Solid papillary carcinoma of the nipple: an in situ carcinoma or an expansive growth tumor?

Papillary breast lesions are a heterogeneous group of tumors which mainly arise in the central mammary region, ranging from benign to malignant. Among them, solid papillary carcinoma (SPC) represents a very uncommon variant with indolent clinical behavior and excellent prognosis. The categorization of papillary lesions as benign, atypical or malignant is often difficult even for experienced pathologists. Furthermore, for prognostic purposes, to decide whether to consider a lesions as in situ when it is not associated with frank invasive foci of carcinoma may be problematic. We present a case of solid papillary carcinoma arising in the nipple with an expansive and circumscribed growth, mimicking an in situ lesion of the breast on the hematoxylin and eosin stained sections, but in which a myoepithelial layer around neoplastic nodules could not be detected by using immunohistochemistry. To the best of our knowledge, primary origin in the nipple is very rare for SPCs and it has been described only once in the literature. The case we herein illustrate is of interest not only because of its origin in the nipple, but also because of its not in situ, but invasive, although expansive and not infiltrative growth. In the differential diagnosis, nipple disorders as adenoma and syringomatous adenoma, usual ductal hyperplasia (UDH), papilloma, intracystic papillary carcinoma, lobular carcinoma in situ, ductal carcinoma in situ and skin adnexal tumors are considered.