Invasive Lobular Carcinoma Arising in Ectopic Breast Tissue: A Case Report

Incomplete regression of the embryonic mammary line occurs in 0.3-6% of the population. Ectopic breast tissue is mostly asymptomatic and can undergo malignant transformation. Ectopic breast cancer accounts for 0.2-0.6% of all breast cancers. Screening breast examinations can miss these lesions due to their location making the diagnosis more challenging. We describe a case of a primary invasive lobular carcinoma in an ectopic breast on the left axilla detected in a 49-year-old woman. Firstly diagnosed as a sebaceous cyst, the lesion was excised under local anesthesia. Histopathology showed breast tissue widely infiltrated by an invasive carcinoma. Excision of the remnant tissue with axillary lymph node dissection was performed. Ectopic breast carcinoma is a rare diagnosis and there is a general lack of awareness. The presence of an abnormal mass along the mammary ridge should raise clinicians' attention. Management of primary ectopic breast carcinoma should be based on a multidisciplinary approach under the same principles as breast cancer. Furthermore, it does not appear to bring a worse prognosis when diagnosed at similar disease stages.