Primary Cutaneous Large B‐Cell Lymphomas: relevance of the 2017 WHO Classification

Abstract

Aims

We applied the 2017 WHO classification criteria to categorize a series of 64 primary cutaneous large B‐cell lymphomas (PCLBCLs), containing a majority (⩾80%) of large‐cells and a proliferative rate ⩾40%, raising the problem of the differential diagnosis between PCLBCLs, leg type (PCLBCLs‐LT) or primary cutaneous follicle center lymphomas with large cell morphology (PCFCLs‐LC). The aims were to determine reproducibility and prognostic relevance of the 2017 WHO criteria.

Methods and results

Morphology and phenotype identified 32 PCLBCLs‐LT and 25 PCFCLs‐LC; 7 cases (11%) remained unclassified. Morphology was less reproducible than immunophenotype. Pertinent markers for the differential diagnosis were MUM1, FOXP1, CD10 and IgM. BCL2 and BCL6 were expressed by both PCFCLs‐LC and PCLBCLs‐LT in substantial percentages. Neither Ki67 nor P63 expression had diagnostic value. MYD88 was only found mutated in PCLBCLs‐LT (n = 22, 69%). Using Hans/Hans modified algorithms, 23 out of 25 PCFCLs‐LC had germinal center (GC) status and the 32 PCLBCLs‐LT, non‐GC status. Overall survival was poorer for PCLBCLs‐LT than PCFCLs‐LC (P=0.0002). Non‐GC cases had poorer overall survival than GC cases (P=0.0007). In PCLBCLs‐LT, MYC expression was associated with cutaneous relapses (P=0.014). Applying GC/non‐GC status for unclassified cases, only a single case remained discordant.

Conclusions

Our results support the 2017 WHO classification criteria for PCLBCL diagnosis. The Hans modified algorithm using CD10 and MUM1 distinguished PCFCLs‐LC from PCLBCLs‐LT with optimal diagnostic value without requiring BCL6 immunolabeling (poor reproducible). Rare unclassified cases may be a provisionally heterogeneous subgroup for which GC/non‐GC status (relevant for prognosis) may guide therapeutic decisions.

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