Cancer is one of the most prominent causes of death worldwide, having accounted for approximately 12.5% of total deaths (over 7 million people) in 2002 (World Health Organization 2004). This percentage is greater in developed nations: in Australia, for example, cancer was responsible for 29.4% of all deaths in 2005 (Australian Bureau of Statistics 2007), and was the leading cause of death.
The term ‘cancer’ refers to various diseases characterised by two symptoms: the unregulated proliferation of cells and the spread of these cells through the body by invasion and/or metastasis. Cell proliferation typically results in the development of neoplasms (more commonly known as tumours). Without the capability to spread, a tumour might not pose a risk, and is considered benign.
Cancer develops as a result of genetic abnormalities in proto-oncogenes and tumour suppressor genes which translate genetic code into proteins that promote and suppress cell growth respectively. These abnormalities can be the result of a mutagen altering genetic information or a spontaneous error in DNA replication. Substances responsible for these mutations are known as carcinogens.
The location and stage of a tumour determines the available treatment options: surgical removal, chemotherapy, hormone therapy, immunotherapy and radiation therapy are the main modalities where cure is the aim. Treatments may also be delivered as part of palliative care, where the aim is to improve the quality of life of the patient.
Radiation therapy (or radiotherapy) is a treatment modality in which a therapeutic dose of ionising radiation is delivered to a tumour. This radiation disrupts the malignant tissue, with the desired result being cell death or an impairment to cellular division. The dose, or level of radiation, is prescribed by an oncologist, and depends on the size, stage and location of the tumour, and the use of any other treatment modalities.
Tumours cannot be treated in isolation, inevitably normal tissue will also be damaged. The aim of radiation therapy is to maximise the dose to the tumour, while minimising the dose to healthy tissue (reducing complication likelihood), that is, to find the greatest therapeutic ratio. This chapter will discuss beam delivery, the physics of radiotherapy, radiobiology and the treatment planning process.
High energy photons, electrons and protons are all capable of ionising atoms or molecules and can be used for radiation therapy. Radiation can be delivered via an external beam, an internal sealed source (brachytherapy) or an injected or ingested radioisotope. Photon-based external beam therapy (sometimes called teletherapy or gamma therapy) is the most frequently used clinical treatment modality. Radioactive sources, orthovoltage units or linear accelerators can be used to deliver this beam. Electron-based therapy is generally reserved for superficial treatment, due to the short range of electrons. Hadron-based therapies are uncommon clinically because, while they offer a highly depth-conformal dose, delivery is more complicated and expensive.
Linear accelerators, which constitute the 70-95% of teletherapy units in the developed world (International Atomic Energy Agency 2008), are described in further detail here. Information about Australian radiation oncology treatment centres is contained here.
Further reading
The following sites provide summaries on aspects of radiation oncology:
- The National Cancer Institute, the US Government’s principal agency for Cancer research, provides a good summary of radiation oncology.
- The Radiation Oncology Wikibook is an free access reference book with detailed reviews of clinical practice.
References
- Australian Bureau of Statistics, 2007. 3303.0 – Causes of Death, Australia, 2005.
- International Atomic Energy Agency, 2008. Setting up a radiotherapy programme: clinical, medical physics, radiation protection and safety aspects.
- World Health Organization, 2004. Deaths by cause, sex and mortality stratum in WHO regions, estimates for 2002.
Last updated 30 June, 2014 by