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This refers to one of the major modalities of cancer therapy. In the commonest form of radiotherapy, an external beam (from a machine that stands off the patient and does not touch the patient) is 'shone' at the area of the patient containing a cancer and, without the patient feeling anything (humans cannot sense x- or gamma rays or electrons - the commonest forms ) treatment is delivered. Radiotherapy utilises ionising radiation, which damages cells by creating ionising events in constituent molecules. The most important interaction is the breakage/disruption of DNA molecules, which cannot be repaired. This may occur as primary ionising events or via the secondary hydrolysis products of water (which is, by virtue of its high prevalence in all human cells, all around the DNA molecules).

There is nothing tumour/cancer selective about ionising radiation and the advantage in terms of killing cancer cells over normal cells is gained by exploiting other differences in behaviour between the two types of cells. Thus, normal cells sense when damage has been inflicted and recruit into service repair and regenerative mechanisms to quickly heal the defect. Cancer cells do not retain this normal physiological 'homeostatic' mechanism to quickly regain the status quo. It is in their very nature that they have lost their responsiveness to all cell commands and function autonomously with disregard to normal cellular signalling mechanisms. This difference is exploited in the usual form of radiotherapy, which takes place over several to many weeks and involves many treatments (which the doctor calls fractions). This is called fractionated radiotherapy and exploits a difference so fundamental that the normal tissues are able to accommodate the damage of ionising radiation during a fractionated course, whilst the damage to the tumour is progressive and cumulative. The net effect is ideally cure of the cancer without damage to the normal tissues. This hope is never fully realised because the normal tissues always suffer some radiation damage, but the radiation tolerance of the various body organs is now well established in radiotherapeutic science and respected in the dose prescriptions delivered in the clinical subject of radiotherapy. However, it is worth noting that the dose to cure a cancer is often close to normal tissue tolerance and also that an organ, once irradiated to doses near to its tolerance , never recovers fully and will always be more vulnerable should radiotherapy to that area of the body be considered again in the future.

Radiotherapy, like surgery, is a regional form of cancer therapy. That is, it can only sterilise the cancer cells in the area which is irradiated (and it is usually too toxic to open up the radiotherapy portals/fields to irradiate the whole body). At first, this migh seem a huge disadvantage as cancers spread, and therefore reliance should be put on modalities that can sterilise the cancer wherever it is (i.e chemotherapy or hormone or immuno- therapies etc.). However, this viewpoint is incorrect in the overall plan of therapy, given the state of efficacy of the therapeutic modalities for most common cancers. Where a cancer has not spread outside its original site of derivation or where any such spread is small bulk compared to the amount of disease in the primary region, then the sterilisation of the disease in that primary region is critical to the overall cure of the patient and this is the area that chemotherapy etc. (i.e the systemic therapies - those that go all around the body) have most dificulty in gaining 'local' cure. In current cancer therapy, chemotherapy is often combined with surgery and radiotherapy to maximise the chance of both local cure (i.e. cure within the primary region) and cure of potential metastases.

The above is all relevant to the strategies for cure of cancers; such strategies in radiotherapy are referred to as radical radiotherapy strategies.

Radiotherapy has another and equally important use in clinical oncology which is a palliative role - i.e. delivering therapy to areas of the body where the cancer is causing pain or threatening to cause imminent damage: for example, the patient with painful bone metastases may be rendered pain free or much improved by radiotherapy to the affected bones. The patient with a lump of cancer pressing on his bronchial tubes or spinal cord threatening lung collapse or paralysis respectively would gain enormous benefit for radiotherapy to these sites and prevention of the imminent catastrophes. However, such therapy is not going to cure the patient; it is for his/her comfort and is referred to as palliative therapy.

Conformal radiotherapy: This refers to the use of modern linear accelerators to deliver the external beam radiotherapy that possess an extra feature called multi-leaf collimation. This extra facility allows the more accurate shaping of the radiation beam to the contours of the patient's tumour and has allowed the treating doctor to increase the delivered dose in some diseases that are routinely treated by radical radiotherapy (e.g. carcinoma of the prostate) - see figure in carcinoma of prostate section.

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