Relative biological effectiveness (RBE) of photon and electron

[Leb]

The title was too long, original question was: Why is the relative biological effectiveness (RBE) of photon and electron the same ? I was suspecting the charge of electron to possibly have some effect...

Also, in biological matter, would there be a difference between photon damage and electron damage in terms of "by products" ? Maybe the damage is the same, but the way the damage is done is different ?

 

[mathman]

I am not sure what RBE means here. However the effects of photons and electrons are very much dependent on energy (especially photons ranging from gamma rays to radio waves).

 

[UltrafastPED]

For information on RBE see: https://en.wikipedia.org/wiki/Relative_biological_effectiveness

For the same energy deposition the question is: which is more effective at ionization?

So a flood of 4.5 eV photons (~250 nm UV = germicidal lamp) is able to break molecular bonds, and ionize many types of molecule. If instead a few higher energy electrons were encountered, but with the same total energy, the electrons would slow via "braking radiation", ionizing molecules, and generating photons at the same time.

The converse is to encounter a few high energy x-rays; these would also slow by "braking radiation": knocking off electrons, and generating lower energy photons.

The result is that the damage profiles are similar because the loss of energy in both cases follows a similar process. Your results may vary when the energy per particle is decreased below the ionization threshold - but RBE is typically concerned with ionizing radiation.

 

[Leb]

mathman, could you give examples where a same energy photon and electron would give different results in terms of biological damage ?

 

[Choppy]

The title was too long, original question was: Why is the relative biological effectiveness (RBE) of photon and electron the same ? I was suspecting the charge of electron to possibly have some effect...

Also, in biological matter, would there be a difference between photon damage and electron damage in terms of "by products" ? Maybe the damage is the same, but the way the damage is done is different ?

The first thing to remember here is the process by which both types of radiation induce biological damage. How photons intereact with biological matter is dictated by their energy, but whether it's photoelectric, Compton, or pair production, in each case the photons give rise to electons which then go on to produce radicals that break bonds or in some cases cause damage through direct ionization of something important.

A beam of electrons incident on some tissue will skip that initial interaction part, but afterwards - an electron track is an electron track.

The electrons set in motion by photons are going to have a lower mean energy than the photons. In the case of the photo-electric effect, you lose the electron binding energy. In the case of a Compton scattering event, some of the energy is carried off by the scattered photon, but generally speaking, the energy difference is not so significant so as to cause any difference in the biological effectiveness of the radiation.

You can reach a point where energy difference induces a difference. For example, kV imaging radiation requires less dose to achieve the same effect as MV therapeutic radiation (both photons). So you can expect the RBE to change between a 6 MV photon beam and 40 kV imaging beam. But even this can't always be resolved since the change is small and bilogical experiments often have large error bars associated with them.

 

[Leb]

Thanks for the reply. I slightly forgot, what we were tought again and again, that photon interaction with matter can only occur via 3 processes (in which electrons occur anyway...). So in some sense, it's not the photon, it's the electron that's the culprit (alhthough it is the photon who "creates" the electron).