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Top Line: Initial abstract reporting of NSAB B-39 last year in San Antonio suggested that local recurrence rates are just as low with partial as with whole breast radiation techniques—it just couldn’t prove it.
The Study: That’s because the number of events were just too small for any meaningful non-inferiority analysis. The full manuscript brings us slightly longer follow-up with more patients reaching the ten-year mark. What does this change? Not the difference between arms in the primary endpoint of in-breast recurrence rates, which was 4% with partial and 3% with whole breast radiation at a median of ten years follow-up. Neither did the fact the upper boundary of the confidence interval for the hazard ratio for local recurrence after partial breast radiation (1.58) surpasses the a priori threshold (1.5). However, unlike patients with cord compression, you can’t lose these women to follow-up if you tried. With over 4100 women with long-term data evaluable, the authors now unequivocally conclude that partial breast radiation cannot be considered statistically equivalent to whole breast radiation given there was more than sufficient statistical power. Importantly, there were no notable differences in toxicity, secondary cancers, nor patient-reported cosmesis.
TBL: The absolute difference in rates of local recurrence at 10 years is <1% when radiating either just part or the entire breast after resection of a favorable breast cancer. | Vicini, Lancet 2019