WHY CANT WE CURE ALL CANCER WITH RADIATION? – THE REACTION OF NORMAL TISSUE LIMITS THE SAFE DOSE (REASONS)
Why is the size an important guide as to whether or not a cancer is likely to be cured? One reason is that, because bigger cancers contain more cells, the chance that they will contain some cells which have a natural resistance to radiation is higher than for small cancers. Another reason is that tumours generally do not develop an efficient blood supply. This means that big tumours contain a high proportion of cells which are getting very little oxygen. This is important because cells which are getting very little oxygen are not as sensitive to radiation as cells which are getting plenty of oxygen. It takes two to three times the dose of radiation to kill the poorly oxygenated cells. A third reason is that big growths contain a higher proportion of cells which are not actively dividing than small growths. As we have seen, cells which are not dividing are less sensitive to radiation.
The bigger the tumour, the more of these relatively resistant cells it will contain. However, some of them are present even in tumours that are only a few millimetres across. The chances of curing growths which contain some poorly oxygenated cells and some cells which are not dividing can be improved by giving the radiation treatment in small doses spread over some weeks rather than giving the whole dose in one treatment session. As the cancer shrinks, cells which were not dividing start to divide and cells which were poorly oxygenated get more oxygen. Thus, as the weeks go by, these cells become much more sensitive to radiation treatment than they were to start with. Although the results are better when the treatment is spread out like this, bigger cancers still need a much higher total dose. Even with a higher dose, there is still a much smaller chance of curing the bigger cancers. Very big cancers simply cannot be cured with doses that are safe.
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