I have been looking for information about the way Radiotherapy works and, needless to say, there is much more involved than just local frying of tissue. I have been told and also read, that the dose received in tissue is not necessarily maximum at the surface. For instance, the dose seems to be a maximum at 0.4cm for 2MV photons and 2.3cm for 10MV photons. I cannot think why, if the absorption is exponential with depth (or so I would expect) the maximum effect (energy from the beam) is not right at the surface. 2.3cms is very deep under the surface. What happens between the surface and that depth to make the dose less? I have looked in all the places on the web that I can think of but, as usual, what's available is either the chatty home-medecine advice or the latest papers which all assume the reader knows this sort of basic thing. Any ideas? I think this is probably the right forum as it's basically Physics.