Do you prefer to use the FAST or FAST-Forward regimen when treating stage I breast cancer with an ultra-hypofractionated approach?

We favor APBI like the Florence regimen but if technically not suitable, then FAST-Forward.

I offer all three (APBI, FAST, FAST-Forward) based on eligibility and let the patient decide.

While I don't make these schedules my primary recommendation, there are patients for whom the logistics make a big difference. In those cases, I will offer these regimens in low-risk disease without adverse features (like things I'd normally boost) and "normal" anatomy. I keep in mind the follow-up times in these studies (5 years for FAST-Forward, and 10 years for FAST), and while I don't necessarily mention this, the comparison "standard" fractionation arms were different with the FAST one being less relevant now. I then let the patient decide on which regimen; most choose FAST-Forward. Both seem to be very well tolerated. We use the FAST-Forward metrics for planning both.

As per the UK guidelines, and drawing on IMPORT-LOW and FAST-Forward, don't you think 26 Gy in 5 fr daily PBI is reasonable (and more compelling) as well for most PBI-eligible patients? In the FLORENCE trial, 30 Gy in 5f EOD was compared to 50 Gy plus boost 10 Gy, and only ~50% of the patients met the ASTRO suitable criteria. I am inclined to believe that the FLORENCE regimen is an overkill for low-risk patients and may be more appealing in eligible cases where a boost is felt to be beneficial, such as close margins, grade 3, or tumors >2 cm. I wonder what are your thoughts on these.

APBI is an excellent option for eligible patients. If they are not eligible for APBI by criteria, but they would qualify for FAST/FAST-Forward, I always prefer the FAST-Forward regimen, especially for patients coming from a distance.

Based on patient preference: either APBI, FAST, or FAST-Forward.

Based on personal preference: FAST (while using FAST-Forward constraints).

Based on eligibility criteria on trials: the FAST eligibility (≥ 50 yo, pT1-2 pN0) more accurately represents the patient population at my clinic receiving ultrahypofractionation, rather than the FAST-Forward eligibility (≥ 18 yo, pT1–3, pN0–1).

Patient selection is important with ultrahypofx and should be discussed when selecting treatment recommendations. Many ultrahypofx candidates could also benefit from no radiation (age > 65, < 3 cm size, margin neg, ER+, either G3/LVI+).

The trouble for me is that not all the research is pointing in the same direction. With some trials demonstrating worse cosmesis and slightly higher IBTR or nodal failures with APBI and no serious downside to treating the whole breast, until we have better evidence, it makes sense to me to continue to treat the whole breast over a partial breast regimen.

I prefer FAST-Forward in general, and especially with their eligibility criteria, it can be generalized to a large percentage of early-stage patients.