Axillary Breast Cancer Masquerading as Extramammary Paget Disease after Bilateral Total Mastectomy: A Case Report

Mammary Paget disease (MPD) and extramammary Paget disease (EMPD) are distinct conditions typically differentiated by anatomical location: MPD occurs on the breast and is indicative of underlying breast carcinoma, while EMPD arises in apocrine gland-rich areas like the genitals or axilla. MPD is almost always associated with underlying breast cancer, whereas EMPD may occur with or without malignancy. Both present as erythematous, scaly lesions with histopathologic features of Paget cells, necessitating clinicopathologic correlation for diagnosis.

We present a diagnostically challenging case of an erythematous, scaly plaque in the axilla of a 56-year-old female with a history of invasive ductal carcinoma and bilateral mastectomy. Initial findings, including positivity for CK7, CEA, and HER2, suggested EMPD. However, given the proximity to the previously afflicted breast, a wide local excision was performed and revealed tumor cells within mammary epithelium and glands, leading to a final diagnosis of MPD (in situ breast carcinoma involving the skin). The lesion's location in the axillary tail, overlapping with her prior breast cancer site, raised concerns for residual disease. Imaging showed no metastatic disease or pathologic mutations. Clear margins were achieved, and adjuvant radiation was deferred due to her history, limited disease extent, and potential toxicity.

This case highlights the diagnostic challenges of MPD in atypical locations and underscores the importance of clinicopathologic correlation, particularly in patients with a history of breast cancer. It also emphasizes the need for individualized treatment strategies and long-term surveillance, even after mastectomy, to detect and manage rare recurrences. Clinicians must maintain vigilance for atypical presentations of recurrent or residual disease, as breast cancer can manifest unexpectedly years after definitive treatment.