Biophysical characterization of a relativistic proton beam for image-guided radiosurgery

Abstract

We measured the physical and radiobiological characteristics of 1 GeV protons for possible applications in stereotactic radiosurgery (image-guided plateau-proton radiosurgery). A proton beam was accelerated at 1 GeV at the Brookhaven National Laboratory (Upton, NY) and a target in polymethyl methacrylate (PMMA) was used. Clonogenic survival was measured after exposures to 1-10 Gy in three mammalian cell lines. Measurements and simulations demonstrate that the lateral scattering of the beam is very small. The lateral dose profile was measured with or without the 20-cm plastic target, showing no significant differences up to 2 cm from the axis A large number of secondary swift protons are produced in the target and this leads to an increase of approximately 40% in the measured dose on the beam axis at 20 cm depth. The relative biological effectiveness at 10% survival level ranged between 1.0 and 1.2 on the beam axis, and was slightly higher off-axis. The very low lateral scattering of relativistic protons and the possibility of using online proton radiography during the treatment make them attractive for image-guided plateau (non-Bragg peak) stereotactic radiosurgery.

Fig. 1.

Simulations of the lateral straggling of the proton beam used in our experiments. Left, a beam's eye view of the position of single protons after traversal of 20-cm PMMA. Simulations by SRIM 2011 [16] at two different energies: typical therapeutic 200-MeV beam, and the 1-GeV beam used in the experiments described here. Right, calculations of the beam shape's FWHM for a monoenergetic incident proton beam in PMMA using Molière's formula [18] or simulated by GEANT4 [17]. The FWHM spread is calculated for beams accelerated at 60 MeV (used in eye therapy), 200 MeV (deep protontherapy), 1 GeV (these experiments), 2 and 4.5 GeV (future FAIR facility).

Fig. 2.

Dose of a 1-GeV proton beam measured along the beam's axis after PMMA blocks of increasing thickness. Dose is normalized to the entrance value measured by a monitor chamber in front of the target. Measurements are pooled from experiments at seven different dose values (ranging from 2–10 Gy), and bars are standard errors of the mean values. Curves are interpolations of GEANT4 simulations. The contributions of primary protons, secondary protons and other ions (neutrons, deuterons, tritons, helium and lithium) are plotted in different colours.

Fig. 3.

Simulated energy spectrum of the 1-GeV proton beam after traversal of 20-cm PMMA. Simulation was performed with 50 000 protons using GEANT4. Red, primary protons; blue, secondary protons; green, neutrons; black, total.

Fig. 4.

Lateral straggling of the beam as visualized by a pixel camera for a broad beam (A) and the actual small beam (B) used in our experiments. White bar is 5 cm. The plot (C) shows the measured dose after 20 cm air or PMMA on the beam axis and at different lateral distances.

Fig. 5.

Survival curves of V79, SQ20B and SCC25 cells after exposure to γ-rays or protons at 1 GeV. Bars are standard errors of the mean values. Lines are best fits using the linear-quadratic model (see Table 1 for the fitting parameters).

Fig. 6.

Survival curves of V79, SQ20B and SCC25 cells after exposure to protons behind the 20-cm PMMA block. The flasks were exposed either on the beam axis, or at 2 or 4 cm from the main axis. Bars are standard errors of the mean values. Lines are best fits using the linear-quadratic model (see Table 1 for the fitting parameters).