Unexpected presentation of accessory breast cancer presenting as a subcutaneous mass at costal ridge: a case report

BACKGROUND: During embryogenesis, bilateral thickening of ectoderm from anterior axillary folds to inguinal folds, called mammary ridges or milk lines, develops into breast tissues. Only a pair in the pectoral area is spared from regression and continuously develops into normal breasts. Accessory breasts can result if the regression process is incomplete. These ectopic breasts can change physiologically and pathologically similar to normal breasts. Unsurprisingly, they are capable of turning malignant. Reported cases show the most common area for accessory breast cancer to be the axillary area. We report a rare case of accessory breast cancer over the costal ridge. CASE PRESENTATION: We present the case of a 51-year-old Asian woman who complained of an enlarged mass lower to her left breast developed over the period of 3 months while on contraceptive pills. Unaware that the mass could be an accessory breast, the primary doctor had prescribed oral contraceptives. After our patient had noticed that the mass was obviously growing, she decided to consult a surgeon as the mass continued to grow. Expected to be benign, the mass was investigated by ultrasonography and then excised surgically. A pathology report identified the mass to be a carcinoma of the ectopic breast tissue. However, sentinel lymph node biopsy showed no nodal metastasis of all four lymph nodes. Following surgery, she received chemotherapy, radiation, and antihormonal treatment. After 2 years of postoperative follow up, she remained free of disease. CONCLUSIONS: This patient was diagnosed as having accessory breast cancer which presented with a subcutaneous mass. As this condition is exceptionally rare but curable by early treatments, we recommend surgeons to consider potential malignancy when patients present with a subcutaneous mass.