Metastatic HER2-positive breast cancer in a 19-year-old woman following a 4-year diagnostic delay: a case report.

Background: Breast cancer in adolescents and young adults (AYA) under age 30 is exceedingly rare, representing a very small proportion of all breast cancer cases. When it occurs, it demonstrates more aggressive biological behavior with higher rates of advanced-stage disease and poorer outcomes compared to older populations. Although breast cancer remains uncommon in this age group, population-based data demonstrate that the overall incidence of carcinomas among adolescents and AYA has increased steadily in the United States since 1975, with the greatest relative increase observed among adolescents aged 15-19 years, highlighting the importance of maintaining clinical suspicion for malignancy even in the youngest patients. Case Description: A 19-year-old woman presented with a significantly enlarged left breast mass that she had first noticed at age 15 years. Initial clinical evaluation presumed a breast abscess; attempted drainage and antibiotics failed to resolve the lesion. Core needle biopsy revealed estrogen receptor (ER)-positive (70%), progesterone receptor (PR)-positive (50%), human epidermal growth factor receptor 2 (HER2)-positive infiltrating ductal carcinoma with a Ki-67 of 51%. Positron emission tomography/computed tomography (PET/CT) confirmed stage IV disease with osseous metastasis to the manubrium. She received neoadjuvant docetaxel, carboplatin, and trastuzumab with radiation therapy, achieving near-pathologic complete response. At age 21 years, she developed recurrent left breast disease and five brain metastases requiring craniotomy, CyberKnife radiosurgery, and bilateral mastectomy. Despite multiple lines of HER2-targeted therapy including ado-trastuzumab emtansine and fam-trastuzumab deruxtecan-nxki in conjunction with tamoxifen, she continues to experience ongoing disease complications at age 25 years. Germline genetic testing identified only a BRCA1 variant of uncertain significance; no pathogenic mutations were found. Conclusions: This case underscores the critical importance of early clinical suspicion and prompt tissue biopsy for persistent breast masses in young patients, regardless of age or presumed benign etiology. Negative genetic testing should not reduce clinical concern. Breast health education targeting AYA patients and their providers represents a modifiable opportunity to reduce diagnostic delay and improve outcomes.