I am not a theorist but in my understanding it is something anologous to what we have in hadron nucleus coherent or incoherent scattering. That is in case of coherent scattering the medium acts as one entity that is individual amplitudes add coherently; 

http://arxiv.org/abs/1405.4241

Dear Oussama Bouzid and Adam Jacholkowski,

Thank you very much for your answer.

The ideia is more clear now... But now I have a new question: Is the coherent effect same as the Landau–Pomeranchuk–Migdal effect?

Thank you very much in advance!

Let me cite from Wiki: "A high energy particle undergoing multiple soft scatterings from a medium will experience interference effects between adjacent scattering sites. From uncertainty as the longitudinal momentum transfer gets small the particles wavelength will increase, if the wavelength becomes longer than the mean free path in the medium (the average distance between scattering sites) then the scatterings can no longer be treated as independent events, this is the LPM effect." So the answer is yes if some conditions (like small momentum transfers) are met.