Accelerated partial breast irradiation (PBI), although not 100% equivalent to whole-breast irradiation (WBI) for disease control in patients with early- stage breast cancer, yields outcomes so similar that it may be considered a viable treatment option for many patients, a long-term follow-up of the largest trial of its kind indicates.
“In an effort to improve quality of life for our patients, we studied whether or not we could reduce overall treatment times significantly down to a week or less by doing a technique known as partial breast irradiation, meaning limiting radiation only to the lumpectomy cavity region and accelerating treatment down to 5 days or less,” Frank Vicini, MD, MPH, Radiation Oncology Institute, Pontiac, Michigan, told a press briefing held here during the San Antonio Breast Cancer Symposium (SABCS) 2018.
“And while [we] cannot declare that WBI and PBI are equivalent in controlling local-in-breast tumor recurrence…the absolute difference in the 10-year cumulative incidence of IBTR [ipsilateral breast tumor recurrence] was only 0.7%…. So PBI may be an acceptable alternative to WBI for a proportion of women who undergo breast-conserving surgery,” he concluded.
The NRG (NSABP B-39/RTOG 0413) trial randomly assigned 4216 women who had recently undergone lumpectomy to receive either WBI or accelerated PBI. Women enrolled in the study had zero to three positive axillary nodes on study entry.
Twenty-five percent of the group had ductal carcinoma in situ (DCIS), 65% had stage I breast cancer, and 10% had stage II disease. The majority of women also had hormone receptor—positive tumors.
Women who were assigned to the WBI arm following adjuvant chemotherapy received daily treatment with 2.0 Gy/fraction of radiation totaling 50 Gy with a sequential boost to the surgical site.
Duration of WBI was 5 to 6 weeks, which was the standard of care at the time the trial was designed.
Those assigned to accelerated PBI prior to adjuvant chemotherapy received twice-daily treatment with 3.4 to 3.85 Gy given as either brachytherapy or 3D external-beam radiation.
Patients who underwent accelerated PBI received a total of 10 treatments over 5 to 10 days.
“The primary endpoint was to determine whether or not PBI results in IBTR — both DCIS and invasive breast cancer — was the same as it was for WBI,” Vicini noted.
At a median follow-up of 10.2 years, Vicini and colleagues documented a total of 161 IBTRs as first events — 90 among women treated with accelerated PBI, and 71 for those treated with WBI.
On the basis of the upper limit of the hazard ratio confidence interval, “PBI did not meet the criteria for equivalence to WBI in controlling IBTR,” Vicini reported.
On the other hand, the 10-year cumulative incidence of IBTR was very low in both groups, at 4.6% for patients in the accelerated PBI arm vs 3.9% for those in the WBI arm.
Furthermore, the difference in recurrence-free interval rates between the two arms — 91.8% in the accelerated PBI group vs 93.4% in the WBI group — was also negligible, at only 1.6%, Vicini added.
There were no differences in distant disease-free interval (DDFI), overall survival (OS), or disease-free survival (DSF).
For example, at 10 years, 96.7% of patients in the accelerated PBI arm were free of distant disease, as were 97.1% of patients in the WBI arm.
At the same follow-up point, 90.6% of patients in the accelerated PBI arm were still alive, as were 91.3% of those treated with WBI.
DSF rates were also very similar between the two treatment arms, Vicini noted.
Rates of grade 3 toxicity were similar between the two treatment groups, at 9.6% in the accelerated PBI group and 7.1% for those who received WBI. Rates of higher-grade toxicities were also very low and were similar between the two treatment groups.
Additional analyses are currently underway to evaluate secondary endpoints, including quality of life and cosmetic outcomes, Vicini noted.
“There have been many studies looking at quality of life [after radiotherapy], and you can imagine that quality of life is better with PBI — it’s pretty intuitive,” Vicini noted.
“And there is a pretty dramatic difference in the treatment interval from 5 to 7 weeks [with WBI] down to a week or less [with PBI], so as long as the control rates are the same, the trend in oncology now is ‘less is better,’ ” he added.
Furthermore, DDFI, OS, and DFS were not different between the two arms. Arguably, these endpoints are more important than recurrence, Vicini suggested, even though recurrence is an important event for patients.
“These findings suggest that the less burdensome radiation method of accelerated PBI may be an acceptable choice for many women,” Vicini said.
“So I think this is a very important study and it certainly is important to patients,” he concluded.
PBI Not Commonly Used
Asked by Medscape Medical News to comment on the findings, press briefing moderator Virginia Kaklamani, MD, University of Texas Health, San Antonio, noted that the majority of patients in the United States who are similar to those in the current study are still undergoing WBI. “PBI is not as common as it should be,” she noted.
“In the end, this is detrimental to patients, because if they have WBI, they have to come in for radiation therapy for at least 5 and sometimes 7 weeks, whereas otherwise [with PBI], they would be coming in for 5 to 10 days,” Kaklamani added.
She felt that findings from the current study “absolutely” should provide reassurance that radiation oncologists can offer accelerated PBI to patients who meet the same criteria as those included in the current study.
“We have many trials evaluating shorter radiation intervals,” Kaklamani noted.
“And the medical community needs to be implementing more PBI, and patients should also be asking for it,” she suggested.
Reshma Jagsi, MD, DPhil, professor and deputy chair, Department or Radiation Oncology, University of Michigan in Ann Arbor, felt that high-quality randomized trials such as this one in which shorter courses of accelerated PBI were compared to WBI are “critically important” to help inform treatment decisions that women who are diagnosed with breast cancer must make each year.
“These decisions depend on the patient’s own values and preferences,” Jagsi told Medscape Medical News in an email.
For example, some women might prefer to come in twice a day for a week rather than daily for many weeks to minimize disruption of their work schedule or because of problems involving transportation, whereas others might quite reasonably decide otherwise.
“The contribution of trials like these is to arm each woman with the precise risk information she needs to make the decision that is right for her,” Jagsi wrote.
“These trialists have made an enormous contribution through this work,” he concluded.