325. Feasibility of a combination of 90Y/177Lu-Radiopeptide Therapy (Peptide Receptor Radionuclide Therapy – PRRT) and External Beam Radiation Therapy (EBRT) in neuroendocrine tumors (NETs)

Abstract

Purpose PRRT with 90Y-DOTATOC and 177Lu-DOTATATE showed up to 35% objective responses in NETs and acceptable toxicity. Given NET indolent behavior, multiple treatments are sequentially applied; EBRT is proposed frequently as oligometastatic/palliative setting. Our aim was to assess whether the combination of PRRT and EBRT (PRRT + EBRT) arise concerns about tolerability. Methods Over our 807 patients receiving PRRT (1997–2014), 17% underwent also EBRT (14% after PRRT, 3% before PRRT). Among these, 25 patients had dosimetry and clinical data of PRRT + EBRT. The primary NETs were pancreas (12), lung (7), others (6), with multiple metastases in liver (13), bone (10), others (11). EBRT was adjuvant in 18 sites, palliative in 22. Bone metastases were treated with 3D conformal EBRT. Oligometastaseswere treated with image-guided IMRT or stereotactic EBRT, accounting for absorbed doses (AD) from PRRT. AD to Organs At Risk (OARs) and tumours were assessed and toxicity investigated (CTCAE v4). For PRRT, OARs were kidneys and red-marrow; AD to the liver was also evaluated. OARs for EBRT depended on the tumour site. Results The table reports the median value (range) of administered activities and absorbed doses to OARs for the PRRT as well as doses delivered with EBRT. Individual metabolism leaded to large AD variability in PRRT (tumour: 1–42 Gy/GBq, 90Y-DOTATOC; 1–56 Gy/GBq, 177Lu-DOTATATE).No severe red-marrow toxicity was observed (14 patients grade I-II; 3 none); no kidney toxicity was shown but in 1 patient (grade I). Median (range) follow-up was 3.5 (0.2–12.3) years. 7 patients died. Conclusions PRRT and EBRT have different OARs, with the great potential to increase AD to tumours without increasing toxicity, especially in red-marrow and liver. This opens the way to future prospective studies. However, EBRT irradiating liver metastases may (rarely) deliver non-negligible AD to kidneys once summed to AD from PRRT. PRRT + EBRT imposes multidisciplinary dosimetry discussion.