Principles of cancer treatment by radiotherapy

Apart from surgery, radiotherapy is the principal curative treatment for patients with cancer. Radiotherapy is also commonly used to palliate distressing local symptoms (painful bone metastases). Radiation kills cells due to its effects on DNA, with the introduction of single-stranded DNA breaks and, to a lesser extent, double-stranded DNA breaks. The five principal factors that determine the response of a tissue (normal and malignant) to fractionated radiotherapy are repair of sublethal damage (or recovery) between treatments; redistribution of cells within the cell cycle; repopulation during a treatment course; tissue oxygenation (hypoxic cells are less sensitive to treatment); and the intrinsic radiosensitivity of the tissue treated. Radiation therapy is now almost exclusively delivered by the linear accelerator. This gives a maximum dose about 1 cm below the skin surface, resulting in skin sparing, and the ability to treat deepseated tumours (e.g. prostate cancer). Treatment planning allows accurate delivery and can potentially increase the dose to tumours and avoid normal tissues. Current studies emphasize the importance of shaping the radiation beam to the tumour and reducing tumour mobility by breath-holding techniques. Chemotherapy and novel targeted agents are being used concomitantly with radiotherapy, thereby improving outcome.