SPECT/CT studies for dosimetry after PRRT: impact of reduced number of studies on organ dose calculation and patient management

Objectives: After each cycle of Peptide Receptor Radionuclide Therapy (PRRT) with [¹⁷⁷Lu]-DOTA-TATE, patient dosimetry is performed to estimate the radiation dose to kidneys and bone marrow, the most vulnerable tissues. Organ absorbed doses are calculated from Single Photon Emission Computed Tomography (SPECT) images corrected by Computed Tomography (CT) acquired after each course of treatment. Following the MIRD guidelines, three SPECT/CT studies, at t₁=24 hours, t₂=96 hours and t₃=168 hours are acquired after the first cycle of treatment in order to model the source organ distribution. For the following cycles, a single SPECT/CT study is performed assuming an unchanged effective half-life of [¹⁷⁷Lu]-DOTA-TATE. The aim of the present study is to assess how a “2 time points” protocol after the first cycle will affect dosimetry results and whether this protocol impacts on patient management.

Methods: The data of twenty-three patients with metastatic neuroendocrine tumors undergoing PRRT with [¹⁷⁷Lu]-DOTA-TATE were retrospectively analyzed. Radiation doses to kidneys and bone marrow after the first cycle of PRRT were calculated using four protocols: the standard protocol with three SPECT/CT studies acquired at (t₁, t₂, t₃), and three different “2 time points” protocols with SPECT/CT studies performed at (t₁, t₂), (t₁, t₃) or (t₂, t₃). In addition, we compared the actual patient management (whether or not treatment was stopped due to dosimetry results) to a hypothetical change in management, had we used the new “2 time points” protocols.

Results: Using the standard protocol, therapy was stopped in 5/23 patients (22%) due to a high expected dose to kidneys after the following therapy cycle. Using t₁ and t₂ or t₂ and t₃ for dose estimation therapy would have been stopped in one additional patient due to unsafe absorbed dose to kidneys while with the (t₁, t₃) “2 time points” protocol, no change in patient management would have been indicated. In all cases, the dose to bone marrow has not been the limiting factor for continuing treatment. The agreement between the dose to kidneys and bone marrow obtained with the standard and the (t₁, t₃) “2 time points” protocols was excellent with a Pearson’s correlation coefficient r = 0.99 (P < 0.0001) and a mean relative difference of 3.35% ± 3.47% and 3.22% ± 4.48%, respectively. The agreement of the two other (t₁, t₂) and (t₂, t₃) protocols with the standard protocol was less satisfactory with respective mean relative differences of 3.00% ± 10.26% and 9.68% ± 37.98% for kidneys and 10.71% ± 60.57% and 5.98% ± 22.91% for bone marrow.

Conclusions: These preliminary results demonstrate that dosimetry calculations using two SPECT/CT studies acquired 24 and 168 hours after the first PRRT cycle are feasible, showing good agreement with the standard imaging protocol and no change in patient management decisions, while enabling improved patient comfort and reduced scanner and staff time. Further assessment in a larger patient cohort is needed before implementation in the clinical practice.