# A Case of Folliculotropic Mycosis Fungoides Initially Misleading as a Poikilodermatous Rash: The Importance of Gene Rearrangement Testing in Early-Stage Disease (Folliculotropic T-Cell Lymphocytosis)

## TL;DR

A case of early-stage folliculotropic mycosis fungoides was correctly diagnosed using gene rearrangement testing after initial misdiagnosis as a benign skin condition.

## Contribution

Highlights the importance of molecular testing in diagnosing early-stage folliculotropic mycosis fungoides when clinical and histopathologic features are ambiguous.

## Key findings

- The patient's persistent skin condition was found to have clonal T-cell receptor gene rearrangements, indicating lymphoma.
- Early-stage folliculotropic mycosis fungoides was diagnosed based on clinical, histopathologic, and molecular findings.
- Treatment with nbUVB phototherapy and topical corticosteroids was initiated following the diagnosis.

## Abstract

Folliculotropic mycosis fungoides (FMF) is a distinct and often aggressive variant of cutaneous T-cell lymphoma (CTCL) that can mimic benign dermatoses, particularly in its early stages. We present the case of a 42-year-old man with a chronic, progressively worsening, intensely pruritic, scaly eruption involving the lower back, buttocks, and proximal extremities. Despite initial management with topical corticosteroids, his condition persisted and evolved into poikilodermatous patches with follicular accentuation and hyperpigmented plaques, prompting further evaluation. Serial skin biopsies from the left anterior distal thigh, right posterior shoulder, and right inferior medial lower back revealed superficial and deep perivascular and perifollicular atypical lymphoid infiltrates. Molecular studies demonstrated clonal T-cell receptor (TCR) gene rearrangement, raising concern for early-stage cutaneous T-cell lymphoma with folliculotropic involvement. Given the clinical presentation and histopathologic findings, a diagnosis of early-stage folliculotropic mycosis fungoides was made. The patient was initiated on narrowband ultraviolet B (nbUVB) phototherapy three times per week in addition to topical corticosteroids, with close clinical follow-up.