The Study: To try this out, NSABP B-39 randomized over 4200 women with stage 0-2 breast cancer to post-lumpectomy conventionally-fractionated whole breast irradiation with a boost versus accelerated partial breast irradiation (APBI). In the APBI arm, the technique utilized was purely physician preference between catheter brachytherapy at 3.4 Gy x 10 or external beam at 3.85 x 10 all delivered twice daily. And this week finally brought us results reported at the annual San Antonio Breast Cancer Symposium. The primary endpoint of local recurrence rates at 10 years were 4.1% with whole breast and 4.8% with accelerated partial breast. The good news? These rates are measly. Bad news is they’re so measly the confidence interval failed to be tight enough to establish non-inferiority. When increasing the number of events by counting either local or distant relapse at 10 years (with death censored), a statistical difference was detected: 6.6% with whole breast and 8.1% with APBI. Finally, rates of grade ≥3 toxicity were higher with APBI (9.6%) than with whole breast (7.1%), which is in contrast to the much less accelerated partial breast regimens on UK IMPORT LOW.
Bottom Line: APBI appears pretty safe when there’s a logistical pressure to speed things up, but otherwise 3-4 weeks of hypofractionation may be the sweet spot. | Vicini, SABCS 2018