An atypical panniculitis

A 33 year old Caucasian male presented with a 6‐week history of purpuric tender nodules, first appearing on his lower legs. Over time, the nodules increased in size and number over his thighs, trunk and later, his face and neck. Initially, he had no associated symptoms and at this time diagnoses such as an atypical erythema nodosum, lupus panniculitis, and infections were considered. Incisional biopsies were taken from the thigh, for histopathology and bacterial, fungal and mycobacterial culture. He also had numerous blood tests and a chest X‐ray. Whilst awaiting conclusive results, the patient became unwell with fevers and a productive cough, precipitating admission for further investigation and management. The biopsy revealed a diffuse lobular panniculitis, characterised by an atypical dermal and subcutaneous lymphohistiocytic infiltrate, with a lymphocytic small and large vessel vasculitis. There were also poorly formed tuberculoid granulomata, more conspicuous in the dermis than the subcutis. The infiltrates of small atypical lymphocytes were interspersed with occasional larger, blastic lymphocytes and histiocytes with bubbly cytoplasm. Some of the histiocytes appeared phagocytic, but this was not a predominant feature. There was no polarisable foreign material and special stains excluded various micro‐organisms. This was supported by negative microscopy and culture of the fresh specimen. Immunocytochemistry demonstrated the diffuse lymphocytic infiltrate was >90% T cells, with majority CD4+. 10% of the T cells were CD56+ and there was no staining with Epstein Barr virus latent membrane protein or CD30. Lymphocyte gene rearrangement studies showed no clonal band on the TCR gamma/delta rearrangement. Based on these findings, it was not possible to either confirm or deny a definite subcutaneous T‐cell lymphoma. The histopathology findings favoured an atypical lymphocytic lobular panniculitis, which usually follows an indolent course. However, further staging revealed lymphomatous infiltrates in both lungs and bone marrow, and flow cytometry confirmed this as a T/NK cell non‐Hodgkin lymphoma, with extensive cutaneous and extra‐nodal involvement.