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In radiotherapy, why don't normal tissiue cells or organ cells in the way of incoming radiation die, but tumours die instead?

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    $\begingroup$ They do. That's why chemotherapy patients lose their hair and don't feel well in general, cancer aside. $\endgroup$
    – DKNguyen
    Feb 24, 2022 at 14:58
  • $\begingroup$ @DKNguyen - and radiation-only patients as well, as I have personal experience with. There is long-term damage to non-cancerous tissue from radiation treatments as well. Just better than dying from the cancer. $\endgroup$
    – Jon Custer
    Feb 24, 2022 at 14:59
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    $\begingroup$ @DKNguyen Chemotherapy is not radiotherapy. $\endgroup$
    – noah
    Feb 24, 2022 at 15:00
  • $\begingroup$ @noah I guess my terminology is wrong. They are so often accompanied by each other I always assumed radiation was intrinsic to chemotherapy. $\endgroup$
    – DKNguyen
    Feb 24, 2022 at 15:01

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Living cells which are in the process of actively dividing i.e., replicating DNA strands, peeling them apart and sorting them out, rebuilding them into duplicate genes, and so on are particularly susceptible to any sort of challenge which might create transcription errors- most of which would lead to the death of the cell by either jamming the transcription machinery itself or leaving the daughter cells unable to function correctly.

Since cancer cells are almost always in the process of uncontrolled growth, at any point in time most of them in a tumor will be actively dividing. This puts them at much greater risk of being killed by chemicals or radiation than the noncancerous tissue nearby.

Note that since the cells lining your digestive tract and the cells that produce hair growth are also frequently dividing, they will be killed too as a side effect of chemo or radiation. This is why your hair falls out and your digestive system is seriously damaged by chemo agents in particular, since those agents circulate throughout your blood stream and are not "beamed" specifically at the tumor.

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    $\begingroup$ FYI, Radiation and Chemo treatments typically are delivered as a regular sequence of doses called fractions. Each fraction is supposed to be not-quite-enough to kill healthy cells, and the interval between fractions (one to two weeks) is supposed to allow enough time for the healthy cells to recover. ISTR there are around eight or ten fractions in a typical treatment. $\endgroup$ Feb 24, 2022 at 19:11
  • $\begingroup$ @SolomonSlow, +1 from Niels. I am a cancer survivor cured by surgery, and fortunate that i needed neither chemo nor radiation. $\endgroup$ Feb 25, 2022 at 0:47
  • $\begingroup$ So basically we just kill the cancer cells with a higher probability than healty cells and "hope" to find the balance to kill the cancer without killing the patient? $\endgroup$
    – kruemi
    Feb 28, 2022 at 13:46
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    $\begingroup$ this is the best we can do right now, except for experimental gene therapy. $\endgroup$ Feb 28, 2022 at 17:28
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There are two main reasons for this. First, there isn't a single direction the radiation is applied from. Instead, beams from multiple directions are directed at the affected body part. The part where all the beams overlap is the volume recieving the highest radiation dose. Ideally this is where the pathological tissue (e.g. a tumor) would be.

Second, healthy tissue is better at regenerating from radiation damage than cancerous tissue. Therefore, over the course of many radiation sessions, the surrounding tissue can heal (to some degree), while the damage in the tumor accumulates over time (also to some degree).

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Healthy tissue will get damaged by radiation, but it's a matter of degree. The goal of Radiotherapy is to limit radiation dose to healthy tissue as low as practicable while maximising the possible dose to the targeted tumour.

One way of doing this is using a moving beam, or multiple intersecting beams.

enter image description here

If you look at the radiation source here, you can see that the whole device is designed to pivot around the patient. This means that as it moves, the tissues it sweeps the beam through get a lower dose than the point along the axis of rotation. This means that the tumour can be targeted to get a much larger dose of radiation than the surrounding tissues.

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  • $\begingroup$ In the most common form of radiation therapy, so-called 3D conformal therapy, the machine does not rotate during treatment. Rather, they shoot dose from four or six different stationary ports (viewpoints), and they use shielding material to shape the cross section of the beam at each port to match the outline of the target as seen from that port. $\endgroup$ Feb 24, 2022 at 19:23
  • $\begingroup$ In a more advanced (i.e., expensive) technique called intensity modulated radiation therapy, the machine does rotate while delivering dose, and the intensities of individual "beamlets" are modulated as it rotates. IMRT works kind of like a CT scan in reverse, and like a CT scan, it has to be delivered in "slices." That makes it take longer/cost more than conformal therapy. IMRT usually is reserved for more difficult cases in which sparing nearby healthy tissue is a high priority. $\endgroup$ Feb 24, 2022 at 19:27
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They do die. That is why radiotherapy can lead to other problems. It kills all cells around the point where the radiation hits. But the human body is good at regenerating.

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    $\begingroup$ Chemotherapy is not radiotherapy. $\endgroup$
    – noah
    Feb 24, 2022 at 18:29
  • $\begingroup$ Thanks for correcting me. I forgot about that. $\endgroup$
    – AstroGuy
    Feb 24, 2022 at 19:02
  • $\begingroup$ I accidentally put that in. $\endgroup$
    – AstroGuy
    Feb 24, 2022 at 19:02
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    $\begingroup$ @noah, but Chemo and radiation have a lot in common. (See Niels Nielsen's answer.) Both treatments indiscriminately damage the DNA of living cells, and the effectiveness of either treatment is due to the different ability of healthy cells vs. diseased cells to repair that damage. $\endgroup$ Feb 24, 2022 at 19:57
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    $\begingroup$ @SolomonSlow That doesn't mean they can be used interchangeably, they have completely different mechanisms to attack the patological tissue. $\endgroup$
    – noah
    Feb 24, 2022 at 20:04
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I suggest you look up "Medical Physics". Cancer is treated locally, by burning "only" the damaged tissue. The role of a Medical physicist is exactly that of studying, case by case, the amount of radiation needed to destroy only the necessary - no more, no less.

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In shooting ranges, why don't people get shot, the shooting targets get shot instead? Because the guns are aimed at targets instead of people. The same is true for radiotherapy, radiation is aimed specifically at tumors.

Actually that was simplification. See, people occasionally do get shot at shooting ranges. Likewise, it is impossible to aim the radiation is such a way that the cancer gets 100% of the dose and healthy tissue gets 0%. But you still could do a lot to minimize exposure to healthy tissue and organs.

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    $\begingroup$ Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. $\endgroup$ Feb 24, 2022 at 22:14
  • $\begingroup$ This is a very bad analogy. A tumor inside the body in this case would correspond to a target being surrounded by people on all sides. $\endgroup$
    – noah
    Feb 24, 2022 at 22:51

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