ESTRO 38 Abstract book

S1211 ESTRO 38

Purpose or Objective Previous literature cites that patients treated with volumetric arc therapy (VMAT) for head and neck cancer necessitates Daily Cone Beam Computerised Tomography (CBCT) in order to ensure treatment accuracy. This is because throughout a course of radiotherapy for head and neck cancer anatomical changes occur within the patient due to the treatment delivered, resulting in geometric uncertainty. It is imperative the imaging dose is minimised in line with the Ionising Radiation (Medical Exposures) Regulations 2018 (IR(ME)R) and that images are fit for purpose. Within the practice environment (PE) the imaging mode for kV CBCT is set by the manufacturer of the Linear Accelerators (Linac), Varian Medical Systems, Palo Alto, CA (Varian). Material and Methods A service evaluation was undertaken to determine whether lower dose kV CBCT head imaging modes provided adequate imaging quality for the purpose of treatment verification, in comparison to the standard manufacturer settings. Optimization of images was completed via a reduction in the mAs. Two alternate settings were produced and tested on a phantom. The novel kV CBCT imaging protocols were then tested on patients who were receiving already justified for daily imaging. Daily kV CBCT scans were acquired and patients received one CBCT per week of a lower dose mode. Overall 304 kV CBCT images were scored independently using a graded scale by three independent multidisciplinary image observers. Qualitative analysis was used to evaluate the results of the study. Results N=38 patients were included in the project. 304 kV CBCT scans were scored using a graded scale. Initial review of images allowed the grouping of image scores into good, moderate and poor categories based on the average image scores for all image reviewers. 56.57% of lower dose images were deemed good in quality in contrast to 1.31% were deemed poor in quality. 42.11% of the lower dose scans were grouped in the moderate quality category. This identified that poor quality images were infrequent. Statistical analysis was undertaken using parametric t-test analysis and provided comparison statistics between standard imaging and lower dose imaging for each individual score. Intra observer variability was also analysed using Kappa analysis. This identified that the physicist results were statistically different to the radiographer and clinician when scoring images. Conclusion Findings identified that low dose imaging produce adequate quality imaging for the purpose that they are intended whilst benefiting the patient by minimising concomitant exposure. The very low dose imaging modes are not suitable for volumes that extend past cervical vertebrae seven, as this produces poor quality images with artefact and image distortion. Calculations were completed to obtain the effective dose that the patient would receive with the differing imaging modes throughout the course of radiotherapy and it was proven that the lower dose imaging reduced the dose received by the patient. EP-2195 Optimization of SABR lung CBCT verification L. Turtle 1 , A. Willett 2 , C. Lee 2 , C. Fitzpatrick 2 , R. Biggar 2 1 Clatterbridge Cancer Centre NHS Foundation Trust, Radiotherapy, Wirral, United Kingdom; 2 Clatterbridge Cancer Centre NHS Foundation Trust, Physics, Wirral, United Kingdom

Conclusion For cases involving higher dose effects on the evaluated structures, 4DOF registration improved plan conformity compared to 3DOF. Clinically relevant dose deviations in the CTV were not found in this patient collective. While mean dose delivery to OARs increased only slightly, up to 6 Gy in additional dose occurred for individual patient cases in this collective because of rotational errors. Safety margins for OARs could be a solution to this matter. EP-2194 Do lower dose KV-CBCT protocols produce adequate quality images for head and neck cancer patients? M. Forshaw 1 , A. Taylor 2 , S. Temple 3 , H. Wong 4 , A. Willet 5 , C. Rowbottom 5 1 The Clatterbridge Cancer Centre, Radiotherapy, Wirral, United Kingdom; 2 Sheffield Hallam University, Radiotherapy and Oncology, Sheffield, United Kingdom; 3 The Clatterbridge Cancer Centre, Physics, Wirral, United Kingdom; 4 The Clatterbridge Cancer Centre, Statistics, Merseyside, United Kingdom; 5 The Clatterbridge Cancer Centre, Physics, Merseyside, United Kingdom