Value of Individualized Dosimetry in Peptide Receptor Radionuclide Therapy (PRRT) by Lutetium-177 [177lu – Dota0, Tyr3] Octreotate

Abstract

Response and toxicity prediction is essential to the implementation of Peptide Receptor Radionuclide Therapy (PRRT) for neuroendocrine tumors. Radiolabelled somatostatin analogue [177Lu-DOTA0, Tyr3] octreotate stands as a promising therapy tool. Specific dosimetry is a crucial factor in patient treatment planning. In our Institution, neuroendocrine tumor treatment, by radiopeptide infusion via intrahepatic arterial catheterization, is a well established technique. Kidney protective agents are also included in our protocol. The individualized patient dosimetry calculations were based on planar and SPECT scintigraphy images. Counts were determined in a region of interest (ROI) around the tumor, liver, kidneys and spleen. In planar technique, the ROIs were drawn in both anterior and posterior images while in SPECT counts measured per slice. For counts conversion to activities, calibration factors were calculated. Planar and SPECT images of cylindrical water - filled phantom, with five different known amounts of activity, were obtained. Corrections for scatter attenuation, collimator efficiency and detector response were calculated. Absorbed doses were calculated using MIRD formalism and S values were calculated using the RADAR system. The absorbed doses to organ per unity administered activity were comparable for both planar and SPECT techniques. On average, the absorbed dose was in tumor [4-40] mGy/ MBq, in kidneys[0.25-1.05] mGy/ MBq, in the spleen [0.3-2.1] mGy/ MBq and in the liver [0.05-0.34] mGy/ MBq. In order to deliver higher dose to tumor and avoid kidneys and red marrow toxicity, accurate individualized dosimetry is obligatory. Furthermore, the results quantitatively confirm the therapeutic efficacy of transhepatic administration and introduce 177Lu labeled peptide as an ideal for peptide receptor radiotherapy