The use of ionizing (high energy) radiation for therapeutic use in oncology dates from the end of the 19th century after the discovery of radium by Marie and Pierre Curie. It is an effective and important treatment modality in head and neck cancer.
It works on the principle of rapidly dividing cells (e.g. cancer cells) being selectively destroyed by exposure to high-energy irradiation. Cells exposed to ionizing radiation form free radicals intracellularly because the energy of the irradiation is high enough to break up molecules in the cell. This causes denaturation of large molecules including DNA and other vital molecular components of living cells. Although most of the changes are not apparent immediately, any attempt at division (to form new cells or repair cell injury) will result in mitotic cell death because the cell’s molecular machinery is no longer functioning properly. Rapidly dividing cells are most vulnerable, in particular most cancer cells and normal fast proliferating tissues such as mucosa and skin.
The unwanted effects of radiotherapy can be divided into acute reactions and delayed effects with long-term late damage. Such effects are dose dependent. The efficacy of irradiation on tumour cells depends on their radiosensitivity (e.g. sarcoma is less sensitive than carcinoma), the oxygenation level of the tissue (higher levels of oxygen help to create free radicals in the cells) and the tumour volume (small volume tumours will be better controlled than large tumours)
There are three ways radiotherapy can be administered – teletherapy (external beam), brachytherapy (radiation source placed within or close to tumour) and radioactive isotopes injected systemically and taken up by cancer cells (e.g. used in thyroid cancer where radioactive iodine isotopes selectively accumulate in the thyroid gland). In oral cancer, only the first two delivery methods are used. In more recent attempts to minimise unwanted radiotherapy effects, sophisticated protocols are being developed to deliver external beam radiation more selectively to the target area (IMRT).
In order to ensure that over a course of irradiation sessions the radiation is consistently and precisely delivered in a highly controlled manner this involves a planning CT scan and the creation of a close fitting transparent mask which is fitted to both your face and the table you lie on ensuring there is no inadvertent movement during the brief period the radiotherapy is being delivered.
Next section: Clinical application (Radiotherapy)