Superficial Anaplastic Lymphoma Kinase ( <i>ALK</i> )-Rearranged Myxoid Spindle Cell Neoplasms: A Case Report and Literature Review

Anaplastic lymphoma kinase (<i>ALK</i>)-rearranged myxoid spindle cell tumors represent a recently recognized subgroup of kinase fusion-positive mesenchymal neoplasms. These tumors often arise in the superficial soft tissues but may occasionally be found in deeper tissues, or, rarely, within visceral organs. The term <i>superficial ALK-rearranged myxoid spindle cell neoplasms</i> (SAMS) comprises a distinct subset of cutaneous soft tissue tumors with superficial location and spindle cell morphology, exhibiting bland cytomorphology and favorable clinical outcomes. <i>ALK</i>-rearranged mesenchymal tumors with high-grade histologic features have been described, but their relationship to SAMS is unclear. Immunohistochemically, SAMS express ALK, CD34 and frequently S100. On the molecular level, they classically harbor <i>ALK</i> gene rearrangements. While <i>ALK</i> fusions are encountered in a variety of neoplastic entities, including inflammatory myofibroblastic tumors and epithelioid fibrous histiocytomas, SAMS are distinct in both morphology and immunophenotype. Since the initial description of SAMS by Dermawan et al in 2021, several single reports and series have been reported, expanding the clinicopathologic and molecular spectrum and highlighting the clinical and morphological heterogeneity within the group of <i>ALK</i>-rearranged mesenchymal tumors.