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Case ReportsAbstract
Subcutaneous panniculitis-like T-cell lymphoma (SPTCL) is a rare cytotoxic T-cell lymphoma that primarily involves subcutaneous fat and often mimics benign panniculitis, creating significant diagnostic challenges. Early recognition is critical due to its potential for systemic involvement. We report a 64-year-old woman with a three-month history of a progressively enlarging, tender soft tissue mass on the right upper arm, followed by a second well-defined, painful nodule (~3 cm) on the posterior arm. Ultrasound revealed an ill-defined subcutaneous lesion measuring 5.5 × 5.3 × 1.4 cm with increased vascularity. Surgical exploration uncovered three discrete subcutaneous nodules grossly resembling fat necrosis.Histologic evaluation demonstrated lobular panniculitis with fat necrosis and prominent granulomatous inflammation that partially obscured an atypical lymphoid infiltrate of small- to medium-sized lymphocytes. Immunohistochemistry showed diffuse CD3, CD43, and CD8 positivity, scattered CD56 and CD4 positive cells, and rare PAX-5–positive B-cells. Ki-67 indicated a high proliferative index. T-cell receptor beta gene rearrangement confirmed clonality, establishing the diagnosis of SPTCL. This case highlights a rare histologic pitfall in which granulomatous inflammation masks SPTCL, mimicking benign panniculitis. A high index of suspicion, coupled with immunophenotyping and molecular testing, is essential for timely and accurate diagnosis.