Rhabdomyosarcoma

Rhabdomyosarcoma, often abbreviated RMS, is a malignant tumour of skeletal muscle.

General

• Most common paediatric sarcoma.
• ~6% of all childhood cancer.

Classification

Histologic

1. Alveolar rhabdomyosarcoma.
   • Usually young adults/adolescents.
   • Early mets common.
   • Usu. arises in regions with skeletal muscle.
2. Embryonal rhabdomyosarcoma.
   • Usual <10 years old.
   • Typically locally invasive.
   • Usu. arises in regions without skeletal muscle.

Molecular and histologic

1. Translocation-positive alveolar RMS.
2. Translocation-negative alveolar RMS.
3. Embryonal RMS.

Notes:

• Translocation-negative alveolar RMS shares characteristics with embryonal RMS.

Microscopic

Alveolar rhabdomyosarcoma:

• Alveolus-like pattern:
  • Fibrous septae lined by tumour cells.
    • Space between fibrous sepate may be filled with tumour: solid variant of alveolar rhabdomyosarcoma.
• Eccentric nucleus (???).
• Cytoplasm - dense pink staining on H&E (if well differentiated).
• Usu. nuclear pleomorphism +++.
• Mitoses common.

Molecular diagnostics

Alveolar rhabdomyosarcoma

Common translocations (~80%):

• t(1,13).
  • PAX7/FKHR fusion gene.
• t(2,13).
  • PAX3/FKHR fusion gene.

Notes:

• Several uncommon translocations exist.

IHC

• Desmin (best marker).
• Actin.

See also

• Small round cell tumours.
• Soft tissue lesions.
• Pediatric pathology.