Abstract: 【Abstract】 Objective To investigate clinicopathological features and prognosis of transformed mycosis fungoides （TMF）. Methods A retrospective analysis was performed on clinicopathological data collected from 24 patients with TMF, as well as on flow cytometry results of 16 peripheral blood samples obtained from 11 of the 24 patients, who visited Hospital of Dermatology, Chinese Academy of Medical Sciences between 2014 and 2020. Results Among the 24 patients, 11 were males and 13 were females. Their average age at diagnosis of TMF was 50.0 years （range: 18 - 77 years）, and patients with early-stage TMF （9 cases） and tumor-stage TMF （15 cases） were aged 44.8 and 52.6 years on average, respectively. The average time interval from diagnosis of MF to large cell transformation was 3.7 years, and 8 patients were diagnosed with TMF at the initial visit. Histopathologically, large cells infiltrated in a diffuse pattern in 20 cases, as well as in a multifocal pattern in 4, and the proportion of large cells in 7 cases was greater than 75%. Immunohistochemically, 18 patients showed positive staining for CD30, and the proportion of CD30-positive large cells was greater than 75% in 9; negative staining for CD30 was observed in 6. Flow cytometry of 16 peripheral blood samples showed the presence of cell subsets expressing clonal T cell receptor （TCR）-vβ in 2 of 4 patients with early-stage TMF and 10 of 12 with tumor-stage TMF, and tumor cells with higher forward scatter than normal lymphocytes were detected in 16 samples. During the follow-up, among the patients with early-stage TMF, 3 progressed to tumor-stage TMF 3.3 years on average after large cell transformation, 1 progressed to erythrodermic MF in stage IIIA, and the other 4 still showed an indolent course; among the patients with tumor-stage TMF, 1 progressed to stage-IV TMF, and 5 died 3.3 （1.5 - 6） years after large cell transformation. Conclusion Large cell transformation may occur in patients with MF in any stage, some patients have poor prognosis, so close follow-up is needed for patients with TMF.

Key words: Mycosis fungoides, Cell transformation, neoplastic, Skin manifestations, Pathologic processes, Diagnosis, Antigens, CD30, Large cell transformation