Individual dosimetry and monitoring response to treatment in tandem PRRT with [¹⁷⁷Lu/⁹⁰Y]-DOTA-TATE

Abstract

Introduction: The Duonen project aims to test the efficacy of tandem PRRT therapy of neuroendocrine tumors with DOTA-TATE labeled with 177Lu and 90Y radionuclides compared to conventional mono-radionuclide therapy using 177Lu with a fixed activity of 7.4 GBq. The paper presents the dosimetric procedure used and preliminary results obtained from the treatment of 40 patients and 113 cycles of therapy. Material and Methods: Dosimetry is performed using the QDose program based on 4 SPECT/CT scans and 5 blood samples taken for bone marrow dosimetry. Calculation of the dose to the kidneys is done by segmenting them on images, fitting the corresponding activity versus time (TAC) curves and calculating the dose using the Voxel-S method. The dose to the red marrow is calculated based on the IDAC2 model using the TAC curve for blood obtained from the measurements (self-dose) and the segmented organs from images (cross-dose). In addition, the time-integrated activity coefficients of activity in selected tumors or high uptake areas in four consecutive cycles of therapy were calculated for several patients as a measure of response to therapy. Results: Large variability of individual doses to kidneys is observed. The average specific dose for 177Lu was equal to 0.76 Gy/GBq, SD = 0.29 Gy/GBq, and for 90Y: 3.26 Gy/GBq, SD = 1.26 Gy/GBq. With the standard activity equal to 7.4 GBq in the first cycle, the activity in the next cycle could be increased by > 10% in 60% of cases and had to be decreased in 16% of cases. Conclusion: Individual dosimetry is an important part of PRRT, leading to significant dose modifications in subsequent cycles in more than 80% of the cases studied.