Malignant Ectomesenchymoma: Series Analysis of a Histologically and Genetically Heterogeneous Tumor.

Aims: Malignant ectomesenchymoma is a rare pediatric neoplasm with dual mesenchymal and neuroectodermal elements. Mesenchymal component is usually rhabdomyosarcoma, particularly embryonal subtype, whereas neuroectodermal derivatives are frequently a neuroblastic tumor. Ectomesenchymoma manifests in various sites given the wide migration of neural crest cells during development, though the pelvis/perineum is most often involved. Moreover, no unique unifying molecular abnormality has been determined.

Methods: We conducted a retrospective study to analyze the spectrum of ectomesenchymal tumors encountered in our pediatric population. Six patients were identified and data pertaining to patients' demographic, tumor size and site, histologic components with immunophenotypic profile, molecular alterations, treatment, and outcome were collected.

Results: Mesenchymal elements, represented by rhabdomyosarcoma in all instances, were the dominant component in the majority of cases (5/6). Embryonal and alveolar morphology had similar distribution (3/6) and all patients with alveolar subtype harbored the characteristic translocations of this entity. The neuroectodermal component was most often a neuroblastic-like neoplasm (4/6); however, 2/6 cases demonstrated primitive neuroectodermal tumor-like morphology. No unifying alterations were found on molecular studies.

Conclusions: Our analysis extends the histologic and molecular spectrum of these tumors and highlights their heterogeneity. The percentage of cases with alveolar rhabdomyosarcoma or primitive neuroectodermal-like tumor components suggests that these types of elements might be underreported. This study is also the first to demonstrate FOXO1 gene rearrangements in malignant ectomesenchymoma with alveolar rhabdomyosarcoma subtype.