Abstract: 【Abstract】 Objective To investigate clinical and histopathological differences between mycosis fungoides with large cell transformation （MF-LCT） and primary cutaneous anaplastic large cell lymphoma（PC-ALCL）. Methods Clinical and pathological data were collected from 16 patients with MF-LCT and 7 with PC-ALCL in the Department of Dermatology, Xijing Hospital, Air Forth Medical University from January 2015 to November 2022, and retrospectively analyzed. Results Similarities between MF-LCT and PC-ALCL were as follows: plaques were observed in 16 MF-LCT patients and 5 PC-ALCL patients, nodules in 2 MF-LCT patients and 8 PC-ALCL patients, masses in 9 MF-LCT patients and 2 PC-ALCL patients, and surface ulcers in 11 MF-LCT patients and 3 PC-ALCL patients; the CD3+ CD4+ CD8- immunophenotype was identified in 14 MF-LCT patients and 4 PC-ALCL patients; no death was found in both groups during the follow-up period. Differences between MF-LCT and PC-ALCL were as follows: MF-LCT showed older ages at onset （55.38 ± 11.98 years, range: 34 - 80 years） and longer disease courses （6.03 ± 4.86 years, range: 0.5 - 16 years） compared with PC-ALCL （ages at onset: 52.43 ± 24.53 years, range: 16 - 80 years; disease courses: 2.20 ± 2.42 years, range: 0.1 - 6 years）; skin lesions were more widespread and mostly affected the trunk （13 cases） in MF-LCT, while the lower leg was mostly affected （5 cases） in PC-ALCL. Histopathological findings in MF-LCT showed that the epidermotropic phenomenon was more common （15 cases）, Pautrier microabscesses were observed in most patients （9 cases）, tumor cell infiltration could only be observed in the superficial to middle dermis （5 cases） or deep into the subcutaneous fat layer （4 cases）, and eosinophil infiltration was found in all 16 cases; in PC-ALCL, tumor cells infiltrated the whole dermis in all 7 cases, which infiltrated the subcutaneous fat layer in 4 cases, necrosis was observed in 6 cases, and mixed infiltration of abundant eosinophils and neutrophils was found in 6 cases. Conclusions MF-LCT and PC-ALCL can only be distinguished by systematically considering the patient′s age, medical history, sites of lesions, disease outcomes, and histopathological and histochemical characteristics. The epidermotropism of tumor cells was more likely to be observed in MF-LCT.

Key words: Mycosis fungoides, Lymphoma, primary cutaneous anaplastic large cell, Disease attributes, Pathologic processes, Mycosis fungoides with large cell transformation