The difference between photon dose distribution and electron dose distribution is how each interacts with tissue. Photons in a photon beam must first have a photon to electron interaction and then the electron deposits energy in the tissue. The penetration of the photons and subsequent electron interaction is dependent upon the photon energy. An electron beam begins energy deposition in tissue immediately at the tissue surface. See

https://www.researchgate.net/publication/260210316_BETA_DOSIMETRY

Chapter BETA DOSIMETRY

In the case of photon beams, the fluence of secondary electrons begins to build up at the surface and the number of secondary electrons passing through the medium gradually increases with depth, what leads to increasing density of ionization tracks, i.e. to increase in the deposited energy (the dose) up to a distance from the surface approximately equal to the range of the secondary electrons. In case of electron beams, the stream of ionizing particles (i.e. electrons) is already produced (by a Linac) so, they deposit their energy starting from the very surface.

Good answers already. May I complete first with a remark to the electrons. Electrons from linacs have a nearly homogeneous energy, say 10 MeV. If hitting on a medium, they loose continuously energy, but they are also scattered. The scattering processes increases with energy loss. So the electron leave the original direction. They loose energy per path length (in water 2 MeV/cm). If path is not straight on due to scattering, the energy deposition along the central ray increases. The DD shows an increase but this phenomenon is no dose built up as for the photons. I add two graphs from my textbooks.

Excellent details are already given, hope to answer the question

As explained very nicely in previous answers, the photons deposit their energy through the secondary electrons and there is a dose build-up region before dmax. Perhaps one may also examine the mean free path of photons. The mean free path of photons is the average distance that they will travel before interactions. It is about 6 cm in water at 100 keV and about 14 cm at 1 MeV. Only the very low energy photons will be likely to deposit the energy near the surface. The higher energy ones will not. Thus, a megavoltage beam is not depositing much energy there.

I can't add anything to the excellent answers above.

Excellent answers given. I would only point out that an excellent discussion is available in the dedicated chapter of the book "radiation oncology physics - a handbook for teacher and students" edited by Podgorsak and published by IAEA. It is available @ the link below reported

http://www-pub.iaea.org/mtcd/publications/pdf/pub1196_web.pdf

Dear Giuseppe,

very good recommendation. I just want to add a short additional source. Based on the cited report 1196 you can find a collection of slides to this report. I add the first, the sixths and eights chapter. Its really worth to be downloaded and read.

Dear Hanno,

Thanks for the additional source added: they are extremely well done and useful.

As I only opened now the QA section, I was amazed to see so many excellent answers to this question. I would like to point out that for photon beams ,the surface dose would decrease for increasing photon energies, arriving to around 20% only, for 10 MV or higher energies. Secondly its just this property of high surface dose and short range of high energy electron beams that makes the electron beams so useful for many situations in radiotherapy  for example breast scars after surgery).

Dear Sergio,

your statement about decreasing of surface dose with photon energy is correct and very easily to understand. It´s just the decrease of scattering coefficients of the high energy secundary electrons. The mass scattering power is proportinal to 1/E2. Detailed infos can be found in ICRU 35 and ICRU 37.

Thanks Hanno. Just to add: the effect of low skin dose in high energy radiotherapy is well-known by the medical physicists as the" skin sparing effect".