Loss of myoepithelium is variable in solid papillary carcinoma of the breast

Aims: Reports on the frequency of myoepithelial loss in solid papillary carcinoma (SPC) of the breast, an unusual variant of papillary carcinoma with a solid pattern of expansile growth, have been strikingly contradictory. The aim was to clarify the frequency of myoepithelial loss in cases of SPC diagnosed at our institution. Methods and results: Eleven cases of SPC with available blocks or unstained slides were retrieved from the M. D. Anderson archives or obtained from outside contributors. Immunohistochemistry for smooth muscle actin (SMA) and p63 was evaluated on the circumscribed nests that appeared to be non-invasive by haematoxylin and eosin morphology. Three of the 11 cases (27%) were positive for both SMA and p63 at the periphery of all such foci, whereas eight cases (73%) lacked staining for both myoepithelial markers in at least one focus. Of these eight cases, one was diagnosed with only microinvasion, yet metastatic tumour resembling the circumscribed primary SPC was identified in two ipsilateral axillary lymph nodes. Conclusions: SPC of the breast frequently lacks myoepithelial markers at the tumour-stromal interface in spite of a circumscribed non-invasive appearance. Metastases from such tumours are infrequent, but can occur in cases that lack myoepithelial marker expression by immunohistochemistry.