Follicular Lymphoma of the Breast With Secondary Central Nervous System Transformation and Concurrent Metastatic Melanoma: Report of a Rare Case.

Follicular lymphoma (FL) rarely presents in the breast and even less frequently transforms with secondary central nervous system (CNS) involvement. The coexistence of transformed FL with metastatic melanoma is exceptionally rare and presents significant diagnostic and therapeutic challenges. We report the case of an elderly female initially diagnosed with primary breast follicular lymphoma (FL), managed with surveillance. Over subsequent years, she experienced multiple relapses, treated with bendamustine-rituximab and later lenalidomide, achieving remission. Fourteen years after initial diagnosis, she developed CNS symptoms with cerebellar lesions; resection revealed transformed diffuse large B-cell lymphoma (DLBCL) of germinal center origin (CD5+, CD10+, BCL2, BCL6, MYC+, Ki-67 ~70%). Concurrent metastatic melanoma was also diagnosed. Given the rarity of CNS-transformed FL and the absence of standard guidelines, her case was managed as primary CNS lymphoma. She received high-dose methotrexate, rituximab, and temozolomide with Stereotactic Body Radiotherapy (SBRT) for melanoma. Despite significant intracranial progression, she remained largely asymptomatic and prioritized quality of life, highlighting the challenges of individualized care in complex dual malignancies.