Effect of Peptide Dose on Radiation Dosimetry for Peptide Receptor Radionuclide Therapy with 177Lu-DOTATOC: A Pilot Study.

Background: Optimal peptide concentration in treatment with 177Lu-DOTATOC/DOTATATE is amatter of debate. Most of the studies with peptide receptor radionuclide therapy mention peptidedose ranging between 100 and 250 µg. The aim of this is to identify possible differences inradiation-absorbed doses (D/Gy) to tumor and kidney as a function of the peptide mass dose in orderto identify the most suitable peptide dose for treatment. The therapeutic index (Dtumor/Dkidneys) wasassessed as a key parameter for the treatment response. Materials and Methods: Five patients withmetastasized Grade 1 to Grade 2 neuroendocrine tumor were analyzed in this study. Patients (n = 4)received two cycles of treatment with intravenously injected 177Lu-DOTATOC containing peptidemass doses of 200 µg and 90 µg, alternatively; one patient was treated with 90 µg peptide massin both the therapy cycles. Whole-body (head to mid-thigh) three-dimensional single-photonemission computerized tomography (3D SPECT)/CT images were acquired at 1, 4, 24, 48, and72 h following the injection of 177Lu-DOTATOC. Attenuation correction for 3D SPECT images wasperformed using CT data acquired and fused with the SPECT data (SPECT/CT). Results: Overall, 28 target lesions (liver n = 17, lung n = 4, lymph nodes n = 1, and bone n = 2) were analyzed after 1st and 2nd therapy cycles. Tumor normalized absorbed doses varied by a factor of 74 between 0.35 and26 mGy/MBq. Averaged over all patients, a higher normalized mean tumor dose (10.51 mGy/MBq)was achieved for a peptide dose of 200 µg compared to 90 µg (4.58 mGy/MBq). Kidneys dosesvaried by a factor of up to 4 between patients (0.25-1.0 mGy/MBq) (independent of dose cycle andpeptide dose) and by a factor of up to 2 between dose cycles. The mean kidney dose was 13.7%higher for the 90 µg peptide dose compared to 200 µg. Given the higher tumor dose, the meantherapeutic index of a 200 µg mass dose was considerably higher (16.95), compared to a 90 µg massdose (9.63). This coincided with the observation, that lesion volume reduction was more pronouncedafter an initial treatment with a 200 µg mass dose. Biologically effective dose was only 5. 1%-19.3%higher than the absorbed dose for individual dose cycles. Conclusions: Higher peptide dose of 200µg appears to be more suitable than 90 µg in terms of tumor dose, kidney dose, and therapeuticindex for treatment with 177Lu-DOTATOC. [ABSTRACT FROM AUTHOR]