When X-rays enter matter, they hit things (parts of atoms actually). Why do the Xrays hit such small things? Because both the Xrays and atoms are very small.
So we say very learnedly that "interactions result". To give you an idea of how many interactions occur, consider a million 1MV photons hitting a 29 gram piece of copper which was 5mm thick. In that slab of copper there are 6.023 x 1023 atoms (that's called Avogadro's Number after a smart Italian guy named …. well, you can guess!). Ask your physicist to tell you how many of those photons will get through, and then work out the attrition rate of the photons. Note that if you do the same calculation for a million 100kV photons, the results will be very different. But you will get the idea that it's a chancy occurrence.
Any physics high school student (and because you are going to be a radiation oncologist, you too!) should be able to tell you that there is energy loss during most collisions, and this also occurs with the radiation beam. Interestingly though, there is one interaction which does not result in any energy loss (more later). The nature of the interaction primarily relies on the energy of the x-ray, and to a smaller degree on the physical properties of the matter - things such a Z number, electron density (${e\over g}$) and mass density ($\rho$).
The important interactions in the radiotherapy range include those unable to displace electrons (Raleigh or coherent scattering) and those with sufficient energy to displace electrons, i.e., ionise matter. Other forms of electromagnetic radiation are not sufficiently energetic to cause ionisation.
The ionisation interactions important in the clinic are the photoelectric effect, Compton scattering, pair production and photonuclear reaction.
| Photon Interaction | Graphic |
|---|---|
| Coherent Scattering (Thomson/Rayleigh type) |
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| Photoelectric Effect |
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| Compton Effect |
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| Pair Production |
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| Photonuclear Desintegration |
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