A 10-year randomized trial could not definitively conclude that partial breast irradiation (PBI) is equivalent to whole breast irradiation (WBI) in controlling local in-breast tumor recurrence, but the absolute difference between the two treatments was very small, suggesting it could be a useful option in some women.
“In the effort to improve the quality of life, we studied whether we could reduce treatment time” for women undergoing radiation, said Frank Vicini, MD, of William Beaumont Hospital in Royal Oak, Michigan. While WBI can mean 5–6 weeks of treatment, a PBI approach could bring that down toward 5–6 days.
Vicini presented results of a trial comparing PBI and WBI in 4,216 women with ductal carcinoma in situ, or invasive N0 or N1 breast cancer, at the San Antonio Breast Cancer Symposium (SABCS), held December 4–8 (abstract GS4-04). The WBI patients received 50 Gy at 2.0 Gy/fraction or 50.4 Gy at 1.8 Gy/fraction to the whole breast, followed by an additional boost of at least 60 Gy; the PBI patients received 10 treatments on 5 days totaling 34 Gy in 3.4-Gy fractions of interstitial brachytherapy or MammoSite balloon catheter, or 38.5 Gy in 3.85-Gy fractions using 3D conformal external beam therapy.
In total, there were 161 ipsilateral breast tumor recurrence (IBTR) events, including 90 events with PBI and 71 with WBI, for a hazard ratio (HR) of 1.22 (90% CI, 0.94–1.58). The trial design required that the upper bound of the 90% CI to be under 1.5, meaning the study failed to meet its primary endpoint. However, the absolute difference in 10-year cumulative incidence of IBTR between the two therapies was only 0.7% (4.6% vs 3.9%).
Similarly, the recurrence-free interval slightly favored the WBI group. The 10-year recurrence-free survival rate was 93.4% with WBI and 91.8% with PBI, for an HR of 1.33 (95% CI, 1.04–1.69; P = .02). The 10-year distant disease–free rate was 97.1% with WBI and 96.7% with PBI, for an HR of 1.31 (95% CI, 0.91–1.91; P = .15). Overall survival was also similar, with 10-year rates for WBI and PBI of 91.3% and 90.6%, respectively, and an HR of 1.10 (95% CI, 0.90–1.35; P = .35).
Though the study did fail to meet its primary endpoint, Vicini said the absolute differences mean PBI should not be ruled out. “Because the differences relative to both IBTR and recurrence-free interval were small, PBI may be an acceptable alternative to WBI for a proportion of women who undergo breast-conserving surgery,” he said.
Virginia Kaklamani, MD, of UT Health San Antonio and co-chair of SABCS, who was not involved with the study, said that this result contains a lesson regarding rational trial design. “It’s important when we design the trials to look at clinically meaningful differences,” she said. “We don’t want to harm our patients. But at the same time, we are harming them by giving them more treatment.” Designing a trial where a 0.7% difference means it fails may not really be helping patients in meaningful ways, she said.
Wendy A. Woodward, MD, PhD, of MD Anderson Cancer Center in Houston, also discussed the trial at the symposium, and she pointed out that standard practices with radiotherapy have changed substantially since this trial began. “In the last decade the length of radiotherapy has been cut almost in half,” she said. “It’s important when discussing with patients that the standard is now 3 weeks, so it isn’t a matter of looking at 5–6 weeks, as the standard becomes more and more convenient.”