ESTRO 2022 - Abstract Book

S97

Abstract book

ESTRO 2022

In the first measurement a shell is used to increase the body diameter with 10mm. The shell is also simulated in the treatment planning software and incorporated in the dose calculation. In the second measurement the first shell is removed and the second shell is added. The outer diameter of this shell is identical to the first shell, yet the position of the inner surface is shifted 2.5mm (see Figure A). The wall is 7.5mm thick at one side and 12.5mm in the opposite direction. The couch (PerfectPitch, Varian) of the linear accelerator (TrueBeam, Varian) is shifted 2.5mm to compensate for the asymmetry of this shell. At start of the second measurement the outer geometry is identical to the first measurement, but the positions of the Delta4 diodes are shifted with 2.5mm. The results of the first and second measurement are combined afterwards to increase the resolution with a factor 2. Results It was possible to add and remove the shells without changing the orientation and position of the Delta4 dosimetry system. Figure B shows the measurement profile of a patient VMAT plan. The points of the first measurements are shown in blue, the second in orange. In the central region (width 6cm) the extra measurements points are situated centrally, since the distance between the diodes is 5mm in this central region of the Delta4. In the outer region, the Delta4 resolution is 10mm.

Conclusion It is possible to increase the measurement resolution using 3D printed shells. This solution is not limited to the Delta4, but can also be applied to other dosimetry systems. Furthermore, different designs (e.g. wall thickness) can be used to shift diodes differently with respect to the outer contour.

OC-0122 Accurate in water electron beam dose measurements using polarization imaging

E. Cloutier 1,2 , L. Beaulieu 1,2 , L. Archambault 1,2

1 CHU de Quebec - Universite Laval, Service de physique medicale et Axe Oncologie du Centre de recherche, Quebec, Canada; 2 Universite Laval, Departement de physique, de genie physique et d’optique, et Centre de recherche sur le cancer, Quebec, Canada Purpose or Objective Cherenkov emission carries the potential of direct, perturbation-free, in water dose measurements. However, until now such measurements suffered from large (up to 60%) uncertainties because of the intrinsic anisotropy of Cherenkov emission. Cherenkov radiation is emitted along a cone whose angle is determined by the charged particles energy and direction which vary with tissue attenuation making corrections non-trivial. This work investigates the use of polarization imaging to precisely measure and correct electron beam dose distributions. Materials and Methods Cherenkov emission produced in a 15 x 15 x 15 cm3 water tank, from 6 MeV and 18 MeV electron beams, is measured by a CCD camera (414EX; Atik Cameras, Norwich, United Kingdom) coupled to a rotating polarizer. Images are acquired from