Endocrine Abstracts

Introduction: Determining the radiation dose to both the kidneys and malignant lesions from therapeutic administrations of Y-90 DOTATOC is critical for optimal management of neuroendocrine cancer patients. Here, a new dosimetric approach uses PET/CT to quantify the absolute activity of Y-90 immediately after administration to calibrate subsequent tissue clearance monitored with bremsstrahlung SPECT. The dosimetric information is used to personalize subsequent Y-90 DOTATOC doses to optimize therapeutic effect while keeping renal dose below toxic levels.

Methods: Patients with nonresectable or metastatic neuroendocrine tumors were treated with three cycles separated by 6 weeks. For the 1st treatment cycle, adult subjects received 4.44 GBq of Y-90 DOTATOC. PET/CT imaging was acquired after amino acid infusion and was used to quantify the renal uptake of ⁹⁰Y-DOTATOC. Bremsstrahlung SPECT/CT was acquired immediately after the PET/CT study and was repeated at 24, 48 and 72 h to obtain the time activity curves from the kidneys and lesions in the field of view. Tissues masses were determined from the CT and the radiation dose to these tissues was determined using OLINDA/EXM. The administered activity in subsequent cycles was modified based on the kidney dose determined from the previous cycle.

Results: Eighteen patients have been treated with at least one cycle of ⁹⁰Y-DOTATOC. The mean kidney uptake at the time of PET/CT imaging was 2.2% of the administered activity resulting in a mean kidney dose of 1.4 mGy/MBq (range 0.6–2.7 mGy/MBq). The dose to the kidney tended to vary between cycles indicating the need to monitor dose with each administration. Based on the measured kidney doses, the administered activity was reduced in 6 and increased in 20 treatment cycles.

Conclusion: PET/CT-based Y-90 DOTATOC dosimetry combined with Bremsstrahlung SPECT allows patient-specific optimized therapeutic dosing. This general dosimetric methodology is applicable to any Y-90 based therapeutic.