Definition
Use of ionising radiation to damage DNA to prevent cell replication
Mechanism
Most rapidly replicating cells affected the most
Radiosensitive tissue
- high turnover tissue
- high blood supply
Give DXRT in incremental radiation
Produce free radical by breakup of H2O
- H2O -> H+ + OH-
- free radicals denature DNA
- damage DNA so that cell can't divide
- cell then dies without dividing
Particles
- bigger particle produces greater damage
- different particles have different RBE (Radiobiological effectiveness)
- each particle works the same way- free radical production
Both tumour & normal cells injured
- normal cells recover
- tumour dies
Need O2 for DXRT to work
- hyperbaric better
- unoperated bed better
Types
1. Photons
2. Gamma Rays
Go through body / not absorbed as particles
- Cobolt 60 - Almost obsolete
- Iridium - Occasionally used in Brachytherapy
- Caesium - Used for Gynaecology tumours
- Radium - Now obsolete
3. X-Rays
- diagnostic 50-150 kv
- deep X-Ray Therapy - 300 kv
- linear accelerators - 4-24 mv
4. Particle beams
Electrons - Linear Accelerators
- particle beams via linear accelerator
- produce electron beams -> absorbed as particles
- depending on energy of beam can dial up depth of beam
- 6MeV - 20MeV
5. Beta - Rays
- electrons given off by ionised substance
- injected locally
- Strontium, Yttrium & Samarium
6. Neutron beams
- very damaging
- experimentally are producing heavy particle beams via cyclotrons as neutron beams
Technique
Fractionation
- 1 large dose vs 60 small doses
- curative / fractionation
- palliative / minimal fractionation
100 Centigray = 100 Rad = 1 Gy
Metastasis
- 1000 Centigray in 10 doses
Bone Tumour
- 6000 Centigray in 30 doses
Curative
Maximum possible dose with acceptable damage to normal tissue
- 60 Gy in 30 fractions over 6/52
- equivalent to 18 Gy in one dose
- fractionation decreases late effects on normal tissue
- increased differential between tumour & tissue damage
- allows repair between treament
Usually given 3/52 postoperatively
- allows wound healing
- minimum delay as tumour interference activates cells in arrest phase
Careful planning
- multiple fields
- minimum normal tissue damage
Palliative
Short course with lower total dose
- 30 Gy in 10 fractions
- 3 CentiGy or 3 Rad or 0.03 mRad
Simple field set ups
- late morbidity less of an issue
- delays callus formation if pathogical fracture
- slows chondroid formation at fracture but not with osteogenesis
- 96% local remission
Method of Delivery
External Beam
Brachytherapy
- old method
- place radioactive agent down tube
- high risk to doctor giving treatment
- now use remote brachytherapy
Intrapoerative radiotherapy
- give dose to site at time of surgery
- give high dose with minimal local effects
Remote Afterloading
- pour radio-active agent down into tube
Timing
6 Week Rule
- start radiotherapy < 6/52 after OT
Preoperative
- better blood supply
- needs oxygen to effect cell kill
- shrinks tissue from neurovascular bundle
- may allow limb salvage
Disadvantage
- impairs healing
Postoperative
- usually preferred
- wait for wound to heal
- start within 6/52 otherwise repopulates with tumour cells
- also easier to identify site of tumour
Morbidity
100% side effects
Early
Erythema / Dry desquamation / Ulceration skin
Lymphopaenia
Telangectasia
Myelosuppresion
GIT effects
Late
Skin Fibrosis
Joint Contracture
Muscle Atrophy
Lymphoedema
Hair loss
Chronic bone changes / fracture
Osteoradionecrosis - eg AVN Femoral head
Transverse myelitis
Lung fibrosis
ST & bony hypoplasia in kids
- physeal arrest
Endocrine suppression
Infertility
Skin cancers
Sarcomatous change
Specific tumours
Osteosarcoma
No indication for preoperative treatment
May be used for
- unresectable
- palliation for metastasis
Ewing's Sarcoma
Very high radioresponsiveness
- but low curability
- effective if combined with chemotherapy
- surgery & chemotherapy have better results
Chondrosarcoma
Relatively radioresistant
May be used for
- recurrence
- inoperable disease
Myeloma
Effective for Plasmocytoma
Combined with chemotherapy for Multiple Myeloma
Soft Tissue Sarcomas
Indications
- doubt about surgical margins
- NV structures close to tumour
- 50 Gy DXRT preoperatively
- 10 Gy Brachytherapy postoperatively
Useful to give radiotherapy preoperatively for sarcomas
- because it develops a rind around the tumour
- makes it a lot easier to excise
- operate at about 6 weeks post radiotherapy