LAS VEGAS—There appears to be minimal difference in 10-year ipsilateral breast tumor recurrence (IBTR) in a specific cohort of patients undergoing breast-conserving surgery whether they have a 1 mm or 2 mm margin width cutoff, raising the question of whether reexcision lumpectomies in these patients is necessary, according to new research.

The intent-to-treat analysis of the National Surgical Adjuvant Breast and Bowel Project (NSABP) B-35 trial, comparing anastrazole with tamoxifen in women with ductal carcinoma in situ (DCIS), was published in 2016. The trial included postmenopausal women with hormone receptor–positive disease who underwent breast-conserving surgery and whole breast irradiation. The researchers found no statistically significant difference in local recurrence rates in the two treatment arms.

As the largest trial to date to prospectively collect margin width data, NSABP B-35 presented an opportunity to look at the effect of margin width on the rates of IBTR and all other breast cancer events.

“One of the unique aspects of this study is that the institutional pathologists were required to submit a pathology form to the NSABP Biostatistical Center within three months of randomization, detailing the status of the margins,” said Irene Wapnir, MD, of the Stanford University School of Medicine, in Calif., at the 2025 annual meeting of the American Society of Breast Surgeons.

They identified 2,707 patients whose tumor margins the pathologist determined as indefinite or close, measuring less than 1 mm but not transected at the edge, or negative, defined as a margin of at least 1 mm.

“Using the 1-mm cutoff as a margin width discriminant, there was no difference in the distribution by treatment arm, age category or race,” Dr. Wapnir said.

There was a slight preponderance of smaller tumors in the margin-negative grouping. Pathologists were asked to qualify the negative (>1 mm) margins further by measuring the distance of the closest surgical margin. “Based on this, we were able to define a second group of 2,546 patients for whom we could perform an analysis using a 2-mm cutoff,” Dr. Wapnir said.

This allowed them to compare patients whose margin width was less than 2 mm versus 2 mm or greater. “Using these two margin discriminants, 1 mm and 2 mm, we looked at the number of events,” she said.

The most common first event was an IBTR for both the 1-mm or 2-mm margin width groups. Approximately one-third of the IBTR cases was invasive in both groups. The second most common event was a contralateral breast cancer.

Looking at the 502 patients whose tumor-free margins measured less than 1 mm, they found the 10-year cumulative incidence of IBTR was 5.6%, compared with the 2,205 patients whose margins measured at least 1 mm, who had a 4% 10-year cumulative incidence. “The absolute difference is quite small, but statistically significant,” Dr. Wapnir said (P=0.04).

Similarly, for the 879 patients with margins less than 2 mm, the 10-year cumulative incidence of IBTR was 5.3%, compared with 3.8% for the 1,667 whose margin width was greater than or equal to 2 mm. This difference was also borderline statistically significant (P=0.05).

“IBTR and contralateral breast cancers accounted for nearly all breast events. Small numerical differences were identified, but in both instances using a 1-mm or 2-mm margin width discriminant, the differences were not statistically significant,” Dr. Wapnir said.

The authors concluded that the small differences in rates of IBTR using 1-mm or 2-mm margin widths cutoffs were small and not clinically meaningful.

“The practice of reexcision lumpectomies for margin widths less than 1 mm or less than 2 mm should be reconsidered. We’d like to underscore that our findings apply only to postmenopausal women with hormone–receptor-positive DCIS receiving post lumpectomy whole breast irradiation and five years of adjuvant endocrine therapy.”

Julie Margenthaler, MD, a professor of surgery at the Washington University School of Medicine, in St. Louis, observed that the study challenges current guidelines, which support a negative margin of 2 mm or more following lumpectomy for DCIS.

“Surgeons should be cautions in abandoning the current ‘2 mm rule’ based on this data alone. The guideline was created based on a meta-analysis of a much larger cohort of patients,” she said. “In addition, all patients in the current study were postmenopausal and received adjuvant whole breast irradiation and five years of endocrine therapy. Thus, reexcision can likely be avoided for this subgroup of patients with less than 2-mm margins following lumpectomy for DCIS, but this should be a decision based on local pathology review and consideration of individual patient factors.”