Focal Anomalous Expression of Cytokeratin and p63 in Malignant Phyllodes Tumor: A Comparison With Spindle Cell Metaplastic Carcinoma

Differentiating between malignant phyllodes tumors and metaplastic spindle cell carcinomas could be problematic, especially on core biopsies. Immunohistochemical staining for cytokeratin cocktail and p63 has been utilized to differentiate between these tumor types. Forty-three phyllodes tumors (27 benign, 6 borderline, and 10 malignant) and 22 metaplastic carcinomas, consisting at least 80% of spindle cells, were identified. At least 4 tissue blocks from each phyllodes tumor were subjected to immunohistochemical staining for cytokeratin cocktail and p63. The immunohistochemical profiles for the spindle cells in metaplastic carcinoma were reviewed. Phyllodes tumor was diagnosed in the younger age group (mean age 41 y) with a larger tumor size (mean size 6.6 cm), compared with metaplastic spindle cell carcinoma (mean age 62.7 y, mean size 3.4 cm). Focal expression (5% of the tumor cells) of cytokeratin cocktail and p63 was identified in the stroma of 2 of 10 malignant phyllodes tumors in a scattered/patchy pattern. The stroma of benign and borderline phyllodes tumors was negative for these markers. In metaplastic spindle cell carcinomas, cytokeratin cocktail was negative in 2 of 15 cases and very focally positive in another 3 cases, whereas p63 was negative in one case and focally positive in another case. There can be anomalous, focal expression of cytokeratin and p63 in the stroma of malignant phyllodes tumors, whereas metaplastic spindle cell carcinoma can occasionally have cytokeratin and/or p63-negative staining or have very focal positivity. Caution should be exercised when relying on these markers for confirming a diagnosis, especially on core biopsies.