ALK Expression is a Rare Finding in Mycosis Fungoides.

Mycosis fungoides (MF) is the most common form of cutaneous T-cell lymphoma. Although indolent in its early stages, MF is more aggressive in advanced stages, particularly if large cell transformation occurs. Histologically, transformed MF is similar to primary cutaneous anaplastic large cell lymphoma (pcALCL) and nodal ALCL (nALCL). MF had not been associated with immunohistochemical expression of anaplastic lymphoma kinase (ALK) until we previously reported a case documenting the presence of ALK by immunohistochemistry in a patient with stage IA MF and concurrent nALCL. This is a unique finding due to the fact that ALK positivity is usually found in nALCL. The goal of this study is to examine the extent to which ALK may be expressed in the different morphologic lesions of MF. One hundred three biopsies from 96 patients, including the patient from the original case, with confirmed MF representing various lesion stages and subtypes, were tested for ALK by immunohistochemical staining. Out of the 103 biopsies, only the original index case of Stage IA MF with concurrent nALCL expressed ALK. From these results, we conclude that ALK is not routinely present in MF in any lesion stage or variant type. Routine staining of ALK is therefore of no additional benefit for the workup of known early stage MF; however, its use may be beneficial in the initial workup of unspecified large cell lymphomas of the skin.