Radiotherapy

The thymus is located in the anterior mediastinum and is surrounded laterally by the lungs which are sensitive to radiotherapy. The spinal cord is located posteriorly which limits the use of simple AP/PA techniques when doses exceed 45 Gy. A combination of anterior oblique fields is usually employed with an additional beam to give adequate dose to the PTV.

Adjuvant Radiotherapy

Pre-Simulation

  • Discuss with surgeon the location of any positive margins, residual macroscopic disease, and whether there are clips marking these locations
  • Informed consent
  • Consider dietician referral

Simulation

  • Simulate prone with arms by side
  • Immobilisation with kneefix and headrest
  • No bolus required
  • CT scan from hyoid to umbilicus
    • Contrast may provide improved delineation of great vessels from residual tumour

Planning

  • Macroscopic residual disease = GTV. 1 cm expansion to form CTV60.
  • Microscopic areas of risk + 1 cm = CTV50 (post-operative bed, sites of positive margins)
  • PTV50/60 = additional 5 mm expansion (department dependent)
  • Dose:
    • Microscopic disease only: 50.4 Gy in 28 fractions
    • Macroscopic disease: 50 Gy in 25 fractions with 10 Gy boost to sites of macroscopic tumour remnant
  • Typically a 3 field technique will suffice
  • Organs at risk:
    • Lung - Mean dose < 20 Gy, V20 < 30%, V30 < 20%
    • Spinal cord - Max dose < 45 Gy

Outcomes

  • Improved local control (absolute 30-40%)
  • Likely improved survival but no randomised evidence
  • Early effects: Fatigue, skin reaction, oesophagitis
  • Late effects: Cosmetic skin changes, oesophageal stricture, radiation pneumonitis, fibrosis of lung, rib fracture, second malignancies, myelopathy

Other radiotherapy techniques

Primary radiotherapy uses similar dose to adjuvant therapy but the entire tumour is dosed to the maximum (60 Gy).
Chemoradiotherapy is delivered with carboplatin/etoposide and dose constraints should be lower.


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