Is there additional concern for late cardiac toxicity when using ultrahypofractionated breast radiation protocols, given that the BED to the heart is higher?

The BED to the heart isn't actually higher in this setting.

Dr. @Dr. First Last explained this below, but I'll just explain it another way. Imagine that you place the block edge so that it is touching the heart (i.e., the heart is completely covered by the MLCs, and there is no margin between the MLC...

Irrespective of fractionation, this would avoid any beam going through and only the dose to the heart is from scatter or transmission, which for early-stage breast cancer is around 1 Gy. At that range and dose per fraction to the heart, the fractionation effect would be minimal.