ESTRO 2025 - Abstract Book

S117

Invited Speaker

ESTRO 2025

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Speaker Abstracts Radioligand therapy: Transformative potential in radiation oncology Thomas Zilli Radiation Oncology, IOSI - EOC, Bellinzona, Switzerland

Abstract: Radioligand therapy (RLT) has emerged as a novel therapeutic approach in oncology, utilizing targeted radionuclides to selectively irradiate tumor cells while sparing normal tissues. Initially established in neuroendocrine tumors and prostate cancer, RLT is now being explored in various malignancies, including gliomas, breast cancer, and hematologic diseases. Its combination with external beam radiotherapy (EBRT) presents a promising strategy to enhance tumor control through complementary mechanisms. The rationale for integrating RLT with EBRT lies in their synergistic potential: EBRT delivers highly conformal local irradiation, while RLT provides systemic tumor targeting. This dual-modality approach could improve local and distant disease control, particularly in oligometastatic and radiosensitive tumors. However, key challenges must be addressed, including dose escalation strategies, cumulative radiation exposure, toxicity mitigation, and the optimization of treatment sequencing. Understanding the radiobiological interactions between these modalities is crucial for refining fractionation protocols and personalizing dose adaptation. Patient selection remains a critical consideration, with molecular imaging techniques, such as PET/CT and SPECT, playing a central role in identifying suitable candidates. Additionally, the timing and sequencing of RLT and EBRT— whether delivered concurrently or sequentially—require further investigation to maximize therapeutic efficacy while minimizing toxicity to organs at risk. This presentation will review the current clinical landscape, including pivotal trials evaluating RLT-EBRT combinations across different cancer types. Emerging radioligands, advances in dosimetry, and novel imaging biomarkers, will also be discussed. Ultimately, integrating RLT with EBRT represents a significant advancement in precision oncology, with the potential to redefine treatment paradigms across multiple malignancies. Speaker Abstracts Regional radiotherapy after upfront surgery for node negative disease Philip Poortmans Radiation oncology, Iridium Netwerk, Wilrijk-Antwerp, Belgium. Faculty of Medicine and Health Sciences, University of Antwerp, Wilrijk-Antwerp, Belgium Abstract: Most data about regional nodal radiation therapy for clinically node-negative disease (RT) stems from older trials, comparing more and less radical surgery. In NSABP B-04, the cumulative incidence of locoregional recurrences was lowest in the group treated with total mastectomy + RT without ALND, notwithstanding that 40% of the patients who underwent axillary surgery had involved nodes. In the patient group with mastectomy without axillary surgery, 21% of the patients without any axillary nodes removed developed an axillary recurrence, compared to 12% in patients who had “incidentally” 1 to 5 nodes removed and none in those with ≥6% incidentally removed. Patients after mastectomy including axillary surgery, nodal recurrences exclusively occurred in case of ≥4 involved nodes. No significant differences in distance metastases rates as first event were seen among the 3 groups. In the Manchester trial, clinically node negative patients after mastectomy without axillary surgery were randomised between RT or not, showing a significant reduction of locoregional recurrences and a non-significant survival improvement. Importantly, one-third of the patients without RT had an axillary nodal recurrence within 5 years. The Edinburgh trial randomised node pathologically negative patients based on axillary sampling to RT or not. Two thirds of the patients had identifiable nodes, leading to two subgroups of histologically proven negative nodes vs. nodes not identified. In the RT-arm, 30% were node positive versus 25% in the no-RT-arm. Locoregional recurrences 4844