ESTRO 2023 - Abstract Book

S578

Monday 15 May 2023

ESTRO 2023

al., 2019). More recently, a new high-risk category termed ultracentral tumors has emerged, in which the planning target volume (PTV) overlaps the proximal bronchial tree or the esophagus. Concerns over especially severe complications following SBRT of ultracentral tumors even with the above-mentioned risk-adapted fractionation schedules were raised when several reports described significantly increased grade 3 toxicity in up to 40% of the patients including fatal pulmonary hemorrhage (Tekatli et al., JTO 2016). The recently published HILUS trial which included 65 patients with ultracentral lung tumors treated with 8 x 7 Gy prescribed to the 67% isodose reported, despite promisng local control, highly increased grade 3 to 5 toxicity noted in 22 patients, including 10 cases of treatment-related death (bronchopulmonary hemorrhage, n = 8; pneumonitis, n = 1; fistula, n = 1). By contrast, others did not detect significant differences in higher grade toxicity when comparing pulmonary SBRT for ultracentral to central tumors (Chang et al., Radiother Oncol 2018). The ongoing SUNSET trial might further shed light on a safe dose-fractionation regimen for treating ultracentral tumors with SBRT (Giuliani et al., Clin Lung Cancer 108) Nevertheless, in order to also balance toxicity in pulmonary ultracentral SBRT, either more defensive dose schedules that avoid critical overdoses in adjacent organs-at-risk (OARs), but potentially underdose the tumor might be applied or modern high-precision image-guided radiation techniques might bear the potential to enable safer dose escalation. fRecently, magnetic resonance guided radiotherapy (MRgRT) has become clinically available allowing for more individualized radiation treatments by daily on-table plan adaptation to the current patient and tumor anatomy. Hence, MRgRT might be a promising tool to offer a balance between keeping the dose to closely located radiosensitive OARs below tolerance levels, while simultaneously maintaining adequate dose coverage of the target. Few studies have already reported promising results (Henke et al., Adv Radiat Oncol 2018, Fianzzi et al., IJROBP 2020, Regnery et al., ESTRO 2023) and prospective trials are recruiting (STAR-LUNG STUDY (NCT05354596) and MAGELLAN (NCT04925583)). JCO

SP-0699 Balancing abdominal toxicity after liver SBRT A.Mendez Romero The Netherlands

Abstract not available

SP-0700 Balancing toxicity in pelvic SBRT N. van As

Abstract not available

SP-0701 Balancing toxicity when SBRT is combined with systemic therapy B. Jereczek-Fossa 1 1 University of Milan and European Institute of Oncology, Radiation Oncology , Milan, Italy

Abstract Text SBRT is more and more commonly employed in the modern radiotherapy, including both primary tumor treatment and management of recurrent and metastatic disease. In all these scenarios systemic therapy is commonly used and in case of metastatic cancer systemic therapy remains a standard approch. There is an increasing number of new agents with heterogenous mechanisms of action and toxicity profile (immunotherapy, target therapy etc.) and only few of them have been formally studied in the combination with SBRT. Therefore the majority of data regarding efficacy and toxicity is coming from the retrospective series and prospective studies and registries (eg. ESTRO EORTC Oligocare Consortium). Althought the preclinical data suggest improved radiotherapy efficacy when new agents are added to SBRT, this effect has not been universally showed in clinical trials. From the toxicity point of view, the majority of side effects are striclty correlated to new drugs and outside SBRT volume. Contradictory data on the pausing rules have been published, with no clear negative effects of pausing. Importantly, different half-lives of target therapy (ranging from 24h to 50 days) request different pausing interval between drug and SBRT. So far very favorurable toxicity profie has been seen for the following combinations: 1) RT + endocrine therapy (both standard and new generation agents); 2) RT + HER2 agents, 3) RT + immunotherapy (Meattini I et al. 2022, Anscher et al. 2022), . The recent pooled analysis of FDA database (16,835 patients treated with RT and immunotherapy) showed that administration of immunotherapy within 90 days following RT was not associated in any increase in toxicity apart from fatigue level in (Anscher 2022). Contrarily, somehow higher toxicity has been been reported for the combinations of RT and bevacizumab, TKI, BRAFi, MEKi, PARPi, CDK4/6i, mTORi, therefor interruption of the systemic therapy in these cases has been wildely suggested. The ongoing initiative and trials will help to define the best strategies to combine safely SBRT and systemic therapies (Kroeze et al. 2023). 1. Anscher MS, et al. Association of Radiation Therapy With Risk of Adverse Events in Patients Receiving Immunotherapy: A Pooled Analysis of Trials in the US Food and Drug Administration Database..JAMA Oncol. 2022 Feb 1;8(2):232-240. doi: 10.1001/jamaoncol.2021.6439 2. Kroeze SGC, et al. Metastases-directed stereotactic body radiotherapy in combination with targeted therapy or immunotherapy: consensus recommendations by the EORTC-ESTRO OligoCare Consortium. Lancet Oncol in press 3. Meattini I, et al. Integrating radiation therapy with targeted treatments for breast cancer: From bench to bedside. Cancer Treat Rev. 2022 Jul;108:102417. doi: 10.1016/j.ctrv.2022.102417. Epub 2022 May 21.

Symposium: Smaller and faster: Updates in dosimetry and detectors

SP-0702 Dosimetry for small fields in radiotherapy; Current status & future requirements M. Aspradakis Switzerland

Abstract not available