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SAN ANTONIO -- A supplemental radiation boost delivered concurrently during hypofractionated whole-breast irradiation reduced treatment time, yet was just as effective in terms of recurrence, for patients with early-stage high-risk breast cancer compared with conventional whole-breast irradiation with a sequential boost, results from the phase III showed.

At a median follow-up of 7.4 years, 56 ipsilateral breast recurrence (IBR) events (the study's primary outcome) had occurred, with 5- and 7-year IBR rates of 1.9% and 2.6% in the concurrent arm and 2.0% and 2.2% in the sequential arm (HR 1.32, 90% CI 0.84-2.05, P=0.039), reported Frank Vicini, MD, of GenesisCare in Pontiac, Michigan, during a plenary session at the American Society for Radiation Oncology annual meeting.

While treatment time was reduced by about a week to a week and a half -- "in other words, delivering the entire treatment, even in high-risk patients, in 3 weeks ... more importantly, the trial met noninferiority," Vicini noted. An HR below 2.12 was needed to define noninferiority.

Treatment-related adverse events were comparable between the two arms, he said. "As a matter of fact, overall toxicity was very low, with only about 3% ≥grade 3 toxicity in either arm of the study."

There was also no difference in physician-rated cosmetic outcomes between the arms at 3 years, with 82% of those in the concurrent arm and 86% of those in the sequential arm with an excellent/good score.

"Thanks to RTOG 1005, we now have more tools in our toolbox that we can effectively treat patients with early-stage breast cancer," said session discussant Lori J. Pierce, MD, of the University of Michigan in Ann Arbor. "We can continue to individualize the treatments for breast cancer and also maximize convenience for patients."

When the trial was launched in 2011, "we were witnessing the slow adoption of hypofractionated whole-breast irradiation in this country," she noted. Much of the hesitation to use hypofractionation was due to "the uncertainty of adding a boost."

Thus, "this was the right trial at the right time," she added. "I think it helped to encourage the use of hypofractionation to the whole breast at a time the uptake was modest."

Vicini pointed out that while randomized trials support a radiation boost to the lumpectomy cavity region after whole-breast irradiation, the advantage gained with the relative reduction in IBR comes with the disadvantage of an extended treatment duration.

"Hypofractionated whole-breast irradiation in 15-16 fractions over 3 weeks is currently used to deliver adjuvant whole-breast irradiation after lumpectomy, with acceptable toxicity and comparable IBR rates as 5 weeks of conventional whole-breast irradiation," he explained. "However, the hypofractionation trials that established this equivalence used a sequential boost when delivered, and the boost was not evaluated as part of the study question."

As a result, he noted, the boost delivery has remained sequential, adding about a week to a week and a half of additional treatment duration. Moreover, while a boost is indicated in patients at high risk of local recurrence, the earlier hypofractionation studies were completed with primarily low-risk patients.

Therefore, Vicini and his team wanted to determine whether the local recurrence rate with a radiation boost delivered concurrently was noninferior to a boost delivered sequentially.

They began enrolling patients in May 2011 and ultimately included 2,262 eligible patients (median age 55) with early-stage high-risk breast cancer treated at 276 different sites across the U.S., Canada, Switzerland, Israel, Hong Kong, Japan, Republic of Korea, and Singapore.

Risk factors for recurrence included higher tumor grade, older age, receipt of pre-surgical chemotherapy, and estrogen receptor-negative status.

"With the caveat that for the concurrent boost we use contemporary dose planning constraints, and we use approaches that are similar to how we would plan a sequential boost so that we don't have an increased interval dose, I would argue that this is the preferred approach for patients who are getting whole-breast moderate-dose hypofractionation as opposed to sequential boost," Pierce said.

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