Background: Little is known about the patient characteristics and tumor characteristics associated with a high level of granulocyte colony-stimulating factor (G-CSF) and leukocytosis. Moreover, the prognosis of G-CSF-producing gastric cancer has been extremely poor.
Case Report: A 72-year-old man presented with fatigue and body weight loss. Laboratory testing showed pronounced leukocytosis (white blood cell count, 34900×106/L). Bone marrow aspiration biopsy excluded leukemia and metastatic leukemoid reaction. G-CSF-producing cancer was suspected as the cause of the abnormally elevated serum G-CSF level (293 pg/ml). Gastrointestinal endoscopy showed type 3 gastric cancer, and the biopsy specimens were histologically proven to include moderately to well differentiated adenocarcinoma with positive expression of G-CSF. Abdominal computed tomography showed a lymph node lesion and multiple hepatic metastatic lesions. This patient was diagnosed as having stage IV gastric cancer that produced G-CSF. We treated the patient with 3 chemotherapy regimens, and he survived for almost 2 years after diagnosis.
Conclusions: The possibility of a G-CSF-producing tumor should be investigated in patients who present with severe leukocytosis in the absence of infection. This unusual gastric cancer should be treated as soon as possible after diagnosis.

Keywords: Liver Neoplasms - secondary, Leukocytosis - etiology, Granulocyte Colony-Stimulating Factor - therapeutic use, Bone Marrow - pathology, Antineoplastic Combined Chemotherapy Protocols, Adenocarcinoma - metabolism