[A case of malignant tumor of the ascending part of duodenum with osteoclast-like giant cells].

Extraskeletal neoplasms with osteoclast-like giant cells are very rare. These tumors are most frequently reported in the breast and pancreas, and but rarely in other sites. We report a case of duodenal malignant tumor with osteoclast-like giant cells. The patient was a 76-year-old man who presented with vomiting. Computed tomography, magnetic resonance imaging, and gastrointestinal endoscopy revealed a giant tumor in the ascending part of duodenum. Biopsy specimens showed an undifferentiated malignant tumor with benign multinucleated giant cells. Immunohistochemical staining indicated that the tumor cells were reactive with vimentin, but not with epithelial markers or the other mesenchymal markers, and the multinucleated giant cells were reactive with CD68. Thus, we diagnosed a malignant tumor of the ascending part of duodenum with osteoclast-like giant cells. To the best of our knowledge, this is the first case of duodenal malignant tumor with osteoclast-like giant cells in Japan.