Giant cell tumor of soft tissue of the breast: Case report with H3F3A mutation analysis and review of the literature.

Giant cell tumors of soft tissue (GCT-ST) arising in the breast are extremely rare. We report a unique case of breast GCT-ST coincident with ductal carcinoma in situ (DCIS), diagnosed with histological, immunohistochemical, and H3F3A (Histone H3.3) mutation analyses. A 59-year-old woman preoperatively diagnosed with DCIS underwent total mastectomy for a cystic mass. Histology revealed a tumor composed of mononuclear cells interspersed with numerous osteoclast-like giant cells, resembling giant cell tumor of bone (GCT-B), with apocrine DCIS in proximity to the tumor. The mononuclear and giant cells were immunoreactive for CD68 and negative for cytokeratins. Granulomatous diseases, carcinomas with giant cells, and giant cell-type sarcomas were excluded by histological and immunophenotypic features. Lack of H3F3A mutation eliminated the possibility of GCT-B metastasizing to the breast. These findings were consistent with GCT-ST of the breast. To our knowledge, this is the ninth reported case of breast GCT-ST, but the first case that accompanied DCIS or involved H3F3A mutation status investigation. For correct diagnosis of this rare tumor, it is important for pathologists to raise the possibility of GCT-ST when encountering giant cell-rich breast lesions and to exclude other differential diagnoses by combining the results of histological, immunohistochemical, and genetic analyses.