ESTRO 2021 Abstract Book

S1486

ESTRO 2021

objective of this work is to determine if pre-treatment rotations are a predictive factor in intrafraction motion for SRS patients. Materials and Methods Over a period of 9 months in 2020-21 we performed pre- and post-treatment CBCTs for 46 stereotactic radiosurgery patients treated using HyperArc on a Varian Edge linac with a 6 DoF couch; patients were immobilised using the Q-Fix Encompass Mask. Moves were applied following the pre-treatment CBCT, therefore by analysing the post-treatment CBCT corrections we can quantify intra-fraction motion. Various factors including the pre-treatment shifts were analysed to find which were predictive of large (>1mm or >1 degree) post-treatment shifts. Results Using a multivariate regression analysis we determined that pre-CBCT pitch corrections were a significant predictor (p<0.01) of intra-fraction motion. The most common result of applying a large pitch correction was a large longitudinal shift on the post-treatment CBCT. This may indicate deficiencies in mask fitting prior to planning. Other factors considered as possible contributing factors were clinical indication, number of targets, pre-CBCT translations and total treatment time. None of these factors was a statistically significant predictor of intra-fraction motion. Further work is underway to reduce the rate of IFM through additional training in mask production from the manufacturer. However the departmental process for SRS pre-treatment verification when large pre-CBCT pitch values are observed has been updated such that a confirmatory CBCT is now acquired following the application of CBCT pitch or roll corrections. This confirmatory exposure agrees with the guidance provided in ICRU report 91. This process has been shown to be successful on a small number of patients thus far in that all have displayed very small shifts (<0.2 mm, <0.3 degrees) on the post-treatment CBCT. Conclusion In stereotactic treatments where positional accuracy is critical and margins are significantly reduced, a confirmatory image after applying large pitch or roll CBCT shifts allows for correction of any induced translational shifts. This finding may be specific to the centre in question; however it does also demonstrate the benefit of multivariable analysis to guide quality improvements. Purpose or Objective For stereotactic radiotherapy accurate patient setup is essential. To adjust both translation and rotation patient setup errors, a six-degrees of freedom robotic couch (6DoF couch) can be mounted on top of the traditional treatment table (couch pedestal) of a linear accelerator. Unfortunately, the range of motion of a 6DoF couch is limited and sometimes the 6DoF cannot fully correct a rotation error (Figure 1) [1]. In such event, a clinician has to decide if it is safe to continue treatment. Besides, the 6DoF couch used its full range of motion in the initial setup correction and a consecutive correction might be problematic. We present a method that combines couch pedestal and 6DoF couch movement. The method determines the optimal move of the couch pedestal for a setup correction to increase the range of motion of the 6DoF couch. Furthermore, the optimal move positions the 6DoF couch such that maximum range of motion of the 6DoF couch is preserved for a consecutive correction. Materials and Methods We used a Protura 6DoF robotic couch (CIVCO) that was installed at Radiotherapiegroep Arnhem. Range of motion was assessed using a dataset of 1785 setup errors (up to 3°) from conventionally fractionated treatments, delivered on the 6DoF couch. This dataset enabled us to reproduce the exact pedestal and 6DoF couch positions prior to the setup corrections. To calculate the optimal move, we simulated the position and hardware limitations of the moving components within the 6DoF couch. The optimal move was determined by maximizing the smallest margin of the moving components to their hardware limits. If a rotation error could not be corrected, the algorithm returned the optimal move for the maximum achievable rotation. CNERGY Exact 6D (Cablon Medical) was used to determine the optimal move and distribute the move over the couch pedestal and 6DoF couch (Figure 2). Results In our dataset, translation errors were 0.0±1.9mm, 0.2±3.0mm, and -0.1±1.6mm in lateral, longitudinal, and vertical direction, respectively, and rotation errors 0.6±1.2°, -0.3±1.2°, and 0.1±1.2° in pitch, roll, and yaw direction, respectively. The stand-alone 6DoF couch achieved 89% of the setup corrections with 21% remaining range of motion, while with optimal move algorithm 98% could be achieved with 33% remaining range of motion. Average residual rotation error for corrections that could not be achieved was 0.42° for stand-alone 6DoF couch and 0.16° with optimal move algorithm. Conclusion Using the optimal move algorithm to combine couch pedestal and 6DoF couch movement significantly increases the range of motion of the 6DoF couch. Furthermore, optimal move preserves maximum range of motion of the 6DoF couch for a consecutive setup correction, enabling full potential of 6DoF couches for stereotactic radiotherapy. The new software is clinically used since February 2021. PO-1760 Optimal Move: increasing the range of motion of a 6DoF robotic couch G. Warmerdam 1 , S. Postma 2 , J. Barnhoorn 2 , M. Luesink 2 , A. Arents 3 , K. Pasma 3 1 Radboud University Medical Center, Radiation Oncology, Nijmegen, The Netherlands; 2 Cablon Medical, Radiotherapy, Leusden, The Netherlands; 3 Radiotherapiegroep, Radiotherapy, Arnhem, The Netherlands

References 1. Guckenberger M. et al., Int. J. Radiat. Oncol., Biol., Phys. 2012; 83, 467–474.