gms | German Medical Science

Purpose: To present the beam optics methodology to reach the requirement needed in terms of beam characteristics at isocenter for a fast and efficient pencil beam scanning.

Method and results: In proton therapy, the delivery of the proton beam to the patient by scanning pencil beams imposes new and demanding specifications for the beam optics. At isocenter, the pencil beam must be small and have a circular cross section. A convenient operation is obtained when all magnet parameters are kept constants, or almost constant for a given energy when the gantry is rotated. This requires to have a beam of circular cross section with the same emittance in X and Y at the entrance of the gantry.

When the accelerator is a cyclotron, the availability of divergence limiting slits after the degrader offers a convenient way to equalize the X and Y emittances. A circular collimator, located in the beam line corridor, before the gantry entrance, was used to wipe out the effects of some sextupolar aberrations observed at the highest energies and coming from the cyclotron extracted beam. Despite the use of this collimator, the beam shapes were found Gaussian at isocenter.

We present here the results of the beam optics optimization in the case of the PBS dedicated nozzle installed on an IBA gantry. The dimension of the beam at isocenter is measured using the combination of a scintillator combined with a CCD camera (the IBA Dosimetry BIS device). This device offers a true 2D analysis of the proton beam fluence, and is thus better than orthogonal axis integration devices such as ionization chambers equipped with strips or wire electrodes. Thanks to an analysis tool, good results have been demonstrated. We have demonstrated circular beam spots with symmetry better than 10% and reproducibility better than 5% for all ranges. The spot size was adjusted to 3 mm (one sigma) at the highest energy, i.e. 229 MeV, increasing to 5 mm at 100 MeV. The beam spot has also been scanned over a field of 200 squared millimeters and properties remained within most constraining requirements.

Conclusions: We will demonstrate that this method can be transposed to other beam line layouts such as universal nozzles, fixed beam lines or inclined beam lines, showing the universality of the solution, its effectiveness to quickly calibrate a beam line and to obtain a good beam spot quality, allowing releasing some constraints on the treatment definition.