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 outcomes. (A European friend told me this is because the START docs were perfect in choosing the higher risk patients to boost... hmm.) You certainly can't make any inferences about the need for boosting, or not, from the FAST/FAST-FWD data; so again, one is left with extrapolating the need for boost in hypofx/ultrahypofx from data which is long in the tooth.

I have done 5fx breast quite a bit; am very pleased with it. For me, because ultrahypofx is...

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.