ESTRO 2023 - Abstract Book

S581

Monday 15 May 2023

ESTRO 2023

on standard cone-beam CT (CBCT) imaging. Breathing motion amplitude must be managed to avoid overly-large planning margins that impact normal tissue doses and inhibit dose-escalation. Voluntary and device-assisted imaging and treatment delivery in breath-hold can suspend motion at a given respiratory phase (e.g. inhale or exhale) though is not suitable for all patients such as those with language barriers and poorer performance status. Even with excellent patient compliance, some patients have of non-reproducible breath hold positions (breath-to-breath, and intra-breath hold drifts). Alternatively, abdominal compression devices can reduce breathing amplitude in some patients though they are very uncomfortable for many. There is a risk of compression devices impacting normal tissue deformation in the abdomen which may prevent dose-escalation (e.g. compression the duodenum near pancreas targets). Possible contraindications exist limiting the use of these devices (e.g. colostomies and ascites for compression, etc) so many patients may ultimately have radiation under free-breathing. Given this variability, patient-specific evaluation of motion management under imaging can allow for optimal, personalised strategies to be used. Breathing motion can be characterized with imaging with a high temporal resolution ranging from simple 2D kV fluoro of the diaphragm, to cine-MR for direct tumor visualization, and 4D CT, CBCT or MR imaging to fully model organ motion and targets in multiple planes. Assessment of breathing motion should occur upfront at planning and daily during IGRT as inter-fraction changes in breathing patterns have been observed. IGRT in the upper abdomen is -particularly challenging with CBCT given the lack of soft tissue contrast and highly deformable organ including the liver, pancreas and luminal gastrointestinal organs that are additional subject to filling and emptying. Consistent dietary preparation can mitigate some of the largest inter-fraction changes however this is ineffective for many patients. In the absence of implanted markers that are visible on CBCT, IGRT largely relies on soft-tissue targeting as baseline shifts in position relative to bony anatomy in excess of several centimeters are common. Other surrogate landmarks can be used to target tumors, including normal tissue boundaries. Oral contrast can be administered prior to IGRT when stomach and duodenum are at-risk and required careful avoidance during CBCT-guided treatment. Deforming tissues, including when multiple tumors are simultaneously targeted, can be partially accounted for with focused image registration and corrections with tilt-and-roll tables. In a minority of patients, deformation necessitates adaptive re-planning, either offline following repeat CT simulation or online with some CBCT and MR-enabled linacs. Successfully clinical application of these strategies allows for high-quality IGRT and safe application of dose-escalation and normal-tissue sparing radiotherapy to deliver optimal outcomes for patients with upper abdomen tumors.

SP-0711 IGART Strategies in Complex SBRT G. Lawler 1 1 Irish Research Radiation Oncology Group (IRROG), IRROG Manager, Dublin, Ireland

Abstract Text Radiotherapy treatment delivery to targets in the lung is complicated by organ motion associated with breathing. Many strategies exist to assist in reducing this impact including immobilisation, coached breathing techniques and image guidance. This session will cover target delineation and lung radiotherapy treatment delivery using 4DCT, free breathing, breath-hold (inspiration/expiration), kV, fluoroscopy, cone-beam CT and surface guidance. The impact of each technique and how a combination approach may be needed in complex cases will be highlighted. A review of complex cases will be discussed and IGART used.

Mini-Oral: Head & neck

MO-0712 Compliance to the PATHOS swallowing OAR atlas and impact on predicted dysphagia for trial patients E. Higgins 1 , N. Palaniappan 1 , R. Webster 1 , Z. Nabi 2 , R. Lad 3 , K. Elliott 3 , E. Miles 3 , J. Canham 4 , L. Nixon 5 , C. Hurt 4 , C. Heiberg 5 , M. Beasley 6 , T. Jones 7 , M. Evans 1 1 Velindre Cancer Centre, Clinical Oncology, Cardiff, United Kingdom; 2 Mount Vernon Cancer Centre, RTTQA Group , Northwood, United Kingdom; 3 Mount Vernon Cancer Centre, RTTQA Group, Northwood, United Kingdom; 4 Cardiff University, Centre for Trials Research , Cardiff, United Kingdom; 5 Cardiff University, Centre for Trials Research, Cardiff, United Kingdom; 6 Bristol Haematology and Oncology Centre, Clinical Oncology, Cardiff, United Kingdom; 7 University of Liverpool, Molecular and Clinical Cancer Medicine, Liverpool, United Kingdom Purpose or Objective PATHOS is a phase III RCT investigating whether de-intensified adjuvant treatment after transoral surgery for patients with HPV related Oropharyngeal SCC, will result in improved swallowing function, whilst maintaining excellent clinical outcomes. As part of the Radiotherapy Trial Quality Assurance (RTTQA) programme, investigators were invited to delineate 9 Swallowing OAR (SWOARs) in a swallowing sub-study. Inaccurate SWOAR delineation may affect plan optimisation, lead to inconsistencies in dose delivery to normal tissues and potentially impact NTCP. This study aims to evaluate compliance with the PATHOS contouring atlas and determine if variation in SWOAR delineation leads to any difference in physician-scored dysphagia at 6 months (NTCPD6). Materials and Methods The dataset consisted of 85 patients recruited between Dec 2015 and Feb 2020 that had SWOAR contours submitted. A gold standard (GS) set of outlines was contoured on all cases by a single investigator (EH) and checked by two senior clinicians. A qualitative retrospective analysis was performed whereby the submitted SWOAR contours (SC) were categorised using the same principles adopted by the RTTQA team: “conforms to protocol”, “acceptable variation where study outcome is unlikely to be affected” and “unacceptable variation where study outcome could be affected.” The predicted NTCPD6 for each case of the GS and SC was calculated by applying the predicted model of Christianen et al [NTCPD6 = 1/(1 + e)-S, where S = − 6.09 + (mean dose PCM_Sup × 0.057) + (mean dose Larynx_SG × 0.037)]. The differences between the NTCPD6 for GS and SC were then compared.

Results