Radiation dosimetry in 177Lu-DOTATATE therapy - Inter and intra patient variability

1376 Objectives Peptide Receptor Radionuclide Therapy (PRRT) with 177Lu-DOTATATE is used to treat neuroendocrine tumors (NETs). Post-treatment images together with blood samples can be used to estimate radiation doses. Cumulative dose to tumor and other organs may guide decisions for personalized therapy, with repeat treatments performed to maximize tumor dose without exceeding threshold dose to critical organs (kidneys and bone marrow). However repeated imaging and image analysis are demanding, and results are not always exact. We estimated radiation dose in 78 treatments to review the role of image-based dose estimation. Methods Radiation dose was calculated for tumors, kidneys, liver, spleen and red bone marrow, using whole organ VOI’s drawn on post-therapy SPECT/CT and blood samples, following 78 treatments in 36 patients (pts) who underwent 1-4 treatments each. Dose to tumor was approximated by the beta dose, while OLINDA dose factors were used in dose estimation for other organs. Results Tumor and organ doses are summarized in the table. Variation was high, and was greater between pts than over serial treatments in the same pt. Average linear trend showed dose in tumor and all organs decreasing in successive treatments. Since 177Lu-DOTATATE binds to somatostatin receptors in NETs, decreasing tumor dose indicates response to therapy. Imaging after each treatment alerted us to absence or reversal of the downward trend in certain pts, suggesting tumor regrowth and changes in other organ uptake; in addition, doses to critical organs approached the limit after only 2 treatments in a few pts. Conclusions Variation in radiation dose, both between pts and over serial treatments, suggests that dose estimation from post-therapy images after each treatment can maximize the potential of PRRT,ensuring high tumor dose while not exceeding accepted bounds for dose to critical organs. The trend to lower doses over serial treatments requires validation on a larger pt sample. Research Support USIEF Fulbright