Last revised by David Luong on 13 Dec 2022

Ectomesenchymomas are biphasic or composite malignant soft tissue tumors with rhabdomyosarcoma resembling parts and neuronal or neuroblastic features.

The term ‘gangliorhabdomyosarcoma’ is no longer recommended.

Ectomesenchymomas are very rare and usually occur in infants and children ≤ 15 years with a slight male predominance 1-3.

The diagnosis of ectomesenchymoma is based on histology and immunohistochemistry.

Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5th edition) 1:

The most common presentation is a superficial or deep soft tissue mass or an incidental finding on imaging studies 1.  

Ectomesenchymoma features a compound biphasic morphology with mesenchymal components resembling embryonal rhabdomyosarcoma and neuroectodermal or neuroblastic elements 1-3.

The etiology is unknown 1.

Ectomesenchymomas have been described in the following locations 1-5:

  • pelvis/perineum

  • intraabdominal and retroperitoneum including urogenital organs

  • head and neck region

  • intracranial

Less commonly affected are the extremities and the mediastinum.

Tumors usually vary widely in size and are multilobulated and encapsulated. They might show features of hemorrhage and/or necrosis and feature a tan to whitish appearance 1.

Ectomesenchymoma feature two histological components:

Ectomesenchymoma is a multi-phenotypic tumor and immunohistochemistry stains are usually positive for muscle-specific actin (MSA), desmin and MYOD1 as well as neuron-specific enolase (NSE) synaptophysin, chromogranin A and S100 1,2.

Imaging findings include a generally non-specific, heterogeneous mass 4,5.

Magnetic resonance imaging features are those of a lobular, heterogeneous, and partially cystic-enhancing mass 2,5.

The radiological report should include a description of the following:

  • form, location and size

  • tumor margins and transition zone

  • relations to other organs

  • contact to bones or neurovascular structures

  • associated lymphadenopathy

Treatment is usually multimodal and based on rhabdomyosarcoma-like management strategies. A superficial location, smaller tumor size and younger age ≤3 years are favorable factors 1,4.

Initial suggestions regarding the entity ectomesenchymoma were made by the American pathologists Samruay Shuangshoti and Martin G Netsky as well as James L. Holimon and William I. Rosenblum in 1971 6,7.

Conditions that can mimic the presentation and appearance of ectomesenchymomas also depend on the location of the tumor and include 3-5:

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