Converging Proton Minibeams with Magnetic Fields for Optimized Radiation Therapy: A Proof of Concept

Abstract

Proton MiniBeam Radiation Therapy (pMBRT) is a novel strategy that combines the benefits of minibeam radiation therapy with the more precise ballistics of protons to further optimize the dose distribution and reduce radiation side effects. The aim of this study is to investigate possible strategies to couple pMBRT with dipole magnetic fields to generate a converging minibeam pattern and increase the center-to-center distance between minibeams. Magnetic field optimization was performed so as to obtain the same transverse dose profile at the Bragg peak position as in a reference configuration with no magnetic field. Monte Carlo simulations reproducing realistic pencil beam scanning settings were used to compute the dose in a water phantom. We analyzed different minibeam generation techniques, such as the use of a static multislit collimator or a dynamic aperture, and different magnetic field positions, i.e., before or within the water phantom. The best results were obtained using a dynamic aperture coupled with a magnetic field within the water phantom. For a center-to-center distance increase from 4 mm to 6 mm, we obtained an increase of peak-to-valley dose ratio and decrease of valley dose above 50%. The results indicate that magnetic fields can be effectively used to improve the spatial modulation at shallow depth for enhanced healthy tissue sparing.

Keywords: Monte Carlo simulations; magnetic fields; proton minibeam radiation therapy; spatial fractionation.

Figure 1

Schematic drawing of the configurations of magnetic field and collimator used in the study.

Figure 2

Dose maps for a 150 MeV beam in the five analyzed configurations and the corresponding transverse profiles at the Bragg peak (bottom-right figure).

Figure 3

Transverse profile of the central minibeam for 150 MeV protons at the entrance of the water phantom and at a 7 cm depth.

Figure 4

(a) Peak-to-Valley Dose Ratio (PVDR) as a function of depth and (b) depth dose of the first central valley in the five analyzed configurations for three different proton energies.

Figure 5

Optimized spread-out Bragg peak in the three configurations. The figure shows the contributions of the individual Bragg peaks (colored continuous lines) to the total peak dose (black continuous line) on the central minibeam axis as well as the valley dose on the first central valley (dashed black line).

Figure 6

Peak-to-valley dose ratio as a function of depth (left image) and depth-dose of the first central valley (right image) in the three configurations.