10-Year Results Confirm WBI Benefit in Low-Risk Breast Cancer

Pam Harrison

April 29, 2019

Results out to 10 years confirm the benefit of whole breast irradiation (WBI) added onto hormonal therapy following conservative surgical management of low-risk hormone receptor positive (HR+) breast cancer.

Both local disease control and disease-free survival (DFS) outcomes were significantly improved in the group of women who received WBI compared with those who did not. 

The finding comes from part 8A of the Austrian Breast and Colorectal Cancer Study Group (ABCSG), and these new longer-term results were presented April 28 at the European Society for Radiotherapy and Oncology (ESTRO) 38th Annual Congress in Milan, Italy.

"In our analysis, the omission of WBI turned out to be the main predictor for in-breast recurrences," Gerd Fastner, MD, University Hospital, Paracelsus Medical University in Salzburg, Austria, said in a statement.

"Our findings show that radiotherapy is highly effective in significantly improving local control and DFS in combination with hormonal therapy compared to hormonal therapy alone, and this remains true after long-term follow-up of breast cancer patients with a good prognosis," he added.

In an interview with Medscape Medical News, Fastner commented that, given recent advances in radiation therapy, findings from the ABCSG 8A study do not necessarily mean that patients with a similar risk profile as those enrolled in the study should undergo WBI, as in recent years, results with partial breast irradiation (PBI) have shown similar outcomes to those attained by WBI.

He suggested that use of PBI, together with intraoperative techniques, perioperative brachytherapy, or postoperative hypofractionated external beam radiotherapy, appears to be an appropriate alternative to WBI, depending on individual patient risks. 

On the other hand, radiotherapy may not be appropriate in frail, elderly patients who are less likely to tolerate it well, Fastner noted.

Commenting on the study, ESTRO President Umberto Ricardi, MD, University of Turin, Italy, noted that in 2006 the same researchers reported results at approximately 5 years of follow-up which showed radiotherapy delivered after surgery and combined with tamoxifen or anastrozole led to a significant reduction of in-breast recurrences compared with hormonal therapy alone.

"These new results, with 10 years of follow-up, show that the beneficial effects of irradiation persist, becoming even more evident for these women," Ricardi said in a statement.

"This is important information for women and their physicians which helps them to choose the best treatment for their breast cancer," he concluded.

Study Details

The analysis was carried out in 869 patients who received antihormonal (AH) treatment with tamoxifen or anastrozole after breast-conserving surgery who were then randomized to receive WBI or be followed by observation only.

"All patients were defined as 'low-risk' with grading G1 or 2, small tumor sizes (< 3 cm), and a node negative status," the investigators explain.

WBI was delivered up to a mean total dosage of 50 Gy in conventional fractionation over a period of 39 days and within 6 weeks of surgery. Some 71% of patients randomized to the WBI arm received an additional tumor bed boost at a mean dose of 10 Gy.

At a mean follow-up of 9.89 years, only 11 in-breast recurrences were found in women given additional WBI versus 31 cases in the non-WBI group, Fastner told Medscape Medical News. This corresponds to a 67% relative risk reduction of in-breast relapses in favor of the WBI arm, he noted. 

This resulted in a local disease control rate of 97.5% at 10 years for women treated with additional radiotherapy compared with 92.4% for women who received hormonal treatment alone (P = .0004), Fastner commented.

"Of all 11 in-breast recurrences detected in the group with radiation therapy, six received an additional tumor boost," Fastner added in an email to Medscape Medical News.

"Therefore, a sequential tumor boost after WBI seems to be of no clinical relevance in terms of local control within the existing trial," he added.

On the other hand, patients who entered the study with an undefined tumor grade had an almost fourfold higher risk for local recurrence compared to those with grade 1 tumors, Fastner also noted.

DFS and WBI

Disease-free survival (DFS) rates, as defined by the occurrence of local or distant relapses or both, were also significantly better among the group who received WBI, at 94.5% compared with 88.4% for those who did not receive any radiation therapy (P = .02).

As Fastner noted, about 60% of study participants underwent axillary dissection where at least 10 lymph nodes were removed.

When these patients were compared with those who had only the first relevant axillary nodes removed — sentinel-node extirpation — DFS rates were almost 75% higher in the WBI group compared with the non-WBI group (hazard ratio [HR], 0.25; P = .007).

However, the same DFS advantage was not seen in women who underwent axillary dissection — a finding which is difficult to interpret and can't yet be answered by findings from the study, he commented.

Interestingly, however, metastases-free survival rates at 10 years were virtually identical between the two groups, at 96.7% for the WBI arm versus 96.4% for the non-WBI group, as were overall survival (OS) rates, at 86.6% and 87.6%, respectively.

Genetic Analysis

Of the 869 patients who took part in the WBI randomization part of the study, 519 patients were also entered into the PAM50 trial, in which gene expression was analyzed, the researchers note.

Results from the gene expression analysis found that women with Ki67 > 20% (a high-risk gene marker) or who tested positive for HER2/neu, or both, were not significantly more likely to experience in-breast recurrences (HR, 2.12; P = .14) compared with women who did not have either of these high-risk features.

The study was funded by AstraZeneca. Fastner and Ricardi have reported no relevant financial relationships.

ESTRO 2019. Presented April 28, 2019. Abstract OC-0270

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