P13 The cellulitis that wasn't: a case of subcutaneous panniculitis-like T-cell lymphoma in a toddler.

Subcutaneous panniculitis-like T-cell lymphoma (SPTCL) is a rare cytotoxic T-cell lymphoma of subcutaneous adipose tissue. It is exceptionally uncommon in children, with fewer than 100 cases reported worldwide and very few under five years. Clinically, it may mimic cellulitis, panniculitis, or lupus erythematosus profundus, leading to diagnostic delay. Diagnosis relies on clinicopathological correlation, as histological features overlap with reactive and autoimmune panniculitides, and confirmation often requires molecular studies and expert review. Paediatric cases generally have a more favourable prognosis than adults. We report a two-year-old girl who developed an erythematous, indurated nodule on her left forearm, initially resembling an insect bite. The whole arm soon became markedly swollen and firm, though she remained systemically well. It was treated as cellulitis but did not respond to antibiotics. Multidisciplinary assessment excluded autoimmune disease, atypical infection, and primary immunodeficiency. Histopathology showed a lymphohistiocytic panniculitis with atypical CD8-positive cytotoxic T-cells rimming adipocytes, high Ki-67, and absence of B-cells or plasma cells. T-cell clonality studies confirmed clonal TCR beta and gamma rearrangements, supporting a neoplastic process, although TCR-beta staining was repeatedly negative. Despite this, the findings favoured SPTCL. Staging investigations did not show systemic involvement. Treatment with high-dose prednisolone and ciclosporin led to complete remission. Both were successfully tapered without relapse. SPTCL should be considered in children with panniculitis nodules unresponsive to antibiotics, but distinction from lupus erythematosus profundus and infection remains a critical diagnostic pitfall. Accurate diagnosis and management require multidisciplinary input, highlighting the importance of an MDT approach in such complex paediatric cases.