Aim To determine the frequency, pattern and distribution of stromal keratin expression in phyllodes tumours if any, which may impact diagnostic approaches.

Methods The clinicopathological features of 109 phyllodes tumours comprising 70 (64.2%) benign, 30 (27.5%) borderline and nine (8.3%) malignant grades were evaluated, and the immunohistochemical expression of a keratin panel (MNF116, 34βE12, CK7, CK14, AE1/3, Cam5.2), p63 and CD34 in their stromal component was assessed.

Results There was focal and patchy cytoplasmic keratin staining in 1–5% of stromal cells in 13 (11.9%), 24 (22%), 31 (28.4%), 2 (1.8%), 9 (8.3%) and 2 (1.8%) cases for MNF116, 34βE12, CK7, CK14, AE1/3, Cam5.2, respectively. CD34 was expressed in 79 (72.5%) cases. There was no stromal staining for p63. Stromal MNF116, 34βE12 and Cam5.2 reactivity was significantly associated with phyllodes tumour grade (p=0.027, p=0.034, p=0.009 respectively), while MNF116 stromal staining was observed in tumours with increasing cellularity (p=0.036), necrosis (p=0.015) and cystic change (p=0.048). Contrary to common understanding, these findings confirm that stromal cells in phyllodes tumours can sometimes express keratins, albeit focal and in a patchy distribution. In comparison, fibromatosis and dermatofibrosarcoma were uniformly negative for the same keratin panel, while spindle cell components of eight metaplastic carcinomas expressed at least two or more keratins in a wider distribution of up to 90% of positively stained spindle cells. All eight spindle cell sarcomas were negative for keratins.

Conclusion The use of keratins as an adjunctive immunohistochemical diagnostic tool in the differential work-up of spindle cell tumours of the breast has to be interpreted with caution especially on limited core biopsy material.