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Case Reports

Abstract

Mycosis fungoides (MF) rarely metastasizes to the CNS, a phenomenon requiring careful consideration of its clinicopathologic features. Here, we report a case of a 75-year-old male patient with CD30+ large cell transformed MF who developed CNS involvement in the setting of limited cutaneous disease. The patient was originally diagnosed with CD4+ folliculotropic MF and achieved a complete response on Brentuximab. Nearly 6 years after primary diagnosis, the patient developed dysarthria, facial droop, and a left temporo-occipital lesion on PET imaging. High-dose Decadron was administered for two days prior to stereotactic biopsy, which showed gliosis, increased small lymphocytes, abundant CD68+ macrophages, and prominent loss of myelin. Although rare CD30+ cells were present, no atypical CD30+, CD4/CD8 double negative population was identified, and therefore an inflammatory/demyelinating process was favored. Approximately 3 weeks later, the patient re-presented to the emergency department with altered mental status. MRI showed enlargement of the prior biopsied lesion and multiple new lesions involving the right parietal and occipital lobes. Meningeal biopsy at this time showed involvement by atypical CD4/CD8 double-negative, CD30+ T cells. Targeted sequencing showed the patient's known TNFAIP3 frameshift mutation, NRAS Q61H, and a frameshift mutation in B2M. Despite blood TCRB sequencing remaining negative, meningeal involvement by the patient's MF was confirmed by the same method. This case highlights the significance of recognizing the post-treatment diagnostic challenges in identifying CNS involvement of MF, an important consideration for timely and accurate diagnosis, and presents a unique mutational profile that may influence metastasis to the CNS.