ESTRO 2022 - Abstract Book

S587

Abstract book

ESTRO 2022

Conclusion The combination of our newly innovated customised immobilisation devices and SGRT system can achieve a sub-millimeter positioning accuracy and stability (intra-fractional motion) for SRS and SRT treatment. Furthermore, SGRT can monitor the patient at non-coplanar field during the treatment which is substantially useful for intra-fractional motion management. It ensures the precision of dose delivery and patient safety before and during the treatment.

PD-0652 Comparison of brain planar kV and CBCT imaging modalities for positional accuracy using 6DoF couch

S. Sirianni 1 , C. Dean 2 , A. Cole 2 , L. Wiles 1 , R. Lewis 1

1 Barts Health NHS trust, Radiotherapy, London, United Kingdom; 2 Barts Health NHS trust, Physics, London, United Kingdom

Purpose or Objective •

To compare 2D planar kV image registration results against three-dimensional cone beam CT (CBCT) for patients undergoing radical radiotherapy to the brain, using 6 Degrees of Freedom (6DoF) couch corrections on Varian™TrueBeam. • To quantify the extent of residual errors following the application of 6DoF couch corrections. Materials and Methods 5 patients undergoing radiotherapy to the brain were immobilised using Civco® Uni-frame IMRT thermoplastic shells with standard clear plastic headrest. Patients were first imaged with a kV orthogonal pair; these images were auto-registered in the Varian™ 2D/3D matching workspace where shifts were recorded but not applied. A CBCT scan was then acquired, auto-registered and 6DoF shifts applied prior to treatment. A second CBCT was taken after the applied couch shifts but prior to treatment to detect whether the patient had counteracted the applied corrections. Results 142 CBCT images and 142 kV planar image pairs were obtained and analysed. The mean ± S.D. differences between kV planar and CBCT registration results were: VERT 0.0 ± 0.3 mm, LONG 0.0 ± 0.4 mm, LAT -0.2 ± 0.4 mm, RTN 0.0° ± 0.2, PITCH -0.1° ± 0.2, ROLL 0.0° ± 0.2. The mean absolute differences between the registration results of the two imaging modalities were all less than 0.4 mm in translation and less than 0.2 degrees in rotation. The mean ± S.D. CBCT registration results following application of couch corrections were: VERT -0.2 ± 0.2 mm, LONG 0.0 ± 0.3 mm, LAT -0.2 ± 0.2 mm, RTN 0.1° ± 0.2, PITCH 0.1° ± 0.2, ROLL 0.0° ± 0.1. The mean absolute differences were all less than 0.3 mm in translation and less than 0.2 degrees in rotation. Conclusion The high degree of concordance between the 2D planar and CBCT registration results confirm that the modalities can be used interchangeably. The small residual deviations following couch corrections indicate that in general, patients did not counteract their application. However, for a small number of fractions larger deviations were detected after application of the corrections. In these cases, shell fit was found to be sub-optimal due to patient contour change. This was prevalently seen towards the end of the treatment course and corrected by improving shell fit and reducing patient movement within the shell. This study has shown that it is possible to significantly reduce the use of daily CBCT for this cohort of patients whilst maintaining excellent levels of patient positional accuracy. The replacement of CBCT with 2D planar kV in this institution has resulted in reduced concomitant doses and shorter overall treatment . This study has also highlighted the importance of good immobilisation and the continuous assessment of shell fit throughout radiotherapy. We recommend the use of weekly CBCT imaging for 3D contour evaluation for dosimetric assessment.