Rare Breast Tumors

The current World Health Organization (WHO) classification recognizes 21 distinct histologic subtypes of breast cancer. The rare subtypes constitute <5% of all breast cancers. The majority of these have been previously recognized with only a few newly described lesions that have been included in the recent edition of the WHO classification of tumors. The relevance of recognizing these rarer subtypes of breast cancer is important due to the prognostic implications. Unique genetic alterations have been described in only a few of these rare histologic types like secretory, adenoid cystic, and more recently in tall cell carcinoma with reverse polarity. These tumors can be diagnosed based on their distinct histological features with aid from ancillary testing. Immunohistochemistry, complemented by molecular studies has become an integral part of diagnosing these rarer cancer types. Information on outcome studies is restricted to small case series and case reports on these rare tumors. As such, clear treatment recommendations are still lacking. Recognizing these unusual subtypes on core needle biopsy specimens can be challenging due to limited histological clues. A lack of familiarity with these rare variants may also complicate definite diagnosis.