Syringotropic Mycosis Fungoides: A Rare Form of Cutaneous T-cell Lymphoma Enabling a Histopathologic "Sigh of Relief".

Syringotropic mycosis fungoides (STMF) is a very rare variant of cutaneous T-cell lymphoma. It follows a much milder disease course than its clinically indistinguishable adnexal counterpart, folliculotropic mycosis fungoides (FMF). We report a case of a 36-year-old man who presented with erythematous, studded papules and plaques on the left upper extremity and right anterior thigh diagnosed as mycosis fungoides (MF) Stage 1A on initial superficial shave biopsy. Lesions recurred after initial improvement with narrow-band ultraviolet light therapy demonstrating a concentration of abnormal lymphocytes around eccrine sweat glands on repeat biopsy consistent with STMF. Although the deeper, periadnexal infiltrate found in both STMF and FMF confers increased resistance to skin-directed therapies effective in classic MF, these entities diverge with respect to their clinical behavior. Syringotropism is a marker for increased disease-specific survival, whereas even FMF carries a prognosis worse than conventional MF. Increased awareness among the dermatopathology community of the histopathologic distinction between STMF and FMF is essential to guide treatment type, duration, and intensity in adnexal disease.