Clinicopathologic Characteristics of Solid Papillary Carcinoma of the Breast.

Solid papillary carcinoma (SPC) of the breast is defined as a "distinctive form of papillary carcinoma characterized by closely apposed expansive, cellular nodules." This uncommon tumor frequently demonstrates neuroendocrine differentiation. SPCs are staged as in situ tumors, except those that exhibit jagged borders within desmoplastic stroma, or if accompanied by conventional invasive carcinoma, which is separately graded and staged. In this study, we review the immunomorphologic characteristics of SPCs diagnosed at our institution, with a view toward elucidating points of distinction from other mammary carcinomas that manifest neuroendocrine differentiation. A total of 250 cases of in situ and invasive carcinomas with immunohistochemically documented neuroendocrine differentiation, spanning 12 years, were retrieved from the records of the Department of Pathology, Singapore General Hospital. Morphologic review identified 108 (43.2%) cases of SPC (in situ and/or invasive) among this group of tumors. Tumors with SPC components were significantly associated with estrogen receptor, progesterone receptor, and chromogranin A expression, spindled tumor morphology, and older patient age (above median age). In addition, invasive carcinomas with SPC components were more likely to be of smaller size (≤20 mm), low grade (grade 1), and to occur in older patients (above median age), compared with cases of invasive carcinoma lacking an SPC component. In situ SPCs were significantly associated with mucin production and demonstrated improved disease-free survival over cases of conventional ductal carcinoma in situ with neuroendocrine differentiation. Presence of an SPC pattern is associated with favorable clinicopathologic characteristics, with in situ SPCs demonstrating improved disease-free survival, emphasizing the importance of accurately diagnosing this uncommon tumor.