ALK Translocation in ALK-Positive Mesenchymal Tumors

Context.-A wide spectrum of mesenchymal tumors harboring ALK gene rearrangements has been identified outside the archetypal example of ALK-positive inflammatory myofibroblastic tumors. Objective.-To evaluate the molecular pathology of unusual ALK-positive mesenchymal tumors and their response to ALK-targeted treatments. Design.-Seven patients with ALK-positive mesenchymal tumors, including inflammatory epithelioid cell sarcoma, undifferentiated sarcoma, histiocytic neoplasm, smooth muscle tumor of uncertain malignant potential (STUMP), and atypical fibrohistiocytic tumor, were included on the basis of aberrant ALK immunoexpression. Patients with inflammatory myofibroblastic tumors were excluded from the study. ALK gene rearrangement was investigated either by fluorescence in situ hybridization or next-generation sequencing. Results.-ALK was immunolabeled in all patients, diffusely (similar to 50%) in 6 patients and partially (10%-50%) in 1 patient. ALK gene rearrangement was discovered in 5 of the 6 available patients. The 30-partners of ALK fusion were identified in 3 of 4 investigated patients as follows: PRKAR1A-ALK (ALK-positive histiocytic neoplasm), TNS1ALK (STUMP), and KIF5B- ALK (ALK-positive atypical fibrohistiocytic tumor). We failed to discover ALK translocation in 1 patient with ALK-positive inflammatory epithelioid cell sarcoma. However, transcriptomic investigation showed that this tumor was significantly enriched with ALK-related pathways, which suggested activation of ALK through a nontranslocation pathway, as a constitutive oncogenic mark in this tumor. ALK-targeted inhibitors, which were administered to 3 patients with metastatic diseases, achieved partial remission in 1 patient with ALKpositive inflammatory epithelioid cell sarcoma and stable disease in patients with ALK- positive undifferentiated sarcoma and STUMP. Conclusions.-Molecular investigation of ALK-positive mesenchymal neoplasms could allow for an accurate diagnosis and personalized treatment.