S273
Clinical - Breast
ESTRO 2026
Kiechle 2 , Marcus R Makowski 3 , Stephanie E Combs 1,4 , Thomas Huber 3 , Kai J Borm 1,5 1 Department of Radiation Oncology, TUM School of Medicine and Health, Klinikum Rechts der Isar, Technical University of Munich, Munich, Germany. 2 Department of Gynecology, TUM School of Medicine and Health, Klinikum Rechts der Isar, Technical University of Munich, Munich, Germany. 3 Department of Radiology, TUM School of Medicine and Health, Klinikum Rechts der Isar, Technical University of Munich, Munich, Germany. 4 Institute of Innovative Radiotherapy (iRT), Department of Radiation Sciences (DRS), Helmholtz Zentrum München, Oberschleißheim, Germany. 5 Bavarian Cancer Research Center (BZKF), TUM University Hospital, Munich, Germany Purpose/Objective: Assessing internal mammary lymph nodes (IMN) in breast cancer patients is challenging. Surgical assessment should not be performed in IMN, and in imaging, IMN appear significantly smaller than axillary lymph nodes (AXN) and diagnostic criteria are difficult to apply. However, identifying IMN metastases is highly relevant due to the associated high risk of distant metastasis and therefore implications for further treatment decisions. Material/Methods: A total of 298 non-metastatic breast cancer patients with Magnetic Resolution Imaging (MRI) before and after neoadjuvant chemotherapy (NACT) at the TUM university hospital between 2013 and 2023 were retrospectively analyzed. The size of IMNs visible on pre-NACT MRIs was measured on pre- and post-NACT MRI scans in the axial and coronal planes (figure axial (A) and coronal (B) plane, before (1) and during/after (2) NACT, measurements next to IMN (red arrows)) and correlated with the histopathological and clinical response of the primary tumor (PT) and AXN, as well as with risk factors for IMN involvement.
dedicated breast stereotactic prototype equipped with rib dampening (BSRD). Treatment planning incorporated four-dimensional computed tomography (4DCT) and magnetic resonance imaging (MRI) for tumour size evaluation, and dose planning was performed with ECLIPSE software. Results: Of the 24 patients recruited, 16 completed SBRT. Among those who did not undergo treatment, one patient was unable to tolerate it, two declined therapy, and five chose alternative fractionation regimens.The median age of the treated cohort was 84 years (range 64–96). Stage distribution was Stage I (18.75%), Stage IIA (56.25%), Stage IIB (18.75%), and Stage IIIC (6.25%). The most common molecular subtype was Luminal B (7/16), followed by Luminal A (5/16), HER2-positive (2/16), and Triple Negative (2/16).Half of the treated patients did not receive neoadjuvant therapy; among the remainder, two received chemotherapy, three chemotherapy plus hormone therapy, two hormone therapy alone, and one immunotherapy. Regarding adjuvant treatment, only one patient who had not received neoadjuvant therapy initiated hormone therapy after SBRT, while the others continued their previous systemic regimens.SBRT was well tolerated in all cases, with no acute or late toxicities observed during treatment or follow-up. At the 6-month evaluation, five patients achieved a complete response (CR), five showed a partial response (PR), two had stable disease (SD), and four remain pending assessment. At the 12-month follow-up, three maintained CR, and one patient with previous PR showed further tumour reduction; the remaining patients have not yet undergone imaging evaluation at
12 months. Conclusion:
This updated analysis reinforces our previous findings, showing that SBRT is a well-tolerated and effective treatment option for breast cancer patients unfit for surgery. The absence of toxicity and the encouraging local responses support SBRT as a viable curative alternative in this challenging patient population. Continued follow-up and patient inclusion are warranted to confirm long-term efficacy and safety outcomes. Keywords: Breast, SBRT Digital Poster Highlight 2363 Nodal response after neoadjuvant systemic therapy: are we missing internal mammary lymph node metastases? Johannes E Wider 1 , Sophie T Behzadi 1 , Jana Nano 1 , Sophia Kiesl 1 , Rebecca Moser 1 , Luisa Allwohn 1 , Sophie Maier 1 , Evelyn Klein 2 , Georg P Schmidt 2 , Marion
Results: On pre-NACT MRI, visible IMNs were detected in 175 out of 298 patients (58.7%), 154 of which showed a clinical and histopathological response of the PT to NACT. Complete or partial IMN response was observed in 62 out of these 154 patients (40.3%) (figure A), but only 22.6% of the patients who later showed IMN response had IMN metastases suspected at initial staging or in tumor board discussion.IMN
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