GATA3-positivity and T follicular helper phenotype in transformed mycosis fungoides with leukemic involvement and prominent folliculotropism: a case report.

Mycosis fungoides (MF) shows a broad clinical spectrum, ranging from indolent disease to aggressive variants with poor outcomes. T Follicular Helper (TFH) marker expression has been reported in a subset of MF, with uncertain prognostic significance, while GATA3 has emerged as a potential adverse prognostic marker in MF and nodal T-cell lymphomas. Herein, we report a case of MF investigated through sequential histopathological and phenotypic analyses, including extended TFH markers and GATA3 immunohistochemistry. Initially diagnosed as conventional MF, the disease evolved after 3 years into MF with large cell transformation and leukemic involvement, characterized by prominent folliculotropism, a TFH phenotype, and diffuse GATA3 expression. Retrospective analysis of the initial biopsy identified a minor TFH/GATA3-positive neoplastic population (10%) already present at onset, supporting a model of progressive phenotypic plasticity evolving over time. This case underscores the biological heterogeneity of MF, suggesting that a TFH phenotype and GATA3 expression may help identify patients with a more aggressive disease course, potentially amenable to mogamulizumab-based targeted immunotherapy. Incorporating extended TFH markers and GATA3 into routine immunohistochemical assessment could refine risk stratification and inform treatment decisions.