Diagnosis: Encapsulated Papillary Carcinoma
• Encapsulated papillary carcinoma is considered by some experts as a low-grade invasive carcinoma with an expansile growth pattern. It is defined as a form of well-circumscribed carcinoma (ductal carcinoma in situ–like in distribution) with papillary architecture lacking myoepithelial cells both in the papillae and at the periphery. Lymph node metastasis without associated conventional invasion (NST) is rare.
• The differential diagnosis includes intraductal papilloma with or without secondary cancerization (ductal carcinoma in situ) and intraductal papillary carcinoma.
• Intraductal papilloma has myoepithelial cells in its papillae and at the periphery. Myoepithelial immunostains must be performed to resolve the differential diagnosis if myoepithelial cells are not clearly identified on hematoxylin-eosin stain or if their presence is in question. Ductal carcinoma in situ may cancerize an intraductal papilloma in part or in whole; this lesion has the same myoepithelial distribution as a papilloma but is lined by carcinoma cells.
• In intraductal papillary carcinoma (whether conventional, cystic, or rarely solid), myoepithelial cells are sparse to none in the papillae and are present at the periphery. Occasionally, the epithelial component may have usual ductal hyperplasia features, and the diagnosis depends on the demonstration of the absence of myoepithelial cells in the papillae only (while being present at the periphery). This form of ductal carcinoma in situ may be associated with conventional invasion.
• Encapsulated papillary carcinoma may be associated with a conventional invasive component (i.e., showing architectural complexity in that component). Rarely, a diffuse aggressive nonlocalized form of this tumor is present that is multifocally invasive; on biopsy, this may be impossible to distinguish from the localized form with a good prognosis.
• When conventional invasion is absent or limited in extent, lymph node metastasis is rare regardless of its association with either encapsulated papillary carcinoma or intraductal papillary carcinoma.
• Encapsulated papillary carcinoma is associated with an expression pattern of invasion-associated markers that is intermediate between ductal carcinoma in situ and invasive cancer. Higher expression levels of matrix metalloproteinases MMP-1 and MMP-9 are seen in encapsulated papillary carcinoma and in invasive carcinoma compared with ductal carcinoma in situ. Expression of matrix metalloproteinases MMP-2 and MMP-7 in encapsulated papillary carcinoma is similar to expression in ductal carcinoma in situ and lower than expression in invasive cancer. These findings support the view of an indolent cancer.
