When Cellulitis Is Not Cellulitis: A Case of Suspected Mycosis Fungoides With Nondiagnostic Biopsies.

Mycosis fungoides, the most common subtype of cutaneous T-cell lymphoma, is characterized by an indolent course and a wide spectrum of clinical presentations that frequently mimic benign inflammatory dermatoses. Early-stage disease is particularly challenging to diagnose, often requiring repeated clinicopathologic correlation due to nonspecific histopathologic findings. We present the case of a 64-year-old female with a several-year history of diffuse pruritic skin lesions who was admitted for acute right lower extremity pain, erythema, and swelling concerning cellulitis. While her acute symptoms improved with antibiotic therapy, dermatologic evaluation revealed chronic, widespread hypopigmented and scaly patches and plaques, raising concern for underlying cutaneous T-cell lymphoma. Multiple prior and current skin biopsies demonstrated subacute spongiotic dermatitis without definitive evidence of lymphoma, and immunophenotypic studies, including flow cytometry, were nondiagnostic. This case highlights the diagnostic challenge of suspected early-stage mycosis fungoides, particularly when histopathologic findings are inconclusive and clinical features overlap with common conditions such as cellulitis. The coexistence of an acute inflammatory process may further obscure recognition of an underlying cutaneous lymphoproliferative disorder. Recognition of persistent, atypical, or treatment-refractory dermatologic findings should prompt continued evaluation and consideration of repeat biopsy. This case underscores the importance of maintaining clinical suspicion and integrating longitudinal clinical and pathologic data in the diagnosis of cutaneous T-cell lymphoma.