Mycosis fungoides in Taiwan shows a relatively high frequency of large cell transformation and CD56 expression.

Mycosis fungoides (MF) is an indolent cutaneous T-cell lymphoma and may transform into large cell lymphoma in the disease course. The incidence of MF in Taiwan is lower as compared to that in the West. In this study we aimed to characterise the clinicopathological, immunohistochemical, and genetic features of transformed MF (t-MF) in Taiwan. We retrospectively collected MF cases from April 2004 to April 2015 from four medical centres in Taiwan, reviewed the clinical history and histopathology, and performed immunohistochemistry, in situ hybridisation for EBV (EBER), and fluorescence in situ hybridisation (FISH) for DUSP22/MUM1 gene translocation. Fifty-one specimens from 32 patients with MF were identified with a male to female ratio of 1.5:1 and a median age of 50.5 (range 16-82). Tumours from 11 patients (34%) underwent large cell transformation, with the median age at 61 (range 26-82). The tumour cells of t-MF expressed CD30 and MUM1 in 82% and 100% cases, respectively. CD56 was expressed in two (10%) of 21 MF cases and two (18%) of 11 t-MF cases, respectively; and all four CD56-positive cases were of a helper T-cell phenotype. All CD56 expressing MF and t-MF tumours tested for EBER were negative. FISH study showed rearranged DUSP22/IRF4 in one (9%) of 11 t-MF cases, but not in any of the 19 non-transformed MF specimens. Four patients with t-MF died of disease and six were alive with disease in a median follow-up time of 25 months (mean 44.7 months). Large cell transformation and aberrant CD56 expression were more frequent in patients with MF in Taiwan compared to those in the West. Larger case series and/or national studies are needed to clarify the significance and impact of large cell transformation on the prognosis of patients with MF.