When do you offer a boost for patients receiving 5 fraction whole breast radiation as per UK FAST Forward?   

Any data (convincingly) supporting the use of boost comes from the non-hypofx era. In the START A/B trials (which included many "traditionally boostable" patients), boost/no-boost outcomes were analyzed (left up to each center's/doc's preference), and boost had an almost perfectly zero effect on out...

I offer boost in cases that are ER negative primarily. I haven't used routinely in women under 50 years old but would also consider for this.The trial used a more standard boost dose/fx but I have used an addition single fraction for boost (5.7 Gy if FAST, 5.2 Gy if FAST-Forward).

I have used the same indication for boost as any other fractionation for 50 and above (high oncotype, high grade) and used 5.2 Gy in 1-2 fractions or 10 Gy in 4 fractions.

In the FAST-Forward trial, 75% of the patients did not have a boost. In the FAST trial, they were not included/ineligible. They also showed they are on the steep part of the dose response curve with this regimen. I don't offer 5 fractions much (yet) but will exclude people who need a boost.