This is a tumor of striated muscle that represents nearly 50 % of soft tissue tumors in children and 10–12 % of childhood cancers. The average age of onset is 6 years. However, the RMS of the limbs are more often encountered in adolescence and are mostly of alveolar type, while head and neck locations are rather embryonic and occur with greater frequency in infants and small children.

A genetic predisposition is reported in the context of neurofibromatosis and Li-Fraumeni syndrome. In the latter, there is a mutation in the suppressor gene p53. The RMS can also be associated with the syndrome Beckwith-Wiedemann with a chromosomal abnormality at the level of the 11p15 region. The RMS is also common as part of the syndrome of Castello. Finally, the consumption of marijuana or cocaine by the mother and also by the father seems to increase the risk of development of a RMS.

The RMS is included among the round cell tumors. The diagnosis and its classification are usually possible by optical microscopy studying the tumor cells and tissue architecture. It may be necessary to resort to immuno-histochemistry to look for muscle markers: actin, myosin, desmin, and myoglobin. The expression of myogenin is also common.

The embryonic type represents almost two-thirds of cases. It takes the appearance of fetal muscle at 7–10 weeks of pregnancy with a rich stroma and fusiform, non-alveolar cells. The preferential location of the embryonic type is the head and neck and genitourinary apparatus. Two variants of this type are described, the botryoidal form and the fuso-cellular form. In the botryoidal form, the proliferation has a polypoid aspect with a sub-epithelial layer of dense tumor cells (cambial layer). The location in this case is naso-pharyngeal, vaginal, or in the urinary bladder, while in the fuso-cellular type, the preferred location is para-testiculaire.

In the alveolar type , representing almost 25 % of the cases, round tumor cells are densely grouped within an architecture resembling the pulmonary alveoli. A variant is described known as “alveolar solid”, in which the alveolar architectural organization is not found, but the genetic study connects it to the alveolar RMS. The usual locations of the RMS of alveolar type are the extremities, trunk, and the perineum.

Undifferentiated sarcoma is made of round cells whose morphological appearance, architecture, and antigenic markers expressions do not classify it. It is a diagnosis of exclusion.

Genetic studies allow us to better characterize the RMS. Thus, the translocation t(2; 13) (q35; q14) is characteristic of the alveolar RMS. This translocation leads to the juxtaposition of the PAX3 gene involved in neuromuscular differentiation and FKHR (or FOXO1a) gene. Testing by PCR, the merging of PAX3 and FKHR genes is very sensitive for the identification of alveolar RMS. Embryonic RMS cases have a recurring loss of heterozygosity 11p15. In addition, the ploidy assessment by flow cytometry shows that hyperdiploidy (>51 chromosomes) has a better prognosis than the diploidy.

The clinical presentation varies according to the anatomical location (Table 13.1). It’s usually an asymptomatic mass sometimes associated with organ dysfunction related to the location of the tumor. Initial metastases, mainly to the lung, have no clinical signs.

In almost 40 % of the cases, the tumor involves the head and neck. The distinct locations are orbital, para-meningeal (nasopharynx, middle ear, para-nasal sinuses, the temporal and pterygo-maxillary fossae), and others (larynx, oropharynx, oral cavity, parotid, cheek, scalp). The orbital location is usually rapidly diagnosed due to the exophthalmos. Metastases are rare in this location. In the para-meningeal locations, patients present with signs of oral or nasal obstruction or symptoms related to the ears. Headache, vomiting, or paralysis of one or more cranial nerve pairs already reflect a brain invasion by infiltration of the base of the skull. In other locations, the tumor often remains localized .

RMS of the members and the trunk is usually a non-inflammatory painless mass increasing in volume. A history of trauma may be found, evoking a hematoma. In some cases with rapid evolution, the tumor may be painful or present signs of inflammation that suggest an abscess. These tumors are often of the alveolar type and have a tendency to locoregional and distant extension .

The other locations are rare and often diagnosed late. The RMS can be retro-peritoneal, intrathoracic, hepatic, perianal, and even more rarely in the biliary tract, brain, breast, ovaries, or the heart. Cases of metastatic RMS whose primary tumor was not found were also reported .

The strategy varies depending on location. As soon as the diagnosis is suspected, a biopsy or excisional biopsy should be envisaged. In cases of cavitary tumors, the pathological examination of tumor fragments, either eliminated or obtained by endoscopy, enables the diagnosis. Any suspicious lymphadenopathy should be biopsied to specify its nature.

Particular care must be given to the clinical evaluation, indicating the tumor mass and clinical signs of metastases particularly in the regional lymph nodes. In the head and neck location, it is recommended to do a specialist ENT consultation and possibly an ophthalmic examination. Radiological examinations are required for a proper assessment of the tumor mass and search for metastasis. This assessment will allow monitoring of response to treatment and a better definition of the tumor mass for a possible local treatment by surgery or radiotherapy. Echography is a good first approach, but in the majority of cases a CT scan is necessary. MRI is a better choice in the locations of the head and neck, at the level of the limbs, and in the case of pelvic tumors. The search for lung metastases is done by radiographs and especially by the CT scan. The ⁹⁹Tc scintigraphy is very sensitive in the search for bone metastases. The place of the PET scan in the initial assessment and to monitor the response to treatment is a subject of study. Although isolated metastases in the bone marrow are exceptional, it is recommended to do this research by myelogram and bilateral bone marrow biopsy even when the blood count is normal.