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Radiotherapy

  • Radiotherapy is the use of ionising radiation to treat malignancy
  • Attempts to deliver a measured radiation does to defined tumour volume
  • Need to limit dose to surrounding normal tissue
  • Radiotherapy may be:
    • Curative or radical
    • Palliative
    • Adjuvant
  • Brachytherapy is the use of intracavity irradiation

Physics

  • Radiation may be electromagnetic or particulate
  • Linear accelerators used to generate high energy x-rays (electromagnetic)
  • Generate by electrons hitting a fixed target
  • Depth of penetration depend on x-ray voltage
  • 10-125 KeV x-rays absorbed by superficial tissues
  • 4-24 MV x-rays absorbed in deeper tissues
  • Use of MV x-rays avoids significant skin toxicity
  • High energy electrons may be used instead of x-rays (particulate)
  • Electrons have limited tissue penetration
  • CT planning reduces dose delivered to normal tissue

Biology

  • Radiation damages DNA
  • Either causes direct damage to DNA or acts via the production of free radicals
  • Double-stranded DNA breaks prevents cell replication and induces cell death
  • Tissue response depends on degree of cellular differentiation
  • Terminally differentiated cells (e.g. muscle and nerves) are resistant to damage
  • Most significant effects seen in rapidly dividing cells (e.g. gut, bone marrow)

Toxicity

  • Acute toxicity
    • Occurs within days
    • Depends on overall treatment time
    • Includes mucositis, bone marrow suppression, skin reactions
  • Late toxicity
    • Occurs after weeks or months
    • Depends on total dose and fractionation
    • Includes tissue necrosis or fibrosis

Fractionation

  • Higher total dose of radiation can be given if smaller repeated doses administered
  • Allows a degree of repair of normal tissues
  • High total dose increases the probability of tumour control
  • Hypofractionation = small number of large doses
  • Accelerated fractionation = standard dose over short interval
  • Hyperfractionation = large number of small doses

Roles of radiotherapy

Curative as sole treatment

  • Head and neck cancers
  • Carcinoma of the cervix
  • Seminomas
  • Hodgkin's and non-Hodgkin's lymphomas
  • Bladder cancer
  • Early prostate cancer
  • Early lung cancer
  • Anal and skin cancer
  • Medulloblastoma and other brain tumours
  • Thyroid cancer

Component of multimodality therapy

  • Breast cancer
  • Rectal cancer
  • Soft tissue sarcomas
  • Advanced head and neck cancers
  • Whole body radiotherapy before bone marrow transplantation

Palliative radiotherapy

  • Pain -especially bone metastases
  • Spinal cord compression
  • Cerebral metastases
  • Venous or lymphatic obstruction

Bibliography

Symonds R P.  Radiotherapy.  Br Med J 2001; 323:  1107-1110

 
 

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