Answer: Ductal carcinoma in situ, solid papillary type

Histology: This is essentially a sclerotic lesion with rounded epithelial aggregates. These rounded aggregates represent ducts containing fibrovascular cores, indicating that this is solid papillary proliferation. Nuclei within these nodules are monotonous, consistent with DCIS. Many of these cells have abundant intracellular mucin as demonstrated by mucicarmine stain. The cells also label for neuroendocrine markers, chromogranin and synaptophysin.

Discussion: Infiltrating ductal carcinoma is suggested by the somewhat irregular architecture of the lesion; however, the presence of hyaline sclerosis and the rounded nature of the epithelial nests in this lesion suggests that it is not an invasive process. The lesion also demonstrated immunoreactivity for the myoepithelial markers p63 and actin throughout, essentially excluding invasion. The presence of intracellular mucin does suggest lobular carcinoma in situ; however, even the presence of signet ring cells is not diagnostic of LCIS. The present lesion demonstrates greater cohesiveness and greater cytologic atypism than would be seen in LCIS. Atypical ductal hyperplasia is a cytologically monotonous epithelial proliferation that is confined to a terminal duct lobular unit, and does not usually extend into the major ducts. ADH typically measures less than 3 mm, which is significantly smaller than the present lesion.

The present lesion is an example of what has been described as solid papillary type ductal carcinoma in situ. Some suggest that this in situ lesion may represent the preinvasive phase of invasive mucinous carcinomas of high cellularity which may show endocrine differentiation.