Top Line: Has it been too long since you delved in rad bio theory?
The Study: If so, brush up with this retro commentary. It builds off the moving target for the best αβ ratio estimation for normal breast tissue, recognizing that, as our estimation lowers, the BED for the B-39 and RAPID dose schedule of 3.85 Gy x 10 twice daily fractions approaches, not 50 Gy, but closer to 70 Gy. This may explain the unexpectedly high rates of poor cosmesis as compared to more standardly-fractionated partial breast irradiation. In fact, poor cosmesis mirrors that seen in the boost arm of EORTC 22881–10882 where women received 2 Gy x 33 = 66 Gy to the tumor bed. In other words, the twice daily regimen may deliver too high of a dose over too short of an interval that doesn’t allow for optimal inter-fraction normal tissue recovery. All of this laid the groundwork for a small prospective randomized trial of 113 women with node-negative breast cancers <3 cm who received adjuvant partial breast radiation of 3.85 Gy x 10 delivered either once daily (oAPBI, n=53) or twice daily (tAPBI, n=60). Grade 3 late skin toxicity developed in 4% versus 12%, respectively, primarily grade 3 fibrosis which was seen in 2% versus 8%. That’s in line with the overall rate of fair to poor cosmesis at two years which was more than tripled with twice daily treatment (7.5% → 26.7%). Finally, none of any of this is to say a partial breast approach isn’t sufficient for low-risk tumors. At a median follow-up of six years, only one patient has experienced a locoregional relapse after once-daily radiation and two after twice-daily.
TBL: The first randomized data of its kind is here to show what many have suspected for a long time, that it is the “accelerated” part of accelerated partial breast irradiation that bears the brunt of the responsibility for worse cosmesis. | Boutrus, Int J Radiat Oncol Biol Phys 2021