Useful Parameters for Distinguishing Subcutaneous Panniculitis-like T-Cell Lymphoma From Lupus Erythematosus Panniculitis.

Some cases of subcutaneous panniculitis-like T-cell lymphoma (SPTCL) and lupus erythematosus panniculitis (LEP) demonstrate clinical and histopathologic overlap, raising the possibility that they represent opposite ends of a disease spectrum. SPTCL, however, is typically associated with greater morbidity and risk for hemophagocytic lymphohistiocytosis (HLH); therefore, diagnostic distinction is clinically important. We present the histopathologic, immunophenotypic, and molecular findings with long-term clinical follow-up of 13 patients with SPTCL (median, 64 mo follow-up) and 7 with LEP (median, 50 mo follow-up) in our multidisciplinary cutaneous oncology clinic. Six SPTCL patients developed HLH, including 2 under the age of 21 years. In the SPTCL group, 2 of 13 patients died of disease. In contrast, we had no mortality or development of HLH in our LEP cohort. We demonstrate that a limited panel (Ki-67, CD3, CD4, and CD8 immunostains) reveals foci of "Ki-67 hotspots" enriched in cytotoxic atypical CD8+ T cells in SPTCL. Ki-67 hotspots were not identified in LEP, thus aiding the distinction of SPTCL from LEP. Lymphocyte atypia combined with adipocyte rimming of CD8+ T cells within Ki-67 hotspots was also highly specific for the diagnosis of SPTCL. Hyaline lipomembranous change, B-cell aggregates, plasmacytoid dendritic cell clusters, and plasma cell aggregates favored the diagnosis of LEP but were identified in some cases of SPTCL including patients with HLH. We confirm that SPTCL and LEP can show significant histologic overlap, suggest a role for high-throughput sequencing in confirming neoplastic clones, and introduce the concept of SPTCL "Ki-67 hotspots" in evolving disease.