Squamous Cell Carcinoma Arising Within a Mycosis Fungoides Plaque: A Case Report.

BACKGROUND: Squamous cell carcinoma (SCC) rarely develops within mycosis fungoides (MF) lesions. Recognizing a new epithelial malignancy within a longstanding MF plaque is important because clinical change may otherwise be attributed to lymphoma progression. CASE PRESENTATION: A 71-year-old man with a 30-year history of MF, no prior systemic therapy, psoralen plus ultraviolet A phototherapy, or topical chemotherapy, and limited prediagnostic narrowband ultraviolet B exposure developed a rapidly enlarging ulcerated mass within a lumbar MF plaque. Histopathology showed moderately differentiated keratinizing SCC adjacent to plaque-stage MF. Immunohistochemistry demonstrated a mixed CD4/CD8 lymphoid infiltrate; Ki-67 labeling was approximately 5% in the MF lymphoid compartment and 40-50% in SCC epithelial cells. Wide local excision with 1-cm margins, bilateral advancement flaps, and full-thickness skin grafting achieved clear margins. RESULTS: Recovery was uncomplicated, and no clinical recurrence was detected at 6 months. The surrounding MF plaques appeared less erythematous and infiltrated, although the significance of this observation is uncertain. CONCLUSION: A rapidly growing, ulcerated, or otherwise changing lesion within longstanding MF warrants prompt biopsy to exclude a second cutaneous malignancy. Compartment-specific interpretation of immunohistochemical findings is essential, and surgical excision can be feasible in MF-affected skin.