if radiation causes cancer, why does radiation therapy stop cancer and not give you more cancer?
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It can give you cancer
Radiation cause dna damage. The effect of that damage is variable depending on where in the dna (eg what gene) that damage is. Some damage will have no effect on a cell. Some will cause the cell to die or not work properly and likely die early because it damages something that is necessary for the cell to continue to exist. But some will have damage in a gene that leads to the cell becoming cancerous (dividing uncontrollably and able to invade surrounding tissue and spread elsewhere). So that’s how it causes cancer.
For how it stops cancer; chemotherapy and radiotherapy rely on the cancer cells being more prone to death than the healthy normal tissues. It’s why you’ll get cancers that cannot be treated by certain chemotherapies or is insensitive to radiation. Cancer cells often have higher metabolic rates than normal cells, or other processes that are different than normal cells. The chemotherapy and radiotherapy aim to take advantage of this. Radiotherapy actually damage the sensitive cells further to the point that they die, but that the surrounding healthy tissue gets less damage and is able to survive. However because this surrounding tissue has been exposed and will have been damaged a bit, it is more likely to get a further bit of damage later on and become cancerous (it usually takes multiple bits of damage to actually get a call to become cancerous). Within the field of radiation becomes a high risk area for secondary cancers. It’s one of the reasons radiation isn’t used as much in paediatric cancers as adult ones; the child, if they are cured of their cancer, is going to live longer and have longer to develop cancer (eg if you treat a 70 year old with radiotherapy and there is a risk of cancer in 20years time then they would be 90 at that time and quite possibly have died of something else by then. Whereas, you treat and cure a child of 10 years, and they develop a cancer as a result of treatment 20years later, they are only 30 and most likely to be otherwise healthy. This is a very simplified version, if it makes sense at all)
Although we know radiotherapy can cause secondary cancers, it is a balancing act. Could you use radiotherapy to shrink a tumour to the stage of being able to fully remove it, and can then monitor for a second cancer and treat early, or not give radiotherapy and therefore not be able to shrink the tumour and get it all removed?
Also radiotherapy can be used in cases where your not trying to cure it. There will be palliative cases (not curable) but radiotherapy could help with symptoms. This might be there is horrible bone pain that can’t be controlled with other medicine, or there is a tumour on the spine and aim to improve movement for longer.
Sorry that this has been rambling. Hope it makes sense
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