Definition
- Use of ionising radiation to damage DNA in order to prevent cell replication
- Most rapidly replicating cells affected most
- 100 Centigray = 100 Rads = 1 Gy (J/kg)
Types
- Gamma rays
- Radium – now obsolete
- Cobolt 60 – almost obsolete
- Iridium – used occasionally for brachytherapy
- Caesium – used for gynae tumours
- X Rays
- Diagnostic – 50-150 kV
- Deep XR Therapy (DXRT) – 300 kV
- Linear accelerators – 4-24 MV
- Electrons
- Linear accelerators
- Beta Rays
- Electrons given off by ionised material
- Injected locally
- Strotium, Yittrium & Samarium
Mechanism of Action
- Produce free radicals by breakup of H2O
- Need Oxygen for DXRT to work
- Hyperbaric better
- Unoperated bed better
- ? Haematology substitutes
Fractionation
- 1 large dose vs 60 small doses
- Curative DXRT » Fractionation +++
- Palliative DXRT » Minimal Fractionation
Timing
- Preoperative or Postoperative Radiotherapy
- Preoperative radiotherapy is to ↓ size of tumour to improve survival following excision
- There is NO role for preoperative radiotherapy to allow closer margins (ie. if plan wide margins for excision then still need wide margins after preoperative radiotherapy)
Radio-sensitive Tissue
- High turnover tissue
- High blood supply
Technique
Curative
- Maximum dose with acceptable damage to normal tissue
- 60 Gy in 30 fractions over 6 weeks
- Equivalent to 18 Gy in one dose
- Fractionation decreases late effects on normal tissue
- Increases differential between tumour & tissue damage
- Allows repair between treatments
- Usually 2-3 weeks post-op
- Allows wound healing
- Minimises delay as tumour interference activates latent cells
- Careful planning
- Multiple fields
- Minimises normal tissue damage
Palliative
- Shorter course with reduced total dose
- 30 Gy in 10 fractions
- Simple field set ups
- Late morbidity less of issue
- Delays callus formation if pathological fracture
Morbidity
Early
- Erythema
- Dry desquamation
- Ulceration
- Lymphopaenia
- Telangiectasia
- Myelosuppression
- GIT effects
Late
- Fibrosis
- Contracture
- Atrophy
- Lymphoedema
- Osteoradionecrosis
- AVN of femoral head
- Transverse myelitis
- Pulmonary fibrosis
- Soft tissue & Bony hypoplasia (in children)
- Endocrine suppression
- Infertility
- Skin cancer
- Sarcomatous change
Specific Tumours
Osteosarcomas
- No indications pre-op
- Used for
- Unresectable tumours
- Palliation for metastatic disease
Ewings Sarcoma
- Very radiosensitive but low curability
- Effective combined with chemo
- Surgery + Chemo have better results
Chondrosarcoma
- Relatively radioresistant
- Used for
- Recurrence
- Inoperable disease
Myeloma
- Effective for Plasmacytoma
- Chemo + Radio for Myeloma
Soft tissue sarcoma
- Where doubt regarding surgical margins
- NV structures close
- 50 Gy DXRT pre-op
- 10 Gy Brachytherapy post-op