Case report of invasive lobular carcinoma with pineal metastasis.

A 59-year-old woman was diagnosed with luminal-type invasive lobular carcinoma (ILC) of the breast, cT3N3bM0, cStage IIIC. After neoadjuvant chemotherapy, she underwent mastectomy and axillary lymph node dissection, and pathological examination revealed ypT3N0M0, ypStage IIB. She subsequently received postmastectomy radiation therapy and endocrine therapy. Nine months after surgery, she developed multiple bone metastases, followed one month later by headache and diplopia. Brain magnetic resonance imaging revealed lesions in the occipital lobe, cerebellum, and pineal gland. Endoscopic biopsy confirmed pineal metastasis of ILC with subtype conversion to triple-negative breast cancer. Whole-brain radiation therapy with local boost irradiation was administered, followed by systemic chemotherapy. Comprehensive genomic profiling revealed high tumor mutational burden. This case highlights the clinical value of biopsy for atypical brain metastases in guiding therapeutic decision-making.