Concurrent human epidermal growth factor receptor 2-positive breast cancer with axillary nodal metastasis in a patient with pituitary prolactinoma: a case report.

Background: Managing high-risk human epidermal growth factor receptor 2 (HER2)-positive breast cancer (BC) concurrent with a functioning pituitary adenoma is clinically challenging. Current oncological guidelines lack specific clinical consensus protocols regarding the simultaneous administration of intensive dual-HER2 targeted therapy, tyrosine kinase inhibitors (TKIs), and continuous dopamine agonists. Case Description: A 56-year-old postmenopausal woman with a 5-year history of pituitary prolactinoma presented with a painless left breast mass over a 2-year duration. Her serum prolactin (PRL) was strictly pharmacologically controlled within the normal physiological range (8-15 ng/mL). Diagnostic breast magnetic resonance imaging (MRI) revealed a suspicious 3 mm × 5 mm spiculated nodule [Breast Imaging Reporting and Data System (BI-RADS) 4a]. She initially underwent an excisional biopsy which confirmed invasive carcinoma. Due to intraoperative sentinel lymph node positivity, the tumor's proximity to the nipple, and the patient's explicit refusal of breast conservation, she underwent a left modified radical mastectomy. Definitive surgery demonstrated a pathologically paradoxical finding: a minute 0.8 cm (pT1b) invasive ductal carcinoma accompanied by early axillary lymph node metastasis (pN1a, 2/27 positive nodes). The tumor exhibited high proliferative potential (Ki-67 40%) and parallel HER2 amplification (3+) in both the invasive and the high-grade ductal carcinoma in situ (DCIS) components. Adhering to a multidisciplinary plan while maintaining her cabergoline regimen uninterrupted, she received adjuvant chemotherapy, dual anti-HER2 blockade (trastuzumab and pertuzumab), and radiotherapy. She declined extended adjuvant neratinib due to gastrointestinal toxicity concerns. She remains completely disease-free at 36 months post-surgery with stable pituitary function. Conclusions: This case underscores the ongoing safety, feasibility, and practical compatibility of integrating dopamine agonists with intensive BC therapies. Furthermore, the paradoxical early lymphatic dissemination of a sub-centimeter primary tumor under physiological PRL levels suggests profound local crosstalk between HER2 and PRL receptor (PRLR) signaling. This necessitates vigilant management of even small, highly proliferative lesions in patients with concurrent endocrine comorbidities.