Pustular mycosis fungoides has a poor outcome: a multicentric clinicopathological and molecular case series.

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Mycosis fungoides (MF) usually has an indolent course. However, some patients develop more aggressive disease and few prognostic parameters have been identified. Isolated cases of pustular MF (pMF) suggest an unfavourable prognosis. To describe the clinicopathological characteristics and prognostic value of pMF. We retrospectively collected data from all patients with MF with histological pustules diagnosed from 2009 to 2020. The outcomes and clinicopathological characteristics of pMF at diagnosis (pMFD) were compared with those of a cohort of patients with nonpustular MF (NpMF). Thirty-three patients with pMF (including 22 with pMFD) and 86 with NpMF were included. Median age at diagnosis of pMF was 61 years [interquartile range (IQR) 50-75]. The median duration of follow-up for patients with pMFD was 32 months (IQR 14-49). Clinically, 33% of patients with pMF had pustules. Large cell transformation (LCT) occurred in 17 patients. Patients with pMFD had significantly more advanced-stage disease and showed more LCT at diagnosis than those with NpMF [50% vs. 7% (P < 0.001) and 23% vs. 0% (P < 0.001), respectively]. On multivariate Cox analysis, the presence of histological pustules at diagnosis was associated with shorter overall survival (OS) in all patients [hazard ratio (HR) 13.90, 95% confidence interval (CI) 2.40-79.00); P = 0.003] and in patients with early-stage disease (HR 11.09, 95% CI 1.56-78.82; P = 0.02). In multivariate Fine and Gray model analysis, pMFD was associated with a higher cumulative incidence of LCT (subdistribution HR 13.90, 95% CI 2.43-79.00; P = 0.003) in all patients. Median OS after the occurrence of histological pustules during follow-up in all patients with pMF was 37 months, with a 5-year OS rate of 25% (95% CI 0.06-0.50). pMF often follows an aggressive course, with a high risk of LCT and shorter survival, even for patients with early-stage disease. Histological pustules at diagnosis of MF might represent an independent poor prognostic factor, to be confirmed by further studies. As pustules are not always identified clinically, pustules found on histology should be mentioned in MF pathology reports and should prompt discussion of closer follow-up.