There are several reasons for the reputation that the diagnosis of patch-stage mycosis fungoides holds as a difficult and sometimes impossible diagnosis. One can eliminate mycosis fungoides from the differential diagnosis by making a specific diagnosis of one of these conditions. Unless one is intimately familiar with the clinical and pathologic features of all of the common and most of the uncommon inflammatory skin diseases, it is unlikely that one will be able to discriminate between mycosis fungoides and one of its simulants. One must also master the clinical features of mycosis fungoides and its simulants. This is a far more complex knowledge base than one needs to determine if a cervical biopsy specimen represents carcinoma in situ or not.
Mycosis fungoides can assume nearly every histopathologic pattern seen in inflammatory skin disease.
9 To be effective, a pathologist needs to know the differential diagnoses for each of these patterns. Insufficient emphasis on the patterns that mycosis fungoides and its simulants may have at scanning magnification and on the circumscribed differential diagnoses of each pattern leads to many mistakes. The clinical and pathological variants of mycosis fungoides are many granulomatous, eosinophil-rich, verrucous, atrophic, pustular, bullous, hypopigmented, and hyperpigmented, to name a few. The lymphocytes of early lesions of mycosis fungoides are not cytomorphologically very different from those of its inflammatory imitators. If one requires marked cytologic atypia to make the diagnosis of mycosis fungoides, one will not be able to make the diagnosis of mycosis fungoides in most patch-stage lesions.