ESTRO 2022 - Abstract Book

S1482

Abstract book

ESTRO 2022

Results The best trade-off between CT image quality, marker visibility and artefacts was obtained for the pelvic protocol (Twin Beam, AuSn 120kV, 2 mm slice thickness, applied iMAR algorithm). The smallest artefacts were observed for GA-0.28, GA- 0.28 MR+ and GM-0.8. The content of iron did not influence the quality of CT images. All markers, except for GM-0.8 mm, were visible on 3D T2w-MRI scans performed with the clinical pelvic protocol. GA-0.28 MR+ had a better image-contrast in comparison to GA-0.28 (Figure 1). All markers were clearly visible on CBCT and orthogonal kV images. With respect to dose degradations, the largest underdosage was measured at the end of the SOBP and was at the level of 30%, 10% and 5% for GM-1.2, folded GA-0.4 and folded GA-0.28, respectively. The dose degradations were equivalent between markers with different iron content. The shadowing effect was substantial more than 10 mm downstream of markers.

Conclusion All tested markers gave clear signal on CT and CBCT scans. GM-1.2 caused intolerably high dose degradation and therefore was excluded from the usage in the proton part of our trial. The smallest shadowing effect was measured for GM-0.8 and GA-0.28. In connection with the improved visibility on MR scans GA-0.28 MR+ gives promising results for future clinical use in proton RT.

PO-1683 Mechanical and dosimetric accuracy of Dynamic Trajectory Radiotherapy delivery on a C-arm linac

J. Bertholet 1 , P. Mackeprang 1 , H.L. Loebner 1 , S. Mueller 1 , G. Guyer 1 , Y. Wyss 1 , D. Frei 1 , W. Volken 1 , O. Elicin 1 , D.M. Aebersold 1 , M.K. Fix 1 , P. Manser 1 1 Inselspital, Bern University Hospital and University of Bern, Division of Medical Radiation Physics and Department of Radiation Oncology, Bern, Switzerland Purpose or Objective Dynamic Trajectory Radiotherapy (DTRT) extends volumetric modulated arc therapy (VMAT) with table and collimator rotation during beam-on. This technique has shown improved healthy tissue sparing for equivalent target coverage compared to VMAT in head and neck (HN) radiotherapy. This study demonstrates the deliverability of DTRT on a C-arm linac, reporting on the mechanical and dosimetric accuracy. Materials and Methods DTRT plans were created for different HN cases on an anthropomorphic phantom. Six cases were had sequential boost resulting in 13 plans with 2 Gy/fraction and one case was a single vocal cord irradiation (SVCI) with 3.63 Gy/fraction resulting in a total of 27 trajectories (1 trajectory corresponds to 1 full gantry rotation). Gantry-table-collimator paths were determined by an A* path-searching algorithm minimizing beam’s eye view OAR-target overlap within the collision- free space determined using a case-specific collision model. Maximum gradients of 3° table or collimator rotation per degree gantry rotation were allowed and paths were smoothed using a 10-points (20°) moving average to avoid abrupt table motion. All trajectories were delivered on the phantom using developer mode on a TrueBeam linac (Varian Medical Systems) equipped with a 120-leaf MLC and a PerfectPitch 6-degree-of-freedom table. All machine log-files were recorded in order to assess the mechanical deviations for all dynamic axes, calculated as the difference between expected and actual values. Correlation between speed and deviation was evaluated for gantry, table and collimator angles. Dosimetric validation was carried out with transversal plane film measurements on the phantom for one case (2 plans, 4 trajectories). The mean delivery time was 2.4 minutes (range: 1.9-2.9 minutes) per trajectory. The root-mean-square (RMS) deviation were 0.02°, 0.12° and 0.03° for gantry, table and collimator angles, respectively. Maximum deviations were 0.13°, 0.16° and 0.17° for gantry, table and collimator angles, respectively. The Pearson’s correlation coefficient between speed and deviation was high (negative) for table and collimator angles (<- 0.99, p<<0.01) but low for gantry angle (0.16, p<<0.01). Figure 1 shows example trajectories and deviations for gantry, table and collimator angles for one plan. Although gantry angle deviations appear higher when there is a change in direction for table or collimator rotation, correlation between deviation and speed or acceleration of any component was low. The mean RMS deviation for all moving MLC leaves was 0.17 mm (maximum RMS deviation: 0.27 mm). The passing rates between measured dose on film and calculated dose were 93.9% and 95.8% (global gamma, 2%/2mm, 10% dose threshold). Results All plans were delivered successfully without interlock or collision.