Hyperpigmented Mycosis Fungoides Masquerading as Longstanding Lichen Planus Pigmentosus: A Diagnostic Pitfall

Mycosis fungoides (MF) is a rare primary cutaneous T-cell lymphoma, accounting for 50%-60% of all cutaneous T-cell lymphoma cases. It has a prevalence of approximately 5-6 cases per 1 million people annually and a higher incidence in dark-skinned populations.

We report a case of hyperpigmented MF in a 72-year-old dark-skinned man with a 5-year history of progressive, widespread poikilodermatous patches and thin plaques on the back and bilateral legs. The patient had been treated for lichen planus pigmentosus for 5 years without significant response to therapy.

Multiple biopsies revealed a band-like lymphoid infiltrate in the dermis, accompanied by intraepidermal lymphocytes, some of which had larger hyperchromatic nuclei. CD4 + T lymphocytes were predominant over CD8 + T-positive cells located along the epidermis, dermoepidermal junction, and in the dermis.

A diagnosis of hyperpigmented MF was made based on the clinical, histopathological, and immunohistochemical findings.

This case report highlights the importance of considering hyperpigmented MF as a differential diagnosis in patients with longstanding lichen planus pigmentosus, particularly when there is a lack of response to therapy.