Superficial ALK-rearranged myxoid spindle cell neoplasms: Clinicopathologic and molecular analysis of two cases and a review of the literature.

Superficial anaplastic lymphoma kinase (ALK)-rearranged myxoid spindle cell neoplasms are a recently identified subtype of cutaneous soft tissue tumors, distinct for their co-expression of CD34 and S100 and characterized by ALK gene rearrangements. Although 72 cases have been reported primarily as isolated case reports, this tumor subtype has yet to be included in the WHO classification of soft tissue tumors, underscoring the need for further study. In this study, we diagnosed two additional cases, both arising in the dermis and subcutaneous tissue. These tumors exhibited characteristic pathological features, including linear or concentric whorl patterns, prominent myxoid and collagenized stroma, mild cellular atypia, and rare mitotic activity. The presence of infiltrative margins and the potential for recurrence after surgery suggest at least locally aggressive clinical behavior. Immunohistochemically, the tumors diffusely expressed S100 and CD34, with strong ALK-D5F3 positivity, confirmed by ALK gene rearrangement. These findings further expand the clinical and pathological spectrum of ALK-rearranged neoplasms and highlight the need for continued research on their biological behavior and classification.