Mixed energy use is very common in large breast to achieve dose homogeneity. In our dosimetric study, mixed energy use was about 65% with the goal to keep V105 low. The contribution from high energy can vary from 10-50%. Chen et al., PMID 29548340 

I use 10 MV when needed for breast planning for large breasted patients. Typically 6/10 mix is reasonable, occasionally 10/10 on tangents. When using 6/10, I usually keep 6 on the side closest to the cavity to minimize the risk for under-dosing of the cavity. 

In larger breasted patients, I routinely use higher energies. We typically start with a 6 MV plan and see what the hot spots are like and then add in 10 or 15. While uncommon, in very large breasted patients, we may go completely with high energy tangents. In such cases, we may look at adding a smal...

High energy photon beams in breast, and lack of surface dose, is a more complex issue than in other body sites. And it's more complex on the side of: the surface dose is higher than one might expect from looking at a photon depth dose curve from a beam aimed perpendicular to the measurement medium.&...