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CHICAGO -- Internal mammary node irradiation (IMNI) failed to significantly improve survival outcomes for women with node-positive breast cancer, a phase III trial conducted in Korea found.

At a median follow-up of 100 months, the 7-year disease-free survival (DFS) rates were 85.3% for the group who received IMNI versus 81.9% for those that did not, a non-significant difference (HR 0.80, 95% CI 0.57-1.14), reported Yong Bae Kim, PhD, of Yonsei Cancer Center in Seoul.

Overall survival (OS) at 7 years was also no different in the total population, at 89.4% and 88.2%, respectively (HR 0.87, 95% CI 0.57-1.31), according to findings presented here at the annual meeting of the (ASTRO).

"However, women with medially or centrally located tumors can be considered candidates for IMNI," said Kim, as this subgroup saw higher 7-year DFS rates in an unprespecified analysis, at 91.8% with versus 81.6% without IMNI (HR 0.42, 95% CI 0.22-0.82).

Full results of the study were published simultaneously in .

ASTRO discussant Jennifer Bellon, MD, of Dana-Farber Cancer Institute in Boston, noted that the trial was designed to detect a 10% difference in DFS at 7 years between the two treatment groups, which exceeds the results from regional nodal irradiation seen in other studies, such as the randomized and trials.

"This is an important, but not definitive trial," Bellon said. "While it did not reach its primary endpoint, that may have been related to its lack of power, coupled with a very high-risk cohort."

"We will continue to learn how tumor biology helps predict the risk of local regional recurrence, as well as responsiveness to radiotherapy, and how to tailor radiotherapy fields based on response to preoperative systemic therapy," she added. "And we will need to continually re-evaluate nodal radiation as systemic therapies improve."

The current trial was conducted from 2008 to 2020 at 13 hospitals in South Korea and included 735 women (median age 49) who were eligible if they had pathologically confirmed, node-positive breast cancer after breast-conservation surgery or mastectomy with axillary lymph-node dissection. Patients with distant metastasis or who had received neoadjuvant treatment were excluded.

Participants underwent regional node irradiation along with breast or chest wall irradiation, and were randomized 1:1 to radiotherapy with or without IMNI.

Of these patients, 41.4% had N1 disease, 36.6% N2, and 22.0% N3, and just about all (98.9%) received combination chemotherapy.

Regarding secondary outcomes in the total population, Kim reported the following for the with- and without-IMNI groups at 7 years, respectively:

• Breast cancer mortality: 8.4% vs 10.8% (HR 0.74, 95% CI 0.47-1.16)

• Distant metastasis-free survival (DMFS): 85.8% vs 83.2% (HR 0.81, 95% CI 0.56-1.16)

There was a significant interaction found between tumor location and treatment effect, Kim noted. In addition to the higher DFS among patients with mediocentrally located tumors, the following results in this group were also observed at 7 years:

• Breast cancer mortality: 4.9% in the IMNI group vs 10.2% in the without group (HR 0.41, 95% CI 023-0.85)

• DMFS: 91.8% vs 82.3%, respectively (HR 0.44, 95% CI 0.23-0.85)

• OS: 93.2% vs 88.5% (HR 0.51, 95% CI 0.24-1.11)

For adverse events, there were no differences in toxicity between the groups, although there was a non-statistically significant difference in the rate of radiation pneumonitis in the IMNI group compared with the without group (6.1% vs 3.2%).