ESTRO 2025 - Abstract Book

S484

Clinical - Breast

ESTRO 2025

1516

Digital Poster Optimal dose of radiotherapy for patients with confirmed internal mammary lymph node metastasis in breast cancer in the adjuvant setting Daniel J Coyle 1 , Michelle Colfer 1 , Orla McArdle 1,2 1 Radiation Oncology, Royal College of Surgeons in Ireland, Dublin, Ireland. 2 Radiation Oncology, St. Luke's Radiation Oncology Network, Dublin, Ireland Purpose/Objective: The internal mammary lymph nodes (IMN) drain 10-40% of breast lymphatics (1) and may be involved at presentation in 14-33% of cases with non-metastatic breast cancer (2). IMN involvement has been identified in the past as an independent poor prognostic factor associated with higher rates of distant metastases and lower overall survival (3). Modern radiotherapy treatment planning allows effective treatment of involved IMN with an acceptable dosimetric profile, however, clear guidelines for optimal dose regimens are lacking. We aim to identify the optimal radiation dose for involved internal mammary lymph nodes (cIMN) in the adjuvant breast cancer treatment. Material/Methods: We performed a literature review from January 2010 to December 2024, using the databases PubMed and Prospero. Search terms included: (internal mammary lymph node) AND (radiotherapy) AND (breast cancer). Study inclusion criteria were: patients receiving adjuvant breast radiotherapy with cIMN, cIMN diagnosed based on either pathology or imaging, dose and radiotherapy technique described. The primary endpoint was median radiation dose administered to cIMN. The secondary endpoints were IMN recurrence, overall survival, disease-free survival, locoregional disease free survival and toxicity. Results: 487 citations were retrieved. After elimination of irrelevant papers, 101 studies were suitable for analysis. Nine studies met the inclusion criteria; all were retrospective series, two studies included a comparative group. The mean sample size was 80 (14–185). In all studies, adjuvant radiotherapy consisted of 50–50.4Gy in 1.8–2Gy per fraction to the breast or chest wall and loco-regional lymph nodes. In most cases, this was followed by a boost dose of 10–16Gy to the cIMN. Boost was given per institutional preference; 16Gy was used for cases with residual IMN findings after chemotherapy or for IMN nodes ≥1cm in size. The majority of plans were 3D conformal, a minority were treated with IMRT. IMN recurrence occurred in 0–11% of cases overall. One study demonstrated improved disease-free survival with higher dose to cIMN ≥1cm (4). Overall survival ranged from 74-84.2%. Conclusion: Our review reveals that while there were no fixed guidelines for optimal radiation dose, a consistent pattern emerges from available evidence. Adjuvant locoregional regional radiotherapy was most often followed by a 10–16Gy boost to the cIMN. A higher boost dose may provide improved disease-free survival for cIMN ≥1cm in size. It is important to acknowledge the limitations of the review consisting of only retrospective studies. Prospective trials are warranted to define the optimal dose regimen in cIMN breast cancer. References: 1. Choi JE. The metastatic rate of internal mammary lymph nodes when metastasis of internal mammary lymph node is suspected on PET/CT. J Breast Cancer. 2013 Jun;16(2):202-204. 2. Joo JH, et al. Impact of pathologic diagnosis of internal mammary lymph node metastasis in clinical N2b and N3b breast cancer patients. Breast Cancer Res Treat. 2017 Nov;166(2):511-518. 3. Madsen EV, et al. Prognostic Significance of Tumor-Positive Internal Mammary Sentinel Lymph Nodes in Breast Cancer: A Multicenter Cohort Study. Ann Surg Oncol. 2015 Dec;22(13):4254-62. Keywords: intermammary lymph nodes, breast cancer, review