Factors and predictors affecting late external dose rates and isolation period in patients after lutetium-177-labeled DOTA-Tyr3-octreotate treatment for neuroendocrine tumors.

Objective: In peptide receptor radionuclide therapy (PRRT) using lutetium-177-labeled DOTA-Tyr3-octreotate ([¹⁷⁷Lu] DOTATATE), isolation is required until the external dose rate at 1 m (EDR-1 m) from the body surface falls below the regulatory standards of each country. While it is known that renal function influences EDR-1 m reduction within 180 min post-administration, the factors affecting EDR-1 m on the day following administration (Late EDR-1 m) remain unclear. This study aimed to identify factors influencing Late EDR-1 m after PRRT using [¹⁷⁷Lu] DOTATATE for neuroendocrine tumors and to predict Late EDR-1 m using pretreatment [¹¹¹In] pentetreotide single-photon emission computed tomography/computed tomography (SPECT/CT) data.

Methods: This study analyzed 111 PRRT cycles administered to 36 patients between September 2021 and August 2024. Late EDR-1 m was set as the dependent variable, whereas total radiopharmaceutical uptake (LUTtotal), dose per body weight, creatinine clearance (CCr), and albumin-bilirubin (ALBI) score were set as the independent variables in the multiple regression analysis. LUTtotal was calculated using SPECT/CT data acquired after the patient left the radiation therapy room, defining the volume of interest (VOI) as the area with SUVmean + 2SD or higher in the skeletal muscle. The VOI volume multiplied by the SUVmean was used to define LUTtotal. In addition, using [¹¹¹In] pentetreotide SPECT/CT data, the total radiopharmaceutical uptake (OCTtotal) was calculated in a manner similar to LUTtotal, and its correlation with LUTtotal was examined. A predictive equation for Late EDR-1 m was developed using the results of the multivariate analysis, and its performance was tested using subsequent cases between August 2024 and January 2025.

Results: The median measured Late EDR-1 m was 8.0 (range, 4.0-26.0) μSv/h. LUTtotal and dose per body weight were significantly correlated with Late EDR-1 m, whereas CCr and ALBI scores were not. Based on the results of the multivariate analysis, the predictive equation using the dose per body weight, assuming a dosage of 7400 MBq and OCTtotal, achieved a root mean square error (RMSE) of 2.24 μSv/h. In subsequent test cases, the RMSE was 3.47 μSv/h.

Conclusions: Late EDR-1 m is significantly correlated with LUTtotal and dose per body weight. It can be accurately predicted using [¹¹¹In] pentetreotide SPECT/CT data.