In radiotherapy, the goal is to kill as many cancer cells in a localised area without killing normal cells right? So what possible reason would there be to use gamma irradiation over alpha irradiation?

Gamma is not as good at ionising and damaging cells and atoms that make them up as alpha is. it is also very good transmitting through hard AND soft surfaces so gamma irradiation results in much more collateral damage than alpha irradiation. Alpha particles are absorbed easily and cause more damage to cells.

So why are gamma waves used instead of alpha particles in radiotherapy?

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    $\begingroup$ Can you show your source saying that alpha particles are damaging to cells? I don't think this is true. I think beta is usually used $\endgroup$ Apr 14 '19 at 20:31
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    $\begingroup$ Heavy particle (proton, alpha, and even carbon nulei) beam therapies have been a thing for a couple of decades now, but ... they require more demanding standards of the beam generating kit, the radiation physicists who make the treatment plans, and the technician who run the kit. $\endgroup$ Apr 14 '19 at 20:33
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    $\begingroup$ @AaronStevens Heavy particle have two advantages in this area. First and foremost they can (with adequate control) deliver their energy in a more localized way. Secondly the so called "quality factor" of the radiation is higher; that figure quantifies the amount of biological damage done per unit of energy delivered. Combined the two effects mean much less damage to healthy tissue. $\endgroup$ Apr 14 '19 at 20:36
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    $\begingroup$ At a more basic level than dmckee's comment, the OP may not understand that this requires alphas from a particle accelerator rather than alphas from a radioactive source. Only extremely high-energy alphas will make it through the epidermis. $\endgroup$
    – user4552
    Apr 14 '19 at 23:35
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    $\begingroup$ I've been through proton beam treatment. Also got a thorough tour of the superconducting cyclotron, & conversed with the physicist in charge of the cyclotron. The Bragg peak is the real key to ion beam therapy: not only does the beam stop at a specific depth, but it dumps energy at a much higher rate just as it's stopping. While gamma or x-ray beams are attenuated exponentially and thus deposit more dose on their way to the tumor and beyond the tumor than in the tumor, delivered dose by a (monoenergetic )proton beam gradually increases with depth, then sharply peaks, then stops. $\endgroup$
    – S. McGrew
    Apr 15 '19 at 2:44

Gamma radiation is used when the radiation source is outside the body and we need to focus it into a tumor that's inside it. For these situations, if we used alpha radiation, it would just get stopped at the skin, which is definitely not a good thing.

This type of external-beam therapy can also be done with charged particles, known as particle therapy, in which case you have the advantage that the sources can be more consistent and that you have better control over the focusing (since you can use electrostatic lenses and magnetic fields to shape the beam). However, once you're in that arena, proton therapy is likely to have every advantage of helium-ion beams, and it will be much easier to produce.

Alpha emitters are good in situations where you can get them right next to the tumor cells you want to kill, which probably means that you're including the alpha emitter in some biochemically-active molecule (a radiopharmaceutical) that gets preferentially concentrated in the tumor.

This does seem to be used in practice, though it seems that most therapies of this type use beta emitters, which have a slightly larger radius of action.

  • $\begingroup$ i had thought the only way alpha particles were used at all was when alpha emitters would be injected inside the tumour. Couldn't this done be done on an external tumour? and if so, then wouldn't it be better than using gammas? $\endgroup$
    – Hisham
    Apr 14 '19 at 21:00
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    $\begingroup$ @UbaidHassan What's an "external tumour"? $\endgroup$ Apr 14 '19 at 21:01
  • $\begingroup$ a tumour on the skin? $\endgroup$
    – Hisham
    Apr 14 '19 at 21:02
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    $\begingroup$ I suspect that most tumours that are visible from the skin are still too deep for this type of treatment, but that is ultimately a very technical biomedical question, and the choices involved depend on a whole host of non-physics factors. From a physics perspective, yes, that could be made to work. Whether that pans out in practice is a much more focused question than the scope you set out in your original question, so I won't examine it. $\endgroup$ Apr 14 '19 at 21:06
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    $\begingroup$ @UbaidHassan In this special case you are comparing the wrong options anyway. If the tumour is directly on the skin and so shallow and tiny that it can be reached by alpha radiation completely, then there is no need for expensive radiotherapy, a good scalpel will be more than enough. $\endgroup$
    – mlk
    Apr 15 '19 at 7:02

Alpha particles are absorbed too easily; usually within a couple of centimeters. Gammas have no such issue. Protons, on the other hand are excellent for radiation therapy because their energy can be tailored to produce a "Bragg peak" (see Wikipedia) at a selected depth, and they stop there. Any ion (protons are hydrogen ions, alphas are helium ions) shows a Bragg peak.

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    $\begingroup$ Heavy ions and alphas have very short range under threshold energies. To use them as therapy beams you tune the energies with exquisite precision so that they range out just as the get to the tumor. $\endgroup$ Apr 14 '19 at 20:47
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    $\begingroup$ @dmckee It sound like you should just type out an answer :) $\endgroup$ Apr 14 '19 at 21:12
  • $\begingroup$ agreed, dmckee appears to know a lot $\endgroup$
    – Hisham
    Apr 14 '19 at 22:11

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