Abstract

Aims Primary cutaneous marginal zone lymphoma (PCMZL) is considered a lymphoproliferative disorder (International Consensus Classification, ICC) or an overt lymphoma (WHO‐HAEM5). Seeking evidence for a reactive process or true lymphoma, we retrieved recurrent PCMZLs from the French Study Group of Cutaneous Lymphoma (GFELC) database. Methods Histology, phenotype (light‐chain restriction, immunoglobulin, and immune‐receptor translocation‐associated protein‐1 [IRTA1] expression) and B‐cell clonality at diagnosis and recurrence were compared according to recurrence site (local, locoregional, or distant) and outcomes. Results Initial lesions of the 61 patients (mean age 52) were mostly isolated on the trunk (48%) and classified T1 (70%). Times to first recurrence for local, locoregional, and distant recurrences, were 20, 29, and 37 months, respectively. Light‐chain restriction type did not differ significantly between local/locoregional recurrences and distal recurrences ( P = 0.06; n = 60). The same B‐cell clones were identified for 23/42 local/locoregional recurrences, while 5/19 distant recurrences showed different clonal profiles ( P = 0.0003). No tumour expressed IRTA1. Fifty‐eight tumours were heavy‐chain (IgG/IgG4) class‐switched PCMZLs and 3 IgM+/IgD– PCMZLs. All IgM+ tumours underwent either transformation (skin or brain) into diffuse large B‐cell lymphomas (DLBCLs) and extracutaneous spreading. Conclusion As suggested by WHO‐HAEM5, immunoglobulin phenotype assessment (IgM alongside IgD) appears to be a possible valuable tool in the initial diagnosis of PCMZL to differentiate between the indolent class‐switched PCMZL (IgM‐negative) and IgM+ (IgD–) PCMZL, which has an uncertain prognosis. The variation in B‐cell rearrangements and light chain restriction observed in distant recurrences of PCMZL may suggest different antigen‐driven stimulation processes.