A New Breast Cancer Arising Beneath an Accessory Nipple

What happened, in summary

A 44-year-old woman noticed a mass at the inframammary fold beneath a supernumerary nipple five years after treatment for a separate left-breast invasive ductal carcinoma. Ultrasound showed an irregular lesion, and biopsy identified poorly differentiated invasive lobular carcinoma. The new tumor expressed estrogen receptor at 5% and progesterone receptor at 5%, while HER2 was negative. PET imaging found no distant metastasis. MRI showed a 2.5 cm mass extending into the left rectus abdominis muscle.

She received neoadjuvant cyclophosphamide and docetaxel, with a favorable response on repeat imaging. A wide local excision removed the cancer together with the accessory nipple. Final pathology showed 3.3 cm of residual invasive lobular carcinoma with negative margins. The residual tumor remained weakly hormone-receptor positive, with ER at 5% and PR at 5% to 10%, and HER2-negative.

Postoperative treatment included capecitabine followed by endocrine therapy. Proton radiation was delivered to the chest wall and regional lymph nodes in 25 fractions, followed by an eight-fraction boost to the tumor bed along the inframammary fold. Two years later, she underwent revision of her breast reconstruction with a DIEP flap. She was described as currently having no evidence of disease. The different histology, receptor profile, and five-year interval supported classification as a new primary cancer in accessory breast tissue rather than recurrence of her earlier triple-negative tumor.