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

## TL;DR

This study identifies factors affecting radiation levels in patients after a specific cancer treatment and shows how to predict these levels using imaging data.

## Contribution

The study introduces a predictive model for late external dose rates using pretreatment imaging data in PRRT for neuroendocrine tumors.

## Key findings

- Late EDR-1m is significantly correlated with total radiopharmaceutical uptake and dose per body weight.
- A predictive equation using [111In] pentetreotide SPECT/CT data achieved an RMSE of 2.24 μSv/h in training and 3.47 μSv/h in testing.
- Creatinine clearance and ALBI scores were not significantly correlated with late EDR-1m.

## Abstract

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.

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.

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.

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.