Pseudoneoplastic proliferation of histiocytes with paclitaxel-induced ultrastructural changes in a mastectomy specimen.

A 49-year-old Hispanic woman with a T4N1M0 infiltrating duct carcinoma of the left breast underwent four courses of FAC (doxorubicin 86 mg, 5-fluorouracil 860 mg, cyclophosphamide 86 mg, and dexamethasone 10 mg) adjuvant chemotherapy plus four courses of paclitaxel (Taxol; Bristol-Myers Squibb Oncology, Princeton, NJ) and subsequent mastectomy. The tumor shrunk from 6.5 cm to 2.5 cm after the treatment. The residual tumor in the surgical specimen measured 1.5 cm with eight positive out of 24 axillary lymph nodes. The tumor showed typical chemotherapy changes and a massive proliferation of histiocytes that mimicked a neoplasm. A nodular proliferation of the same cells in one axillary node raised the impression of a second malignant tumor in the breast spreading to the node. The histiocytic cells contained lamellar and coarse periodic acid-Schiff-positive material distending their cytoplasm and they were strongly positive for CD68 and negative for CD1a, pan keratin, and S-100. These findings ruled out histiocytoid carcinoma, granular cell tumor, and Erdheim-Chester disease. The proliferating histiocytes had ultrastructural findings of paclitaxel-induced cytotoxicity with disorganized stacks of intermediate filaments positive for vimentin by immunostains and fewer masses of tubulin. The treated breast carcinoma cells were tubulin-positive but the proliferating histiocytes were tubulin-negative.