An advanced granulocyte-colony stimulating factor (G-CSF)-producing tumor is rare, and it exhibits leukocytosis in association with high serum G-CSF levels. A 67-year-old male with a 1-month history of bloody emesis and black stools was revealed to exhibit leukocytosis, anemia and a high serum concentration of G-CSF. During a gastrointestinal endoscopy, an ulcerating tumor was identified in the stomach. Computed tomography and a fluorodeoxyglucose-positron emission tomography scan demonstrated direct invasion of the gastric tumor into the transverse colon, regional lymphadenopathy, lung nodules and diffuse high uptake of FDG in bone marrow. The histological diagnosis was a G-CSF-producing neuroendocrine carcinoma (NEC) (tumor 4b, node 2, metastasis 1, pulmonary, clinical stage IV). Systemic chemotherapy consisting of cisplatin and irinotecan was started. Common terminology criteria of adverse events grade 3 tumor lysis syndrome and gastric penetration appeared. Grade 4 neutropenia lasted for 10 days despite intensive G-CSF administration. Prominent shrinkage of the primary and the metastatic tumors was observed subsequent to 3 cycles of chemotherapy. Total gastrectomy and resection of the transverse colon were subsequently performed. Systemic chemotherapy was effective for a G-CSF-producing advanced gastric NEC with careful monitoring and appropriate supportive care for severe adverse events.
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