Rhabdomyosarcoma

Definition

Malignant tumour of skeletal muscle.

Epidemiology

Males greater than females.

Embryonal rhabdomyosarcoma is the commonest soft tissue tumour in children under 15.

Two-thirds embryonal                    10-20

One-third pleomorphic                   50-70

Classification

1. Pleomorphic or adult
2. Embryonal or childhood
   1. Alveolar
   2. Botryoid
   3. Spindle cell

Pathogenesis

No record of rhabdomyomas transforming into rhabdomyosarcomas.

There is a chromosomal translocation between chromosomes 2 and 13.

There is an association with low levels of p53 tumour suppressor gene.

Clinical 

Mass in soft tissues.

In children the presence of an enlarging intramuscular mass is highly suggestive.

Embryonal rhabdomyosarcoma is more common in the head and neck than the extremities.  It is also found in the genitourinary tract and retroperitoneally.

Pleomorphic rhabdomyosarcoma is more common in the buttock and thigh.

Natural History

Both forms are high-grade malignancies with rapid growth patterns.

The primary site of metastasis is the lung, but unlike other sarcomas lymphatic mets are not rare.

Radiology

Plain

Soft tissue swelling without bony change

Intralesional ossification and calcification is not seen

CT          

Iso-dense with skeletal muscle hence poorly visualized on CT

MRI

Well visualized.  Intramuscular location

Medium density on T1, high density on T2

Isotope scans

There is increased uptake in the early vascular phase (this is the case for virtually all high-grade soft tissue sarcomas).

There is no increased uptake in the late bone phase (there is increased uptake with liposarcomas).

Diagnosis

Gross pathology

Infiltration of surrounding tissues with no distinct capsule.  Readily distinguishable from skeletal muscle.

Firm, grey to pink colour, bulges when cut.

Larger lesions may have extensive areas of haemorrhagic necrosis that may obscure the neoplastic tissue.

Histology

Largely homogeneous lesion with an infiltrative pseudocapsule.

1. Embryonal
   1. Vast majority of cells are small round blue cells clustered together in clumps with no particular pattern.  The clusters of cells are separated by thin wisps of myxoid like matrix.
   2. Well-differentiated cells have an eosinophilic cytoplasm with cross striations.
   3. Poorly differentiated lesions are difficult to tell apart from other small round blue cell tumours and require EM (elctron microscopy), or positive immunohistochemical studies.
2. Alveolar
   1. This is an uncommon variant of embryonal rhabdomyosarcoma.
   2. Composed of well differentiated round cells arranged in an alveolar pattern separated by thick fibrous septae.
3. Botryoid
   1. This is another uncommon variant of embryonal rhabdomyosarcoma.
   2. Grape like clusters of round cells are massed beneath the mucosal lining of hollow organs – particularly the bladder.
4. Pleomorphic
   1. Lesional cells are spindle shaped with marked variation in size and shape.
   2. Bizarre gigantic cells with huge hyperchromatic nuclei are scattered around.  These cells can resemble a wristwatch with the watch representing the nucleus and the band representing the cytoplasm.
   3. Other configurations are tadpoles and belt buckles.

Electron microscopy

Cross fibrils

Immunohistochemistry

Myoglobin, desmin, actin, vimentin

Treatment

Extremities

1. Staging – MRI, CT, plain XR, CXR, CT chest, PET scan, bone scan
2. Neoadjuvant chemotherapy; this makes limb salvage practicable in most cases
3. Closely followed by wide surgical excision
4. If adequate surgical margins are not obtained postoperative radiotherapy is given
5. Postoperative chemotherapy is then continued for up to 2 years
6. Five year survival rate is 80% for stage I and II lesions

Radiotherapy

Definitive radiotherapy provides excellent palliation but is seldom curative.

Prognosis

Botryoid best; then embryonal, pleomorphic and alveolar.