Linac booster for high energy proton therapy and imaging

For an optimal exploitation of the benefits of proton therapy the most accurate dose delivery system should be used. The TERA Foundation has extensive experience in the field of high gradient high frequency linacs. This paper describes a particular design of a 3 GHz linac boosting the typical cyclotron beams for proton therapy of 230–250 MeV up to 350 MeV. Such an upgrade of a typical proton therapy facility enables performing proton radiography, as well as extending therapeutic capabilities with high energy proton therapy (HEPT). The recent studies and measurements in high-gradient linac technology demonstrated that average fields in the accelerating structures of up to $25–30\mathrm{MV}/\mathrm{m}$ can be achieved, which results in a total linac length of less than 7 m. To test several characteristics of such a linac as a booster of a cyclotron beam, a design has been made of a linac unit accelerating from 250 MeV to 275 MeV, which could be built and inserted for tests in an existing beam line at the PSI proton therapy facility. The feasibility considerations, along with the design of the linac booster and the issues related to a possible integration in an existing cyclotron beam line are detailed in this study.

Figure 1

Left: In a schematic presentation of proton radiography it is shown how the remaining energy of protons with initial energy E1 is measured by means of a range telescope. In this way, the total stopping power along the proton track in the patient is measured. This information is used for a very accurate calculation of the range of lower energy protons (E2) used in therapy. Right: A tumour irradiation with Bragg peaks will also give a dose next to the tumour due to multiple scattering. By using high energy, when irradiating the lateral edge of the tumor, the lateral penumbra is decreased substantially. Behind the tumor only a small volume is receiving extra dose due to the longer range of high energy protons.

Figure 2

Top: Picture of the LIBO module cut-away. Bottom: Schematic image of a CCL tank showing on-axis accelerating cells and off-axis coupling cells.

Figure 3

Example of CCL linac booster.

Figure 4

Beam losses and beam transmission with respect to the bore radius [18]. The values of the effective shunt impedance for the first cell are also reported.

Figure 5

Horizontal (top), vertical (middle), and longitudinal (bottom) acceptance of the linac [18]. The coordinates of the beam at the entrance of the first linac tank are plotted in the three phase spaces. In blue are the particles used as input beam, in yellow the particles that arrive to the end with the correct energy within $\pm 5\mathrm{MeV}$.

Figure 6

Schematic representation of the linac longitudinal acceptance of a particle bunch coming from the cyclotron. About 20% of protons arrive at the right phase for acceleration in the linac as indicated by the section of sinusoids highlighted in red around the linac synchronous phase of $-25\mathrm{deg}$.

Figure 7

Schematic time structure of linac and cyclotron. By chopping the cyclotron beam with the same repetition rate of the linac rf pulses (200 Hz) the losses are reduced. Within the cyclotron beam pulses (green line) a transmission of about 20% is expected.

Figure 8

Beam pulse intensity measured at PSI with a plastic scintillator. Left and middle: The beam pulse structure (200 Hz, $5\mu \mathrm{s}$) is shown. Right: A rise/fall time of less than $1\mu \mathrm{s}$ could be achieved by mounting the existing power supply immediately next to the cyclotron.