The use of single-timepoint images to link administered radioiodine activity (MBq) to a prescribed lesion radiation-absorbed dose (cGy): a regression-based prediction interval tool for the management of well-differentiated thyroid cancer patients

Abstract

Purpose: To introduce a biomarker-based dosimetry method for the rational selection of a treatment activity for patients undergoing radioactive iodine ¹³¹I therapy (RAI) for metastatic differentiated thyroid cancer (mDTC) based on single-timepoint imaging of individual lesion uptake by ¹²⁴I PET.

Methods: Patients referred for RAI therapy of mDTC were enrolled in institutionally approved protocols. A total of 208 mDTC lesions (in 21 patients) with SUV_max > 1 underwent quantitative PET scans at 24, 48, 72, and 120 h post-administration of 222 MBq of theranostic NaI-¹²⁴I to determine the individual lesion radiation-absorbed dose. Using a general estimating equation, a prediction curve for biomarker development was generated in the form of a best-fit regression line and 95% prediction interval, correlating individual predicted lesion radiation dose metrics, with candidate biomarkers ("predictors") such as SUV_max and activity in microcurie per gram, from a single imaging timepoint.

Results: In the 169 lesions (in 15 patients) that received ¹³¹I therapy, individual lesion cGy varied over 3 logs with a median of 22,000 cGy, confirming wide heterogeneity of lesion radiation dose. Initial findings from the prediction curve on all 208 lesions confirmed that a 48-h SUV_max was the best predictor of lesion radiation dose and permitted calculation of the ¹³¹I activity required to achieve a lesional threshold radiation dose (2000 cGy) within defined confidence intervals.

Conclusions: Based on MIRD lesion-absorbed dose estimates and regression statistics, we report on the feasibility of a new single-timepoint ¹²⁴I-PET-based dosimetry biomarker for RAI in patients with mDTC. The approach provides clinicians with a tool to select personalized (precision) therapeutic administration of radioactivity (MBq) to achieve a desired target lesion-absorbed dose (cGy) for selected index lesions based on a single 48-h measurement ¹²⁴I-PET image, provided the selected activity does not exceed the maximum tolerated activity (MTA) of < 2 Gy to blood, as is standard of care at Memorial Sloan Kettering Cancer Center.

Trial registration: NCT04462471, Registered July 8, 2020. NCT03647358, Registered Aug 27, 2018.

Keywords: Differentiated thyroid cancer; Dosimetry; Iodine-124; PET/CT; Radioactive iodine therapy.