Cutaneous T-Cell Acute Lymphoblastic Leukemia and the Expression Pattern of Terminal Deoxynucleotidyl Transferase Immunostaining in Mycosis Fungoides and Spongiotic Dermatitis

BACKGROUND/AIMS: T-cell acute lymphoblastic leukemia (T-ALL) is an uncommon, aggressive malignancy that rarely presents in the skin and is generally not considered as part of the differential diagnosis by dermatologists and dermatopathologists. We describe an unusual case of T-ALL presenting with folliculocentric, erythematous papules on the face, histologically resembling mycosis fungoides (MF). Immunostaining for terminal deoxynucleotidyl transferase (TdT) was positive in tumor cells, supporting the diagnosis of cutaneous involvement by T-ALL. TdT is a nuclear enzyme expressed by immature lymphoid malignancies, but the expression pattern of this marker is not well characterized in the skin. We aimed to assess TdT staining in skin biopsies with similar-appearing lymphocytic infiltrates. METHODS: We evaluated the immunostaining profile of TdT in a cohort of 23 patients, including 13 cases of MF and 10 cases of spongiotic dermatitis. RESULTS: The lymphocytes in the MF and spongiotic dermatitis cases lacked nuclear staining for TdT. Nonspecific, granular, cytoplasmic staining was observed in a small number of background cells. CONCLUSIONS: TdT may assist dermatopathologists in discriminating malignant infiltrates of T-ALL from other conditions.