The Study: The RAPID trial was another accelerated partial breast irradiation (APBI) study with results reported at the annual San Antonio Breast Cancer Symposium. As opposed to B39, this one employed external beam for both arms, making the APBI arm less invasive and more readily applicable across radiation clinics. Also different was the option for hypofractionation of 42.56 Gy in 16 fractions +/- a boost in the whole breast arm. Here again external beam APBI was delivered to 38.5 Gy in 10 fractions twice daily. Over 2K women with node-negative early-stage (≤ 3 cm) breast cancer were enrolled. And where the power analysis for the primary endpoint anticipated 64 local recurrences within 5 years, this instead took 8.6 years. Now we can finally say that APBI using external beam with 2 cm spherical margins on the tumor bed achieves “non-inferior” local recurrence rates compared to standard whole breast techniques: 3% versus 2.8%, respectively, at almost 9 years—slightly less than in B39 where up to 3 involved nodes were allowed. Granted, the same can’t be said for cosmesis with rates of fair/poor outcomes nearly doubled at 7 years following APBI (36%) versus whole breast (19%).
Bottom Line: APBI still appears safe when there’s a logistical reason to speed things up, but at the cost of marginally more recurrences when using brachy and of worse cosmesis when using external beam. | Whelan, SABCS 2018