Carcinoma in situ involving adenosis (both ductal carcinoma in situ and lobular carcinoma in situ) is a difficult diagnosis to make on core needle biopsy and therefore must be forefront in the mind of the diagnostician.

Awareness of the lesion and the characteristic morphology presented herein can aide in rendering the appropriate diagnosis and providing recommendations for management of this localized lesions.

When present, the recognition of additional foci of carcinoma away from the adenosis is a helpful feature.

Importantly, the presence of stromal desmoplasia does not preclude the diagnosis of carcinoma in situ involving adenosis.