Mimicry in Cutaneous Malignancy—Rare Forms of Mycosis Fungoides as Diagnostic Pitfalls: A Narrative Review

Mycosis fungoides (MF) is a rare primary cutaneous T-cell lymphoma (pCTCL) that generally has an indolent course with a favorable prognosis. However, numerous clinical variants have been described that differ substantially from classic Alibert–Bazin MF, resulting in altered prognosis, treatment response, and patient outcomes. This narrative review considers rare MF variants—bullous, ichthyosiform, hypopigmented, folliculotropic, poikilodermatous, granulomatous, granulomatous slack skin, pagetoid reticulosis and syringotropic MF—with emphasis on practical diagnostic approaches for clinicians. Given that MF can mimic more than 50 different dermatoses and is frequently associated with prolonged diagnostic delay, we provided detailed clinical and dermoscopic features that should raise diagnostic suspicion and guide biopsy decisions. We discussed extensive differential diagnoses for each variant and highlighted MF’s status as dermatology’s “great imitator.” Additionally, we addressed the risk of second primary malignancy in patients with MF, as well as the genetic and microenvironmental factors proposed to contribute to its clinical heterogeneity. Furthermore, we evaluated existing classification systems and suggested future directions that integrate molecular data and tumor biology to improve prognostic assessment and guide therapeutic decision-making.