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Ectomesenchymoma

Ectomesenchymoma is an exceedingly rare multiphenotypic sarcoma consisting of both mesenchymal and neuroectodermal lines of differentiation. This biphasic neoplasm is composed of areas resembling rhabdomyosarcoma intermixed with variable neuronal/neuroblastic components.

Symptoms & Causes

Introduction

Ectomesenchymoma is a rare, dual-component soft tissue sarcoma featuring both mesenchymal and neuronal differentiation, primarily affecting children.

Reference
WHO Classification of Tumours Editorial Board. Soft tissue and bone tumours [Internet]. Lyon (France): International Agency for Research on Cancer; 2020 [cited 2024 09 11]. (WHO classification of tumours series, 5th ed.; vol. 3). Available from: https://tumourclassification.iarc.who.int/chapters/33.

Related Terminology
Not recommended: gangliorhabdomyosarcoma.

Subtype(s)
None

Symptoms

Tumors present as a superficial or deep soft tissue mass.

Localization

The common sites include pelvis/perineum, urogenital organs, and intra-abdominal or retroperitoneal soft tissue; less commonly, the head and neck, extremities, and mediastinum are affected.

Epidemiology

Ectomesenchymomas are exceedingly rare, with approximately 50 cases reported to date if only composite rhabdomyosarcomas and neuroblastic tumors are included. Most patients are infants or children in the first two decades of life (range: birth to 15 years), frequently aged < 5 years, and there is a slight male preponderance (M:F ratio: 1.38:1).

Etiology

Unknown

Diagnosis & Treatment

Staging

Ectomesenchymoma is staged according to the American Joint Committee on Cancer (AJCC) staging system for rhabdomyosarcoma or the Union for International Cancer Control (UICC) TNM system.

Pathogenesis

Cytogenetic and array comparative genomic hybridization findings of ectomesenchymoma show overlap with those of embryonal rhabdomyosarcoma, with common trisomies of chromosomes 2, 8, and 11. Frequent HRAS mutations at codon 13 or 61 and upregulated myogenesis-related genes, such as MYOG, CHRND, and MRLN, have been demonstrated, further underscoring the shared alterations seen in embryonal rhabdomyosarcoma.

Macroscopic Appearance

The masses are multilobulated and thinly encapsulated, with a tan cut surface and variable necrosis and hemorrhage. The tumors can vary widely in size (range: 3–18 cm), with most being > 5 cm in diameter.

Histopathology

The prototypical morphology is that of an embryonal rhabdomyosarcoma with intermixed neuroectodermal elements. The latter cover the entire spectrum of neuroblastic phenotype, ranging from scattered ganglion cells to mature ganglioneuroma, intermediate ganglioneuroblastoma, and primitive neuroblastoma. The appearances of embryonal rhabdomyosarcoma vary, spanning botryoid, classic fascicular spindle cell with alternating myxoid-cellular areas, and the so-called dense pattern of primitive round cells. Immunophenotypically, ectomesenchymoma is a multiphenotypic tumor, with the rhabdomyosarcomatous component expressing MSA, desmin, MYOD1, and myogenin, whereas the neuroblastic component is highlighted by NSE, synaptophysin, and chromogranin A, and the Schwannian or satellite cells by S100. Pure rhabdomyosarcoma cases with aberrant expression of neuroendocrine markers should not be considered ectomesenchymomas.

Cytology

Not clinically relevant

Prognosis and Prediction

Patients treated with multimodality strategies including rhabdomyosarcoma-based protocols have a comparable outcome to patients with embryonal rhabdomyosarcoma. Favorable prognostic factors include age ≤ 3 years, size < 10 cm, and superficial location. Localized stage and the presence of a primitive neuroblastic element do not appear to have a substantial impact on outcome.

Clinical Features

Diagnostic Molecular Pathology

Not clinically relevant

Essential and Desirable Diagnostic Criteria

Essential: composite morphology with areas mostly resembling embryonal rhabdomyosarcoma, intermixed with unequivocal neuroblastic elements (neurons, ganglioneuroma, ganglioneuroblastoma, or neuroblastoma); immunoprofile highlighting the two components, with positivity for desmin/myogenin and synaptophysin, respectively.

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