Cytokeratin-positive rib osteosarcoma metastasizing to the small intestine.

Osteosarcoma (OS) is a malignant tumor in which osteoid or bone is produced directly by tumor cells. Some OS cells are positive for cytokeratin (CK) and epithelial membrane antigen by immunohistochemistry (IHC) and this may lead to a misdiagnosis of metastatic carcinoma, particularly when the tumor location is unusual. On the other hand, gastrointestinal metastasis of OS is rare. We present the case of a 67-year-old Japanese man with a small intestinal intussusception due to metastasis of a CK-positive rib OS. The tumor cells were positive for CK, osteopontin and osteonectin by IHC and a diagnosis of a CK-positive chest wall OS metastasizing to the small intestine was considered. Osteoid or bone formation was histologically absent and therefore chest wall OS had to be differentially diagnosed from metastatic carcinoma of unknown origin. A postmortem histological analysis confirmed a rib OS. Awareness of CK-positive OS is important for making a correct diagnosis and for disease management and an immunohistochemical analysis of the tumor for expression of osteopontin and osteonectin may be used to support the diagnosis. In addition, this case shows that rib OS can metastasize to the gastrointestinal tract, albeit rarely, which may induce an intestinal intussusception.