Unusual paraneoplastic thrombocytosis in small bowel neuroendocrine tumor responding to ¹⁷⁷Lu-DOTATATE

Neuroendocrine tumors (NETs) are often associated with paraneoplastic features including carcinoid syndrome in up to 30% to 40% of cases. However, paraneoplastic thrombocytosis is a rare NET-associated condition. We report the case of a 59-year-old woman referred for significant and unexplained weight loss. Biological evaluation revealed thrombocytosis (900 × 10⁹/L). A thoracoabdominal and pelvic computed tomography (CT) scan revealed multiple liver and bone lesions, suggestive of metastatic disease. Colonoscopic evaluation identified a well-differentiated NET as the primary tumor. Hematologic explorations revealed no underlying hematological condition explaining thrombocytosis. Cold somatostatin analogue treatment was started and resulted in initial stabilization of the disease. However, follow-up CT scan showed liver progression after 2 years concomitant with worsening thrombocytosis. ⁶⁸Ga-DOTATOC positron emission tomography showed intense uptake of all known tumoral lesions; the patient was then treated with ¹⁷⁷Lu-DOTATATE. Under ¹⁷⁷Lu-DOTATATE treatment, platelet counts gradually decreased until normalization 8 weeks after the first injection, remaining within the normal range for up to 18 months, in parallel with clinical improvement. In conclusion, we report a rare case of paraneoplastic thrombocytosis associated with a small bowel NET showing rapid biological resolution from the first cycle of ¹⁷⁷Lu-DOTATATE.