- 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 |