Rhabdomyosarcoma

Rhabdomyosarcoma is a malignant tumour with skeletal muscle cell morphology. It is one of the tumours of muscular origin. 

This article focuses on a general discussion of rhabdomyosarcomas. For location specific details, please refer to:

• rhabdomyosarcomas of the biliary tract
• rhabdomyosarcomas of the genitourinary tract
• rhabdomyosarcoma of the heart
• rhabdomyosarcomas of the head and neck
• rhabdomyosarcomas of the orbit

Epidemiology

Rhabdomyosarcomas are the most common soft tissue tumour in children and account for 5-8% of childhood cancers ^6-7, and 19% of all paediatric soft tissue sarcomas ⁷. 

In general, they are found in young patients, less than 45 years of age ⁶, with ~65% diagnosed in patients under 10 years old ⁷. 

There is a slight male predilection (M:F 1.67:1 ⁷) with Caucasian children affected more often than children of other races.

Clinical presentation

Clearly, clinical presentation will vary depending on the location of a tumour (see below); however, in general, rhabdomyosarcomas are rapidly growing masses. They cause localised pressure effects on neurovascular structures and have a predilection for infiltrating bones ⁶. Pathological fractures could therefore occur.

These tumours can occur anywhere, and not necessarily where the skeletal muscle is normally found. In children and adolescents, they occur predominantly in the head, neck and pelvis.

Distribution 

Rhabdomyosarcomas are found essentially anywhere in the body ^4,7:

• head and neck: ~50% *
  • orbit: ~20%
  • oropharynx/nasopharynx, palate: ~15%
  • sinuses, mastoid, middle ear: ~15%
• genito-urinary: ~25%
  • paratesticular: ~20%
  • bladder: ~5%
• extremities: ~15%
• other: ~10%
  • trunk and thorax: 7%
  • gastrointestinal tract: 1%

*: see note on figures/percentages

Pathology

Rhabdomyosarcomas are thought not to arise from skeletal muscle, but rather to differentiate into a tumour which resembles skeletal muscle ⁷. This accounts for it arising in locations where no skeletal muscle is present. It is divided into three subtypes ^6-7,7:

1. embryonal rhabdomyosarcoma
   • spindle cell rhabdomyosarcoma: 50-66%
   • botryoid rhabdomyosarcoma: 5-10% (best prognosis)
   • anaplastic rhabdomyosarcoma
2. alveolar rhabdomyosarcoma: 20%
3. pleomorphic rhabdomyosarcoma: 5%

Associations

Although the vast majority of cases are sporadic, increased incidence of rhabdomyosarcomas is seen in patients with a variety of syndromes and congenital anomalies, including ^7,8:

• neurofibromatosis type I (NF1)
• Beckwith-Wiedemann syndrome
• Li-Fraumeni syndrome
• DICER1 syndrome
• Costello syndrome
• maternal use of cocaine and marijuana

Staging

Please refer to Rhabdomyosarcoma staging.

Radiographic features

Unfortunately, the appearance of the mass itself is non-specific and indistinguishable from other sarcomas. The location and demographics of the patient are most useful in narrowing the differential.

Plain radiograph

Although entirely non-specific plain films are a useful first step as they can give a quick global view of the region and identify calcifications in the mass, bony involvement and metastases. The mass appears of soft tissue density. 

When present in the extremities in children, embryonal rhabdomyosarcomas may cause bowing of the adjacent long bones. This should not be thought of as suggesting slow growth or indolent behaviour ⁶.

Ultrasound

• heterogeneous well-defined irregular mass of low to medium echogenicity

CT

• soft tissue density
• some enhancement with contrast
• adjacent bone destruction is seen in over 20% of cases ⁶

MRI

Signal characteristics include:

• T1
  • low to intermediate intensity, isointense to adjacent muscle
  • areas of haemorrhage are common in alveolar and pleomorphic subtypes
• T2
  • hyperintense
  • prominent flow voids may be seen particularly in extremity lesions ⁷
• T1 C+ (Gd): shows considerable enhancement

Embryonal rhabdomyosarcomas tend to be more homogeneous, whereas alveolar and pleomorphic rhabdomyosarcomas frequently have areas of necrosis ⁶. The latter is associated with ring-like enhancement ⁶. 

Treatment and prognosis

Unfortunately, up to 20% of patients have metastases at the time of diagnosis ⁷. These are typical to lung and bone marrow.

Treated with combination surgery, chemotherapy, and radiation:

• surgery: resection of a primary tumour, if necessary after down-staging chemoradiotherapy
• chemotherapy: common agents include vincristine, cyclophosphamide, dactinomycin, adriamycin, ifosfamide, VP-16
• radiotherapy: external beam radiation is used in some cases of rhabdomyosarcoma 

Survival varies dependant on primary location, histological type, local invasion and metastases. Overall 5-year survival is approximately 75% ⁷.