Can absorbed radiation doses by organs and tumors after PRRT be estimated from a single SPECT/CT study?

Objectives: After each cycle of [¹⁷⁷Lu]-DOTA-TATE Peptide Receptor Radionuclide Therapy (PRRT), dosimetry is performed to enable calculation of the radiation absorbed dose to tumors and normal organs. In our center absorbed doses are typically calculated from three quantitative Single Photon Emission Computed Tomography (SPECT) studies corrected by Computed Tomography (CT) acquired at t₁=24 h, t₂=96 h, t₃=168 h after the first cycle of treatment and from a single SPECT/CT study performed at t₁ after the following cycles. The aim of the present study was to assess the feasibility of using a single quantitative SPECT/CT study after each PRRT cycle and a trained multiple linear regression (MLR) model for absorbed dose calculation, and to evaluate its impact on patient management.

Methods: Quantitative [¹⁷⁷Lu]-DOTA-TATE SPECT/CT data after PRRT of seventy-two consecutive patients with metastatic neuroendocrine tumors were retrospectively evaluated. A set of 40 consecutive studies was used to train the MLR model. Independent variables included the time of imaging t_s after administration of [¹⁷⁷Lu]-DOTA-TATE (i.e., t₁, t₂ or t₃), the volume and counts in a given tumor/organ VOI. The dependent variable was the dose absorbed by the organ/tumor of interest obtained with the standard protocol. For bone marrow dosimetry, the MLR model uses as independent variables the time of imaging t_s, the blood activity concentration at t_s and the volume and counts in the remainder of the body VOI. The model was evaluated in 32 consecutive patients. Management decisions (whether or not treatment was stopped due to dosimetry results) were compared to hypothetical patient management, based on the MLR model for absorbed dose calculation.

Results: There was no difference in management decisions in 32 patients of the test dataset, between the standard protocol with three SPECT/CT studies and MLR model-based protocol using a single SPECT/CT study at t_s = t₃ after the first cycle and at t_s = t₁ after the following cycles. The coefficients of determination r²_test on the test population between the absorbed doses after the first cycle calculated using the standard protocol and those obtained from the MLR model at t_s = t₃ were 0.93, 0.97, 0.95, 0.99 and 0.98 for kidneys, bone marrow, liver, spleen and tumors, respectively. Cumulative doses were respectively obtained with a mean relative difference of -0.5% ± 5.4%, 1.6% ± 15.1%, -6.2% ± 7.3%, -5.5% ± 5.8% and 2.9% ± 12.7% and a coefficient of determination r² of 0.94, 0.83, 0.96, 0.97 and 0.96.

Conclusions: Dosimetry calculations using a simple MLR model with a single quantitative SPECT/CT study are feasible and are in good agreement with the standard protocol with three SPECT/CT studies with no change in patient management decisions, while avoiding the use of dosimetry software and enabling improved patient comfort and reduced scanner and staff time.

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