SHARP: A compact focusing system for medical applications using a diverging plasma lens

TL;DR

The SHARP project will develop and test a system that can be used with very high energy electrons, creating a Bragg-peak-like spot using novel accelerator technology, and potentially be useful for FLASH radiotherapy.

Abstract

Cancer therapy for deep-seated tumors requires precise irradiation of a small target deep within the patient while minimizing radiation exposure to surrounding tissues. This can be accomplished with a round beam sharply converging towards a single spot, requiring a large beam size in both planes at the exit of the focusing system. Achieving this over a short distance using only quadrupole lenses is challenging; but by using a linear active plasma lens (APL) in defocusing mode, the beam can be quickly and non-destructively enlarged before focusing using quadrupoles. The position of the irradiation spot can also be scanned in three dimensions by changing magnet settings. The SHARP project will develop and test this concept. Such a system can be used with very high energy electrons (hundreds of MeV), creating a Bragg-peak-like spot using novel accelerator technology. This could lead to more compact radiotherapy facilities, not requiring a bulky infrastructure typically associated with proton radiotherapy machines. If successful, SHARP will enable precision conformal radiotherapy, spatial fractionation, and potentially be useful for FLASH radiotherapy.

Figures (9)

• Figure 1: Left: Photograph of the Active Plasma Lens (APL) at CLEAR, while discharging in with argon. Right: Principle of operation for an APL, showing the direction of the electrical current in the plasma driven by the APL's power source, and the resulting magnetic field and its effect on a negatively charged beam.
• Figure 2: Simulated beam size ($\sigma$) in the horizontal ($x$) and vertical ($y$) plane as a function of distance along the beam line ($s$), with SHARP focusing optics. Log scale is used for the vertical axis to clearly see the depth of the focus point. Top plot: Focus at symmetry point of the APL relative to the center of the final focusing triplet, at $s^*=0$, and beamline layout above. Bottom plot: Focus 1 m behind the symmetry point, at $s^*=1$ m. Both plots show four cases of the APL current $I_\text{APL}$: Off ($I_\text{APL} = 0$), low (100 [A]), moderate (500 [A]), and high (1000 [A]).
• Figure 3: Quadrupole strengths in the final focus triplet as a function of different focal point position $s^*$, for different APL currents, optimized by minimizing Equation \ref{['eq:r-quads']}. The first column of the legend shows how the different quadrupole magnets are represented by different colors, and the right column shows how different APL currents correspond to different line dash patterns.
• Figure 4: Beam size $\sigma$ and opening angle $\sigma'$ in the focal spot, as a function of focus position $s^*$ and APL current $I_\mathrm{APL}$. Data for the horizontal ($x$) plane is shown; the vertical plane is very similar. This simulation assumes no scattering, linear optics, and Gaussian beams.
• Figure 5: Particle fluence as given in Equation \ref{['eq:fluence']}, normalized to 1.0 at the peak inside the APL, for four different APL currents. The focal point is located at $s^* = 1.0$ m, and the peaks of the fluence are indicated with circles of the same color.