2755 - Is the Strategy of Regional Nodal Irradiation (RNI) in cT1-2N0M0 Breast Patients with Limited Positive Sentinel Nodes (SLNs) Forgoing ALND Associated with the Risk of Non-SLNs Metastasis: A Multi-Cent

S. Zheng^1,2, W. Qi^1,2, L. Cao^1,2, K. W. Shen³, Z. Sun⁴, J. Liu⁵, C. Jiang⁵, Y. Wang⁶, and J. Chen^1,2; ¹Department of Radiation Oncology, Ruijin Hospital, Shanghai Jiaotong University School of Medicine, Shanghai, China, ²Shanghai Key Laboratory of Proton-therapy, Shanghai, China, ³Department of General Surgery, Comprehensive Breast Health Center, Ruijin Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, China, ⁴Department of Breast Surgery, Jiangxi Cancer Hospital, Nanchang, China, ⁵Department of Radiation Oncology, Jiangxi Cancer Hospital of Nanchang University, Nanchang, China, ⁶Breast Cancer Center, Shandong Cancer Hospital and Institute Affiliated to Shandong University, Jinan, China

Purpose/Objective(s): De-escalation of axillary surgery in cT1-2N0M0 breast patients with limited positive sentinel nodes (SLNs) has been recommended by international guidelines, although the strategy of regional node irradiation (RNI) is not fully established. To quantify the risk and burden of non-SLN metastasis, a nomogram was generated in these subsets of patients from 3 tertiary hospitals in China. Clinical practice of RNI in patients with 1-2 positive SLN(s) without axillary lymph node dissection (ALND) over the past decade was then analyzed. Materials/

Methods: In total, 832 cT1-2N0M0 breast cancer patients from 3 tertiary hospitals who underwent mastectomy or lumpectomy with 1-2 SLN(s) followed by ALND from 2011 to 2022 were enrolled and were randomized at a ratio of 7:3 into a training cohort and an internal validation cohort. The external validation cohort consisted of 264 patients in the z0011 study. Ordinal logistic regression analysis was performed to identify predictors of any pN stage and pN2-3 with non-SLN metastasis. Then, a total of 240 cT1-2N0M0 patients with 1-2 SLN(s) who underwent BCS omitting ALND between 2014 and 2023 from 2 centers were compared for RNI strategy using the Chi-square.
Results: Of the 565 patients in the training group, 375 (66.4%) had no non-SLN metastasis, 110 (19.5%) were of pN1 with non-SLN metastasis, and 80 (14.1%) were of pN2-3 with non-SLN metastasis. Multivariate ordinal logistic regression showed that more non-SLN metastasis was significantly associated with higher BMI (p=0.046), more positive SLNs (p < 0.001), fewer negative SLNs (p < 0.001) and LVI (p=0.003). A nomogram was constructed based on these 4 variables. The AUC of non-SLN metastasis was 0.695 and 0.676 for the internal and external validation set, and the AUC of pN2-3 was 0.814 and 0.775, respectively. A risk stratification system was established based on the cutoff value of median total risk score being 99.7. In low-risk group, the proportion of pN1patients without non-SLN positive, pN1 with non-SLN positive and pN2-3 with non-SLN positive was 78.4%, 17.0% and 4.6% respectively, which was 54.4%, 21.9% and 23.7% respectively in high-risk group. There were 141 and 99 patients from 2 of the 3 centers respectively, who met the criteria of ACOSOG Z0011. The application and the extent of RNI differs significantly in these 2 centers (both p<0.001).
Conclusion: Along with the recommendation axillary surgery de-escalation, there exists a non-homogenous practice of RNI in cT1-2N0M0 breast patients with limited positive SLNs without ALND. Our nomogram could predict both the likelihood of any non-SLN metastasis and 4 or more positive ALNs, which could potentially help radiation oncologists to optimize the decision and extent of RNI based on individual risk.

Abstract 2755 – Table 1

Low risk	Total	AxRT+SC/IMN	WBI±AxRT	P value
Center A	105	66 (62.9%)	39 (37.1%)	p<0.001
Center B	54	6 (11.1%)	48 (88.9%)
High risk	Total	AxRT+SC/IMN	AxRT+SC/IMN	P value
Centre A	36	29 (80.6%)	7 (19.4%)	p<0.001
Centre B	45	10 (22.2%)	35 (77.8%)