Although the “Alibert–Bazin” (often referred to as “classical” in many texts) type of mycosis fungoides (MF) is typically characterized by progressive stages of patches, plaques, and tumors, as was initially described in 1876, patients with MF commonly demonstrate a chronic, indolent clinical course over years, possibly decades, and many patients in fact do not demonstrate a progression beyond the plaque stage. A “premycotic” period often precedes a definite diagnosis of MF, during which patients may have nonspecific, chronic, finely scaling lesions that may wax and wane over years, which may result in nondiagnostic biopsies. These lesions can also clinically resemble common skin conditions such as eczema, psoriasis, or drug reactions and thus may be misdiagnosed early in the course of the disease (see next section: “Diagnosis and Differential Diagnosis”). For these reasons, patients with early stages of MF presenting with these nonspecific dermatoses can have lesions for some length of time before a definitive diagnosis can be made.

Early MF lesions commonly present as scaly patches that are variable in size, shape, color, pigmentation (hyper- and hypo-), and degree of epidermal atrophy. These lesions can be pruritic and preferentially affect the buttocks and truncal areas [1]. Women frequently display the first signs of MF in their buttocks, thighs, and breasts. Patches may evolve into plaques, which have a more generalized distribution, and annular or arciform configuration. Plaque lesions also have a more infiltrated reddish-brown appearance.

A small percentage of patients will progress to developing firm, violaceous nodules and/or exophytic tumors, which can undergo necrosis and ulceration. Lymphadenopathy may be appreciated on physical exam in this stage. Extensive tumors and indurated plaques affecting the face may result in prominent brows and leonine facies. These lesions can also be found in uncommon sites such as the oral and genital mucosae [2]. In the late stage disease, confluence of lesions may result in erythroderma.

Generalized erythroderma represents more advanced disease, in which patients can have generalized, severe skin involvement with intense pruritus and scaling. Cutaneous manifestations in the tumor and erythroderma stages of MF can be intensely symptomatic. Luckily, the majority of patients do not progress to have this advanced cutaneous disease. Of note, erythroderma is also a characteristic feature of the aggressive leukemic cutaneous T cell lymphoma (CTCL) variant, Sézary syndrome (SS) . In rare cases, SS may follow classic MF. However, SS is considered separate from MF [3]. Erythrodermic MF is distinguished from SS insofar as the former has absent/minimal blood involvement, whereas SS is characterized by circulating atypical T cells (Sézary cells).

MF has been said to mimic more than 50 different clinical entities [4]. The differential for the scaling patches and plaques seen in MF is broad. The lesions in MF commonly resemble other skin disorders such as eczema, psoriasis, parapsoriasis, superficial fungal infections, photodermatitis, and drug reaction s—although a thorough examination of histologic features and skin findings can generally exclude these diagnoses. A solitary MF lesion can resemble nummular eczema, lichen simplex chronicus, erythema chronicum migrans, and tinea corporis [5]. In addition, tumorous lesions in MF can mimic those of other cutaneous lymphomas. Erythrodermic MF can resemble generalized atopic dermatitis, erythrodermic psoriasis, and contact dermatitis.

For these reasons and more, diagnostic algorithms have been generated to facilitate an accurate diagnosis of MF. The International Society for Cutaneous Lymphomas (ISCL) has proposed a point-based algorithm for the diagnosis of early MF, which incorporates clinical, histopathologic, molecular biologic, and immunopathologic criteria as outlined in Table 4.1 [5].