If radiation exposure kills people, why is it an effective cancer treatment? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

If radiation exposure kills people, why is it an effective cancer treatment?

So, to paraphrase - If water drowns people, why is it essential for life? -or - If knives kill people, why are scalpels used in surgery? Because too much of anything can be dangerous, but the right amount can be life saving.

Radiation therapy for the purpose of cancer treatment is given with sub-millimeter accuracy, in the exact dosage needed to kill cancer cells, yet leave the healthy ones unharmed.

So why are cancer cells killed by a certain dose of radiation, but healthy ones are not? Primarily because normal cells will only divide a finite number of times during the patient’s remaining lifetime, while cancer cells will continue to divide indefinitely if allowed to. If a population of healthy cells is exposed to a potentially lethal dose of radiation, some of those will die as they try to reproduce, others will be unaffected, and some will die after a few cycles. We have learned what dose of radiation we can “get away with” and still allow an organ to continue to function for the rest of the patient’s life.

We try to give as much radiation as possible to the cancer and still allow the normal tissues to survive. In general, we know what dose we can give to what percentage of each organ without doing permanent harm. We call these “Normal Tissue Complication Probabilities”. Thanks to high powered computers, we can perform thousands of iterations based on multiple beams traversing innumerable paths to give as high a dose as possible to a target volume (containing a population of cancer cells - often as both a mass (tumor) and as an infiltrative process) in order to maximize the therapeutic ratio.

Radiation therapy is every bit as complex as brain surgery, it just uses more technology.

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