Physical principles (Radiotherapy)

Our pages about the clinical application of radiotherapy in the treatment of head and neck cancers discuss a fair amount of relevant technical and physical details. Without including these details in that discussion it would be very difficult to explain some of the clinical applications and procedures.

The term radiotherapy does not really specify what is being done, it merely says that some form of radiation is applied to living tissues with the intent to cure or improve disease – almost always a malignancy.

Here we take a step back from the practicalities and put into context how a force with enormously destructive potential works in principle and how this can be turned to good use. In order for radiation to be able to destroy malignant cells, the applied radiation has to have high enough energy to cause lasting and irreversible damage to cells. At the same time it is necessary to protect healthy tissue as much as possible from bio-damage caused by exposure to high-energy irradiation. Photons, electrons and protons are most commonly used to create high-energy radiation but some heavier ions as well as neutrons can also be used. These different sources of high-energy radiation have different properties (with different strengths and weaknesses) and have all been explored to deliver high-energy irradiation in a controlled manner.

Photons with energies in the X-ray region are the most commonly used source of radiation but also photons with even higher energies (gamma rays), usually resulting from radioactive decay processes or generated in an accelerator, are used for therapeutic purposes. A whole range of different technical schemes to deliver X-ray radiation selectively to a target area / tumour volume while sparing nearby healthy tissues as much as possible have been developed, and continue to be further developed. This includes methods such as implanting a small source of radioactive radiation into, or very near to, a tumour (brachytherapy) or more commonly some modulated, multi-beam schemes of irradiation by an external source (for example, intensity-modulated radiation therapy (IMRT)).

Damage to cells by high-energy radiation depends on the type of radiation source, on the overall dosage received as well as on the type of tumour. It is extremely rare to deliver the full dose in a single session. Accordingly, case-dependent time- and dosage-schemes are worked out individually.

The relationship between high-energy radiation and living cells, healthy and malignant ones alike, is a complicated one. It would certainly be wrong to reduce the discussion to a merely mechanical picture with simple cause / effect relationships, omitting a multitude of biological and biochemical issues and considerations.

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