ESTRO 36 Abstract Book

S926 ESTRO 36 2017 _______________________________________________________________________________________________

Radiation-induced lung fibrosis is an unwanted side effect of curative radiotherapy. Radiological findings include changes in the mediastinum such as shift toward the ipsilateral lung due to treatment related volume loss. In this work we investigate building a standardized and semi- automatic method to quantify mediastinum changes as indicator of lung fibrosis. Material and Methods 31 patients treated with conventional chemoradiotherapy were included. This was a sub-group of a non-randomized phase I/II isotoxic trial which enrolled stage II and III NSCLC (IDEAL-CRT). Each patient underwent a baseline PET-CT or diagnostic CT before treatment, and a diagnostic CT for follow-up after 12 months (average: 433 days, range: 358-496 days). Rotation and thickening of the anterior junction line were taken as measures of mediastinal change. β was defined as the angle between the posterior-anterior direction and the line that connects the centroid of the spinal canal to the centroid of the anterior junction line at carina level in the follow-up scan; similarly, the thickness of the junction (t) was defined as the minimum distance between the two lungs at the same level (Fig. 1). Mediastinum shift was then quantified in terms of the absolute difference between the β measured on the co-registered baseline and follow-up scans (i.e., Δβ= β F - β B ), and in terms of the ratio of the thicknesses (i.e., Δt = t F /t B ). This was implemented as a semi- automatic workflow in Matlab using the open-source Pulmonary Toolkit (github.com/tomdoel/pulmonarytoolkit).

Fig. 2 – Colour overlay of baseline (red) and follow-up (blue) scans. Cases of grades (a) 0 and 0, (b) 0 and 2, (c) 2 and 0 for rotation and thickening, respectively. Conclusion Chronic lung fibrosis manifests radiologically as mediastinal shift. We propose a standardized method to characterize these changes based on the positioning and thickness of the anterior junction line. Further measures are needed to fully describe mediastinum change. We aim to correlate these changes with measures of lung fibrosis. EP-1713 Feasibility of low dose 4D CBCT in patients with lung cancer. A. Bryce-Atkinson 1 , A. McWilliam 1 , T. Marchant 2 , C. Faivre-Finn 1 , G. Whitfield 3 , M. Van Herk 1 1 University of Manchester, Division of Molecular and Clinical Cancer Science- Faculty of Biology- Medicine and Health, Manchester, United Kingdom 2 The Christie NHS Foundation Trust, Christie Medical Physics and Engineering, Manchester, United Kingdom 3 The Christie NHS Foundation Trust, Clinical Oncology, Manchester, United Kingdom Purpose or Objective Respiration correlated cone-beam CT (4D CBCT) provides information on the respiratory motion, providing accurate tumour localisation in the lung taking into account breathing motion. This project aims to investigate the potential for 4D CBCT dose reduction, evaluating its effect on image quality, amplitude estimation and registration accuracy. Material and Methods 4D CBCT images were reconstructed from 9 lung cancer patients with a minimum of 800 projections (120kV, 16mA, 10-40ms), phase sorted to the nearest projection over 10 phases. The amplitudes of tumour motion ranged from 3.9-20mm. Dose reduction was simulated by reconstructing using 50%, 25%, 16.67%, 12.5% and 10% of projections and varying the number of phase bins, sorting all remaining projections. The image quality of each reconstruction was assessed visually and by testing registration accuracy for bone (3D) and tumour (4D). Registration accuracy was evaluated by comparing results to the standard reconstruction, for both amplitude of motion in the superior-inferior direction and correctable (mean) position. Results Reducing the number of projections had little effect on registration accuracy overall for both bone and soft tissue, despite very poor visual image quality at low number of projections. Reducing the number of reconstructed phases recovered visual image quality (Figure 1) at the cost of underestimating amplitude and 10 phases yielded best registration accuracy (Table 1). When using 10% of projections, motion was reported at less than 5mm for 89% of patients at 3 and 5 phases. Increasing to 10 phases recovered the tumour motion to approximately that of the standard reconstructions. Automatic soft tissue registration required manual preregistration for two patients: one consistently across all reconstructions, and the other only for the 10 phase reconstruction with 10% of projections.

Fig.1 – Measures of rotation (β) and thickening (t) of the anterior junction line. Results The changes were characterized using the following grading systems: for Δβ, 0- no/small rotation (<4°), 1- moderate (4-8°), 2- large (>8°); and for Δt, 0- no/small change including moderate shrinkage (<1.2), 1- mild enlargement (1.2-3), 2- moderate enlargement (>3) (Fig. 2). In the presence of disease or toxicities at the junction, the definition of its centroid is unclear; hence six patients were excluded. Significant rotation of the junction line toward the ipsilateral lung occurred in 64% of the patients (36% and 28% for grades 1 and 2, respectively). Significant enlargement of the junction was measured in 56% of the patients (equal occurrence of grades 1 and 2); shrinkage was measured in 16% of the cases. There was no correlation between rotation and thickening of the junction (ρ=0.17, Pearson’s correlation coefficient).