In radiotherapy, why do normal tissiue or organ cells not die of radiation? In radiotherapy, why don't normal tissiue cells or organ cells in the way of incoming radiation die, but tumours die instead?
 A: 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.
A: 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).
A: 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.

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.
A: 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.
A: 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.
A: 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.
