Case Report: A rare triad of neoplasms: navigating synchronous accessory breast cancer, papillary thyroid carcinoma, and inflammatory nasal papilloma.

Background: The differential diagnosis of an isolated axillary mass in a woman with normal breast imaging presents a significant clinical challenge, with accessory breast carcinoma being a critical yet often overlooked entity. The concurrent management of multiple primary tumors further complicates therapeutic decision-making. Case presentation: A 60-year-old woman presented with a 1-year history of a painless, enlarging left axillary mass. Initial imaging revealed a BI-RADS 5 lesion in the left axilla with suspicious lymph nodes, but no primary breast abnormality; subsequent breast MRI confirmed the absence of an orthotopic primary. Core needle biopsy confirmed grade II invasive ductal carcinoma of mammary origin. Subsequent evaluations incidentally identified a Bethesda V thyroid nodule and a benign nasal papilloma. Following six cycles of neoadjuvant TAC chemotherapy, she underwent sequential surgeries: prophylactic ipsilateral nipple-sparing subcutaneous mastectomy with axillary lymph node dissection, endoscopic nasal tumor resection, and total thyroidectomy with central compartment dissection. Final pathology confirmed primary accessory breast carcinoma (ypT3N2, Luminal B), papillary thyroid carcinoma (pT1aN1a), and squamous epithelial papilloma of the nasal cavity. Adjuvant treatment included locoregional radiotherapy, endocrine therapy (letrozole plus abemaciclib), and radioiodine ablation. Conclusion: This case underscores that primary accessory breast cancer must be a leading consideration in the differential diagnosis of an isolated axillary mass with negative standard breast imaging. It also illustrates a pragmatic, multidisciplinary strategy for the integrated management of synchronous primary tumors, guided by the principles of oncologic prioritization and sequenced intervention.