Subcutaneous Panniculitis-Like T-cell Lymphoma With Scalp Involvement: A Diagnostic Challenge With Lupus Panniculitis.

Subcutaneous panniculitis-like T-cell lymphoma (SPTCL) is a rare primary cutaneous cytotoxic T-cell lymphoma that may show marked clinical and histopathological overlap with lupus panniculitis. We report the case of a 22-year-old woman with a three-year history of slowly progressive depressed hyperpigmented lipoatrophic plaques involving the thighs, back, and left upper limb, associated with an erythematous alopecic scalp plaque. An initial thigh biopsy showed interface dermatitis, dermal mucin deposition, and lobular lymphocytic panniculitis with positive direct immunofluorescence, leading to a diagnosis of lupus panniculitis. Immunological investigations showed positive antinuclear antibodies and anti-double-stranded DNA antibodies. A second thigh biopsy demonstrated lobular panniculitis with atypical lymphoid cells rimming adipocytes. Immunohistochemistry showed CD3+, CD8+, CD4-, and CD56- cells with positive perforin and granzyme B expression and a Ki-67 index of 40%, consistent with SPTCL. Re-evaluation of the first biopsy supported the same diagnosis. Subsequent biopsy of the progressively enlarging ulcerated scalp lesion confirmed persistent scalp involvement by SPTCL. Oral corticosteroids induced an initial response, but relapse occurred one year later, and methotrexate was added. This case highlights the marked diagnostic overlap between lupus panniculitis and SPTCL and the uncommon scalp localization of SPTCL.