Evaluation of Manually Sculpted Trajectories and a New Trajectory Optimizer Dedicated to a Novel Patient Positioning System on a Truebeam Linac

Abstract

The new patient positioning system is a robotized couch of Varian associated to a TrueBeam Linac This new robotic couch enables Volumetric Modulated Arc Therapy (VMAT) treatments optimization with new dynamic trajectories, thanks to precise, simultaneous and continuous movements of gantry, couch and collimators. The aim of this study was to evaluate the dosimetric improvement of these VMAT treatments compared with our common ones, by manual design of more general treatment trajectories in the treatment planning system (TPS). This study exposed the first results obtained for the treatment of intracranial localization, particularly for re-irradiation of relapsing glioblastoma, for which the proximity of organs at risk is a critical issue due to a previous irradiation or de novo glioblastoma irradiation. First, trajectories were designed manually in the beam’s eye view of the TPS. A script was used in which each control point of the trajectory contained a combination of gantry angle, collimator angle, couch angle, isocenter position and jaws size chosen to spare at best organs at risk while targeting the tumor. Photon Optimizer and dose calculation algorithms were adapted to take into account the movement of the couch. Second, dosimetric improvement of these new VMAT treatments realized thanks to new dynamic trajectories were evaluated and compared to VMAT or Dynamical Conformal Arc treatments usually performed in our center. Dynamic trajectories were first tested on six cases of glioblastoma and four cases of reirradiation. The comparison was made by estimating the relative differences of dosimetric criteria from the ICRU report 83 for VMAT and the ICRU report 91 for DCA. The first results showed for glioblastoma a reduction of the maximal dose to optical structures (optical nerves, chiasma and lenses) from 5% to 80%. Brainstem was also better protected by reducing the maximal dose from 3% to 40% according to the position of the target. In some cases, the D40% delivered to the contralateral hippocampus was reduced from 10% to 40%. In cases of reirradiation, the mean dose delivered to the brain was decreased by 30%. In all cases, the dose reduction at organs at risk did not impact the planning target volume (PTV) coverage. Trajectories defined with the new patient positioning system are very promising as they show a PTV coverage equivalent to the common treatments achieved in our center with a better sparing of the organs at risk. New studies are currently ongoing using a new Trajectory Optimizer (TO) developed at Varian, which provides patient-specific trajectories in an automated fashion, and progressively extended to other localizations as vertebra, lung and liver.