based 177Lu-DOTATATE dosimetry

  • 期刊论文

  • 发表日期：

    2026年3月28日

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  • Shu Kimura,
  • Noriaki Miyaji,
  • Kenta Miwa,
  • Tensho Yamao,
  • Masaki Masubuchi,
  • Naochika Akiya,
  • Kaito Wachi,
  • Arata Komatsu,
  • Ryuichi Nishii,
  • Kohei Nakanishi,
  • Yuto Kamitaka,
  • Kosuke Yamashita,
  • Takuya Furuta,
  • Tatsuhiko Sato

摘要

Objective

Targeted radionuclide therapy (TRT) requires patient-specific dosimetry to optimize treatment efficacy while reducing toxicity. A RadioTherapy package based on PHITS (RT-PHITS) is a full Monte Carlo (MC) simulation-based dose calculation platform. This platform enables individualized 3D dose assessment using single photon emission computed tomography (SPECT)/computed tomography (CT) images of patients who undergo TRT with the peptide receptor DOTATATE labeled with 177Lutetium (177Lu). The present study aimed to investigate the influence of uncertainties arising from the limited number of histories in MC simulation, the integration range of time-activity curves (TACs), and the numbers of imaging time points on the accuracy of dosimetry based on MC simulation.

Methods

SPECT/CT images acquired at multiple time points from two patients (Patients A and B) treated with 177Lu-DOTATATE in the 177Lu Dosimetry Challenge dataset were used for dosimetry analysis. Absorbed dose calculations were performed using the Monte Carlo–based dosimetry software RT-PHITS and an in-house dosimetry system. We then evaluated the dependence of statistical uncertainty on the total number of simulated particles and on a newly defined index–the volume-normalized histories for each lesion and organs at risk (OAR) in the MC simulation. Absorbed doses for lesions and OARs were calculated under a TAC integration range of 124–9,999 h. Reduced datasets at three and two time points were also analyzed.

Results

Accuracy was reliable when 107 histories were achieved under our simulation conditions, although the optimal number of histories largely depends on total source volume and target size. In contrast, the coefficients of variation of calculated absorbed doses versus the volume-normalized histories were consistent across all lesions and OARs. Lesion dose estimates increased with longer TAC integration, whereas kidney doses reached a plateau > 300 h. Absorbed doses estimated based on three imaging time points were generally within approximately ± 10% of those obtained using four time points for most evaluated lesions and OARs, whereas the use of two time points resulted in larger deviations.

Conclusion

Accurate 177Lu-DOTATATE dosimetry with RT-PHITS requires careful selection of TAC range and number of histories, particularly for lesion dose estimation.