Certain subtypes of in-situ carcinoma can be more difficult to recognize than others. Solid papillary carcinoma in-situ can show cytological and architectural features that overlap with those of usual ductal hyperplasia, and apocrine proliferations are often challenging due to less familiarity with the criteria for apocrine atypia. Non-classical examples of lobular carcinoma in-situ, such as florid or pleomorphic, may be difficult to distinguish from ductal carcinoma in-situ. The unique histologic features, diagnostic criteria, ancillary immunohistochemical findings, and clinical management of these challenging subtypes of in-situ carcinoma will be reviewed.

The classification of cellular fibroepithelial lesions of the breast can be diagnostically challenging.  In this lecture, we will review histologic features that enable the distinction of cellular fibroadenoma from phyllodes tumor and discuss the criteria for phyllodes tumor grading and current grading controversies. Potential diagnostic pitfalls, such as those related to proliferative stromal changes in adolescent patients, massive stromal overgrowth, architectural mimicry of neoplastic fronds, or aberrant immunohistochemical reactivity for epithelial markers, will also be presented. 

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