Central Nervous System Progression in Folliculotropic Mycosis Fungoides.

Background: Central nervous system (CNS) involvement by cutaneous T-cell lymphomas (CTCL) is exceptionally rare and is associated with a poor prognosis. Folliculotropic mycosis fungoides (FMF) is a rare subtype of CTCL and often has more aggressive clinical behavior compared to classic MF. The risk factors for CNS progression in patients with primary CTCL are not well understood. Case Summary: We present the case of a 75-year-old male with a history of Stage IA FMF, who developed progressive neurologic and functional decline over six months. Workup revealed marked eosinophilia, and CSF analysis showed monoclonal T-cell population with positive T-cell receptor (TCR) gene rearrangement. Flow cytometry further showed an atypical CD4+ T-cell population with loss of CD7, consistent with the immunophenotype of patient's cutaneous MF, supporting secondary CNS involvement. An MRI of the brain showed T2 hyperintensity in the pons and right middle cerebellar peduncle, with low signal intensity throughout the bone marrow of the skull, concerning for CNS progression of FMF. The patient was started on high-dose steroids, which led to improvement in eosinophilia. Due to transaminitis, high-dose methotrexate therapy was deferred, and the patient was initiated on intrathecal (IT) chemotherapy with methotrexate and cytarabine. Following five cycles of IT chemotherapy, the patient's neurologic status significantly improved, and CSF analysis showed resolution of the atypical T-cell population. Conclusion: This case highlights the rare occurrence of CNS progression of FMF, even in the absence of lymphatic involvement and with well-controlled skin manifestations, and underscores the role of IT chemotherapy in managing this complication.