Ductal carcinoma in situ arising within a fibroadenoma in a 21-year-old woman: A case report.

Carcinoma arising within a fibroadenoma is exceptionally rare, particularly in young women, and may be underestimated in lesions classified as probably benign. We describe a 21-year-old woman with a 3 cm palpable breast mass categorized as Breast Imaging Reporting and Data System category 3, for which ultrasound-guided vacuum-assisted excision was performed due to lesion size and patient anxiety. Histopathology revealed low-grade cribriform ductal carcinoma in situ completely confined within a fibroadenoma, without invasion, consistent with malignant transformation of the epithelial component. A 113-gene germline panel identified no pathogenic variants, supporting a sporadic etiology of the disease. The patient subsequently underwent nipple-sparing mastectomy with sentinel lymph node biopsy and immediate reconstruction; final pathology confirmed pure ductal carcinoma in situ with clear margins and negative nodes. This case underscores the diagnostic limitations of imaging-based risk stratification in very young women with large category 3 fibroepithelial lesions and highlights vacuum-assisted excision and oncoplastic mastectomy as valuable options in individualized management.