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SAN ANTONIO -- Omitting regional nodal irradiation (RNI) did not increase the risk of disease progression or death in patients whose breast cancer changed from lymph node-positive to lymph node-negative after neoadjuvant chemotherapy, a randomized study presented here showed.

For the primary endpoint of invasive breast cancer recurrence-free interval, there were 50 events in the patients who received RNI versus 59 events in the group assigned to observation (HR 0.88, 95% CI 0.60-1.29, P=0.51) at a median follow-up of 59.5 months, reported Eleftherios P. Mamounas, MD, MPH, of the Orlando Health Cancer Institute in Florida.

The 5-year estimates 0f invasive breast cancer recurrence-free interval were 92.7% and 91.8% in the two groups, respectively, Mamounas noted during a press briefing at the San Antonio Breast Cancer Symposium.

The two groups also had similar rates of loco-regional recurrence-free interval, distance recurrence-free interval, disease-free survival, and overall survival.

"These findings suggest that downstaging involved axillary nodes with neoadjuvant chemotherapy can optimize adjuvant radiotherapy use without adversely affecting oncologic outcomes," Mamounas said.

"There are two schools of thought," he noted. "These patients have positive nodes up front so they should be treated as node positive with radiotherapy. The other approach is they were node negative at the time of surgery, so the rate of loco-regional recurrence is lower, so maybe radiotherapy can be omitted."

He said that in his institution, the usual practice had been to use regional nodal radiotherapy. However, based on these results, "that will probably change," he added. "We will omit regional nodal radiotherapy for patients who have the criteria for this study, [if] their nodes are negative after neoadjuvant chemotherapy."

Press briefing moderator Kate Lathrop, MD, of the Mays Cancer Center at UT Health San Antonio, said the results from the trial were "eagerly awaited," since the question of RNI or no RNI was "open ended."

"In tumor boards we didn't have the data to make these decisions on whether patients were going to benefit from radiation therapy after pathologic complete response to neoadjuvant therapy," she said. "And we are using so much more neoadjuvant therapies, and those regimens are getting so much better as we develop more targeted therapies."

"And this is important for patients, too," she noted, adding that not only is there a benefit to avoiding the toxicities associated with radiotherapy, there are time and financial implications as well. "Knowing that the patients did just as well without radiation ... and I think it will change a lot of opinions on how much radiation we can avoid in our patients."

Mamounas also pointed out that patients who have mastectomies and undergo reconstruction are known to experience more complications when they received radiotherapy. "So for those patients I think this will be a significant benefit if radiation can be avoided," he said.

This included 1,556 patients diagnosed with lymph-node positive, nonmetastatic breast cancer whose lymph nodes were found to be cancer free after neoadjuvant chemotherapy and either mastectomy or breast-conserving surgery.

Patients who received RNI also received chest wall irradiation or whole breast irradiation, while those randomized to observation had whole breast irradiation.

Patients had a median age 0f 52, and 69% were white; 21% had tumors classified as T1, 59-61% as T2, and 18-20% as T3; 21-23% had triple-negative breast cancer, 20-22% had hormone receptor (HR)-positive/HER2-negative disease, 24-25% had HR-negative/HER2-positive disease, and 31-33% had HR-positive/HER2-positive disease.

A majority of patients (58%) had breast-conserving surgery, 55% had sentinel lymph node biopsy (SLNB), and 45% had axillary lymph node dissection with or without SLNB; 78% had breast pathologic complete response.

There were no study-related deaths and no unexpected toxicities. Grade 4 toxicities were rare (0.5% with RNI vs 0.1% without), and 10% of patients developed grade 3 toxicities in the RNI group versus 6.5% in the no-RNI group. The most common grade 3 toxicity was radiation dermatitis (5.7% vs 3.3%, respectively).

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