Primary cutaneous methotrexate-associated B-cell lymphoproliferative disorders other than EBV-positive mucocutaneous ulcer: clinical, pathological, and immunophenotypic features.

Methotrexate (MTX)-associated B-cell lymphoproliferative disorders (B-LPD) may first present in the skin. Epstein-Barr virus (EBV)-positive mucocutaneous ulcer (EBVMCU) is now a well known disease listed in the 2017 World Health Organization classification. However, primary cutaneous MTX-associated B-LPD (pcMTX B-LPD), other than EBVMCU, appear to be underestimated, and their distinctiveness remains unproven. This study aimed to document the clinicopathological characteristics of nine patients with pcMTX B-LPD that were not EBVMCU to extend our understanding of this peculiar disease. The cohort included three males and six females, with a median age of 74 years (range 54-83 years). All patients were treated with MTX for RA. Of nine patients, four presented with a solitary lesion, and five had multiple lesions. Histologically, five cases showed a polymorphic pattern, and four showed a monomorphic pattern. Immunohistochemically, four cases showed positive EBER staining, and one showed positive CD5 staining. In eight cases, once pcMTX B-LPD was diagnosed, methotrexate was immediately withdrawn. All eight of these patients experienced spontaneous regression and achieved complete remission (CR), without relapse. The patient with CD5 positivity received cytotoxic chemotherapy as the initial treatment. This patient achieved a CR after the initial treatment, but eventually experienced disease relapse resulting in death. We also revealed that pcMTX B-LPD and MTX-associated EBVMCU exhibited similar biological behaviours. We concluded that most pcMTX B-LPD cases could be cured by stopping MTX treatment. We also highlighted the fact that pcMTX B-LPD and MTX-EBVMCU had overlapping features. This finding suggested that pcMTX B-LPD and MTX-EBVMCU might share an underlying mechanism.