c-kit Immunocytochemical Staining in the Cytologic Diagnosis of Metastatic Gastrointestinal Stromal Tumor

BACKGROUND: Gastrointestinal stromal tumor (GIST) is a distinct group of mesenchymal neoplasms recently shown to exhibit differentiation toward interstitial cells of Cajal (ICC). C-kit (CD117), an immunocytochemical marker consistently expressed in normal ICC, is demonstrable in 81-100% of GISTs. We report two cases wherein immunocytochemical staining for c-kit aided in the diagnosis of metastatic GIST in the liver. CASES: Two patients, a 37-year-old female (case 1) and a 76-year-old male (case 2), presented with multiple nodules in the liver. They had a history of small bowel GIST resected 11 and 1 year earlier, respectively. Fine needle aspiration of the liver nodules showed loose aggregates or fascicles of spindle cells with elongated to oval nuclei, rare paranuclear vacuoles and eosinophilic cytoplasm. The spindle cells showed minimal (case 1) to moderate nuclear pleomorphism (case 2), with occasional mitotic figures seen in case 2. Immunocytochemical staining revealed strong and diffuse staining for c-kit; it was negative for actin, desmin, CD34 and S-100 protein. Thus, a diagnosis of metastatic GIST was rendered. Histologic review of the primary small bowel GISTs of both cases and the subsequently resected liver nodules in case 1 confirmed the diagnosis. CONCLUSION: Metastatic GIST may pose diagnostic problems due to its broad morphologic spectrum and variable cytologic atypia; in particular, distinction from leiomyosarcoma and other mesenchymal tumors is difficult. The diagnostic difficulty is compounded when the prior history of gastrointestinal tumor is not available or forgotten and when GIST is the initial presentation of the tumor. C-kit is a highly sensitive and reliable immunocytochemical marker that can aid in the diagnosis.