Rhabdomyosarcoma (RMS) of the orbit

Rhabdomyosarcoma

“A child with sudden, rapidly progressing unilateral proptosis with an orbital mass → think Rhabdomyosarcoma until proven otherwise.”

Malignant tumour of mesenchymal origin showing skeletal muscle differentiation. Most common primary orbital malignancy in children.

Epidemiology

  • Accounts for ~4% of all paediatric cancers.
  • 10% of all RMS cases occur in the orbit.
  • Age: usually 5–7 years (rare after adolescence).
  • Sex: slight male predominance.

Sites of Origin

  • Orbit (most common intraocular adnexal site).
  • Can arise from extraocular muscles, connective tissue, or blood vessel walls.

Histological Subtypes

  • Embryonal (60–70%) – most common in orbit; relatively better prognosis.
  • Alveolar (20–25%) – more aggressive, seen in older children/adolescents.
  • Pleomorphic (rare) – in adults, very aggressive.
  • Botryoid variant – polypoid form under mucosal surfaces.

Clinical Features

  • Rapidly progressive, unilateral, painless proptosis (hallmark).
  • Eyelid swelling, chemosis, orbital mass.
  • Globe displacement (may cause strabismus or diplopia).
  • Decreased ocular motility.
  • Pain is uncommon unless secondary infection or bone invasion.
  • Mimics orbital cellulitis – so a high index of suspicion is needed in children with "sudden proptosis."

 Investigations

  • Imaging
    • CT scan orbit: well-defined, extraconal/intraconal soft tissue mass; may cause bone erosion.
    • MRI orbit: superior for soft tissue extent and intracranial spread.
  • Biopsy
    • Incisional/excisional biopsy confirms diagnosis.
    • Histopathology: small round blue cells with skeletal muscle differentiation.
  • Immunohistochemistry
    • Positive for desmin, myogenin, MyoD1.

 Staging (IRS – Intergroup Rhabdomyosarcoma Study)

  • Stage I: Favourable site (orbit, head/neck, GU tract except bladder/prostate).
  • Stage II–IV: Progressive extension, nodal or distant metastases.
  • Common metastasis sites: lungs, bone marrow, bones, liver.

 Treatment

  • Multimodal approach (no more exenteration as primary treatment).
    1. Chemotherapy – backbone of treatment.
      • VAC regimen: Vincristine + Actinomycin D + Cyclophosphamide.
    2. Radiotherapy – indicated if residual tumor post-chemo.
      • Doses: ~45–50 Gy localized.
    3. Surgery – limited; biopsy or excision if localized and operable.
      • Orbital exenteration only in non-responsive, recurrent cases (rare now).

Prognosis

  • Orbit is a favourable site → survival rates >90% (5-year survival) with modern treatment.
  • Prognosis worse with: alveolar type, older age, metastatic disease at presentation.

 Key Differential Diagnosis

  • Orbital cellulitis
  • Lymphangioma
  • Neuroblastoma metastasis
  • Ewing sarcoma/PNET
  • Lymphoma

 


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