Principles of cancer radiotherapy

4 Principles of cancer radiotherapy


Radiotherapy is an extremely effective treatment modality in the management of cancer patients, either as a single treatment or as part of a multimodality protocol. Ionizing radiation kills cells via the deposition of energy in, or near to, the cellular DNA, either causing DNA damage directly or via the production of free radicals within the cell. As with chemotherapy, however, ionizing radiation cannot differentiate between normal cells and neoplastic cells, so any proliferating cells will be damaged. Tumour cells are ‘selectively’ targeted by their increased growth fraction, whereas normal cells that divide more slowly may not be affected so acutely. It is therefore important to realize that radiotherapy can cause both ‘early’ and ‘late’ side effects in non-cancerous tissues. Cells that divide rapidly and frequently as part of normal physiology, such as epithelial stem cells, are likely to show the side effects of radiotherapy earlier than tissues comprised of cells that divide more slowly. As with chemotherapy, the total dose that can be given to a patient depends upon the side effects, but with radiotherapy it is very important to always be aware of the possible late side effects that could occur. Early side effects can be minimized by dividing the total dose into a number of ‘fractions’, administered over a period of time.


In 1975, Withers described the ‘Four Rs’ of radiotherapy (Repair, Redistribution, Reoxygenation and Repopulation) to explain the fact that giving small doses more frequently seemed to be more effective than giving fewer large doses. In human medicine, most patients will receive at least 20 fractions, often given at daily intervals to enable a higher total dose to be given without the risk of horrendous early side effects (e.g. deep blistering and cutaneous burning). This approach is now being used in canine patients in the USA, where many centres now use a Monday–Friday treatment schedule and give between 18 and 21 fractions over a 3–4-week period, depending upon the tumour type and location. In the UK, however, it has been more commonplace to use a different approach by utilizing what is known as ‘hypofractionated’ regimens, where most patients receive just four or five treatments at weekly intervals, although hyperfractionation is becoming more commonplace. The reasoning behind the use of hypofractionated radiotherapy is several-fold:


• It is often more convenient for owners to come once a week.

• It means the patients only have to receive 4 or 5 anaesthetics, rather than 18–21.

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Principles of cancer radiotherapy

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