New Annular, Indurated Plaques During Mogamulizumab Therapy in Cutaneous T-Cell Lymphoma: A Clinicopathologic Pitfall

New annular, indurated plaques arising during mogamulizumab therapy for cutaneous T-cell lymphoma (CTCL) present a diagnostic challenge, as they may raise concern for treatment-related eruption or disease progression. We report two patients with darker skin types who developed plaques that were morphologically distinct from their baseline presentations during mogamulizumab therapy. These morphologic changes may be subtle, manifesting as increased induration, textural change, or plaque thickening, and can be easily overlooked if not assessed in the context of prior disease morphology. Despite clinical changes suggestive of alternative pathology, histopathologic evaluation in both cases demonstrated persistent or progressive mycosis fungoides. Immunophenotyping and T-cell receptor gene rearrangement studies further supported lymphoma involvement rather than drug-induced eruptions. These cases illustrate clinicopathologic discordance, in which changes in clinical morphology do not reliably reflect the underlying pathology. Accurate classification requires a structured, algorithmic approach incorporating repeat biopsy, histopathologic assessment, immunophenotyping, molecular studies, and comparison with prior clinical and pathologic findings over time as well as appropriate restaging. Recognition of this diagnostic pitfall, particularly in patients with darker skin types, is essential to guide appropriate management during mogamulizumab therapy.