ESTRO 2025 - Abstract Book

S4407

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

Clinical history and therapeutic characteristics were collected. Quality assessment of the treatment plans was based on dose volume histograms (DVH) analysis in terms of organs at risk (OAR) dose and volume parameters.

Results: The mean age was 52.4 [29-75]. 51.4% of patients had right-sided tumours. All patients had a radical surgery, and 90% had axillary lymph nodes dissection and 10% had sentinel lymph node dissection. Sufficient dose coverage of target volumes was achieved for 78.57% of cases (PTV-V 95% >95%). The mean PTV-V 95% was 92.42% [68.16-100]. However, the maximum acceptable dose was exceeded (>110%) in 32.86% of cases, and the average Dmax was 44.6Gy (110.7%) [106.8-117%]. For the heart, the mean Dmoy was 1.68Gy [0.3-4], and the dose was greater than 2.5Gy in 24.3% of cases. The average heart volume receiving 2% (V2%) and 10% (V10%) of the prescribed dose was 13.37% [0-30.44] and 3.4% [0 10.3], respectively. The heart receives fewer doses in case of right RT compared to left RT. The difference was significant for all studied parameters : Dmoy (1.19Gy versus 2.6Gy, p = 0.0001), V2% (9.8% versus 18.2%, p = 0.0001) and V10% (2.1% versus 5.9%, p = 0.03). Regarding the dose constraints in the ipsilateral lung, the mean lung volume receiving 12% (V12%) of the prescribed dose was 33.7% [23.9-47.9%], with a volume greater than 30% in 82.85% of cases. The mean V17% and V28% were 29.42% [20-33.11] and 18.21% [11.5-23.2], respectively. The analytical study showed that V12% was significantly greater in the case of left-sided tumors (p=0.003). Conclusion: In the case of locoregional breast irradiation, it is often difficult to comply with dose constraints in the OAR, especially in the absence of respiratory-gated techniques. In our study, compliance with protocol dose constraints was found in 70% of cases. Digital Poster Delineation of non-small cell lung cancer gross tumour volume on cone beam CT images: a feasibility study including auto-segmentation Sarah Barrett 1,2 , Laure Marignol 1 , Gerard G Hanna 3,2 , Conor McGarry 4,5 , Gerard M Walls 4,5 1 Applied Radiation Therapy Trinity, Trinity College Dublin, Dublin, Ireland. 2 Trinity St. James’s Cancer Institute, Trinity College Dublin, Dublin, Ireland. 3 St. Luke's Radiation Oncology Network, St. Luke’s Hospital Rathgar, Dublin, Ireland. 4 Patrick G. Johnston Centre for Cancer Research, Queen’s University Belfast, Belfast, Belfast, United Kingdom. 5 Northern Ireland Cancer Centre, Belfast Health & Social Care Trust, Belfast, United Kingdom Purpose/Objective: Gross tumour volume (GTV) delineation on radiotherapy (RT) cone beam CT (CBCT) has been posited as a research tool for delta radiomics and tumour volume modelling research, yet there is a dearth of guidance on CBCT segmentation. This study aimed to identify criteria to guide case selection for GTV structure delineation on lung cancer CBCTs, and to evaluate the potential of auto-segmentation in this setting. Material/Methods: RT imaging of patients with T1–3 N0 NSCLC from the NI-HEART cohort were accessed [1].Patients were treated with 55Gy/20# or 66Gy/33# over 4-6 weeks with CBCTs on days 1–3 of week 1 and once weekly thereafter. The planning CT average intensity projection (AVIP) and first CBCT were screened for feasibility of GTV delineation and reasons for exclusion collated (Table 1). GTV auto-contours (AC) were generated for suitable cases using Varian Eclipse™ Smart Segmentation, previously validated in lung cancer 4DCTs [2]. AC review and manual adjustment was performed by a Keywords: Breast cancer, hypofractionation, OAR doses 4574