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

Terminology

The term ‘gangliorhabdomyosarcoma’ is no longer recommended.

Epidemiology

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

Diagnosis

The diagnosis of ectomesenchymoma is based on histology and immunohistochemistry.

Diagnostic criteria

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

• composite or intermixed histomorphology of two tumour components:

  • embryonal rhabdomyosarcoma components

  • neuroblastic tumour elements (neurones, ganglioneuroma, ganglioneuroblastoma, neuroblastoma)

• respective immune profile with reactivity to desmin and/or myogenin and synaptophysin

Clinical presentation

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

Pathology

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

Aetiology

The aetiology is unknown ¹.

Location

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.

Macroscopic appearance

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

Microscopic appearance

Ectomesenchymoma feature two histological components:

• skeletal muscle component similar to embryonal rhabdomyosarcoma

• neuroectodermal components as ganglioneuroma, intermediate ganglioneuroblastoma or primitive neuroblastoma

Immunophenotype

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

Radiographic features

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

MRI

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

Radiology report

The radiological report should include a description of the following:

• form, location and size

• tumour margins and transition zone

• relations to other organs

• contact to bones or neurovascular structures

• associated lymphadenopathy

Treatment and prognosis

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

History and etymology

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.

Differential diagnosis

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

• rhabdomyosarcoma

• embryonal rhabdomyosarcoma

• neuroblastoma