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Calculation of radiation dosimetry in targeted nuclear medicine therapies is traditionally resource-intensive requiring multiple post-therapy SPECT acquisitions. An alternative approach is to take advantage of existing pharmacokinetic data from these smaller cohorts to enable dose computation from a single post-treatment scan in a manner that may be applied to a much broader patient population.  In this work, a technical description for simplified dose estimation is presented and applied to assessment of Lu-PSMA-617 therapy (Prostate-Specific Membrane Antigen) for metastatic prostate cancer. By normalizing existing time-activity curves to a single measurement time, it is possible to calculate a mean and range of time-integrated activity values which relate to radiation absorbed dose. To assist with accurate pharmacokinetic modelling of the training cohort, a method for contour-guided image registration was developed.  Tissue-specific dose conversion factors for common post-treatment imaging times are reported along with a characterization of added uncertainty in comparison to a traditional serial imaging protocol. Single time point dose factors for tumor were determined to be 11.0, 12.1, 13.6, and 15.2 Gy per MBq/mL at image times of 24, 48, 72, and 96 hours, respectively. For normal tissues, parotid gland factors were 6.7, 9.4, 13.3, and 19.3 Gy per MBq/mL and kidneys were 7.1, 10.3, 15.0, and 22.0 Gy per MBq/mL at those times. Tumor dose estimates were most accurate using delayed scanning at times beyond 72 hours. Dose to healthy tissues is best characterized by scanning patients in the first two days of treatment owing to the larger degree of tracer clearance in this early phase.  The work demonstrates a means for efficient dose estimation in Lu-PSMA-617 therapy. By providing methods to simplify and potentially automate radiation dosimetry we hope to accelerate the understanding of radiobiology and development of dose-response models in this unique therapeutic context.

the journal of nuclear medicine

Radiation Dosimetry in <sup>177</sup>Lu-PSMA-617 Therapy Using a Single Posttreatment SPECT/CT Scan: A Novel Methodology to Generate Time- and Tissue-Specific Dose Factors

Radiation Dosimetry in 177Lu-PSMA-617 Therapy Using a Single Posttreatment SPECT/CT Scan: A Novel Methodology to Generate Time- and Tissue-Specific Dose Factors

Radiation Dosimetry in ¹⁷⁷Lu-PSMA-617 Therapy Using a Single Posttreatment SPECT/CT Scan: A Novel Methodology to Generate Time- and Tissue-Specific Dose Factors