An international research team has demonstrated a proton CT (pCT) system for patient alignment prior to proton therapy. The researchers tested the technique by capturing fast, low-dose pCT pre-treatment images of a human head phantom and verifying them against an initial planning pCT image. Using protons instead of X-rays for planning and patient-positioning scans could improve treatment accuracy, as it avoids the need to convert X-ray attenuation to proton stopping power (J. Appl. Clin. Med. Phys. 10.1002/acm2.12565).
The concept of using protons for CT goes back to the 1960s. Early experiments were conducted in the 1970s, but only in the last few years have particle detectors with fast readout — developed for particle colliders such as CERN’s LHC — made it a practical possibility. Another recent trend that has enabled the technology is the widespread use of dedicated graphics processing units for image reconstruction, which are necessary to interpret the hundreds of millions of individual proton paths.
Combining expertise in the fields of high-energy physics, medical physics and high-performance computing, Reinhard Schulte of Loma Linde University and a team of researchers from Italy, Germany, Australia and the US constructed a device consisting of 16 individual silicon-strip detectors arranged in two pairs of planes. With a pair on each side of the patient (in this case, the head phantom), each set of detectors recorded the position and direction of protons before and after they passed through the imaged volume. Downstream from the phantom, a stack of five scintillators measured the protons’ residual energy, indicating the particles’ water-equivalent path length, a crucial measurement used to determine the range of the protons in the patient or phantom.
Radiotherapy typically begins with a treatment plan based on a detailed CT image. In this study, measuring the particle paths and energies for the planning pCT image required six one-minute rotations of the phantom within the scanner, with a further seven minutes for image reconstruction. The researchers expect future versions of the device to need just two or three rotations, and increasing computer power should bring the length of the reconstruction step down too.
With a plan ready to implement, clinicians acquire another CT image immediately before treatment commences, to ensure correct patient alignment. For the pre-treatment pCT images, Schulte and colleagues used a quicker, more approximate reconstruction algorithm and tested it under a range of radiation-dose scenarios. By discarding different proportions of the proton measurements, the researchers simulated pre-treatment pCT scans that delivered between 100% and 12.5% of the dose of the planning pCT.
To reproduce the kind of variation that arises from patient motion in the clinic, the team introduced rotational and translational discrepancies between the planning and pre-treatment images. Applying rigid and deformable image-registration algorithms to correct for these discrepancies, the researchers found that even the lowest-dose scan (with the smallest signal-to-noise ratio) yielded residual errors generally smaller than 1 mm, and never greater than 2 mm.
The most important advantages of using protons for planning and pre-treatment images are not in their imaging performance, however. When proton-therapy plans are produced from X-ray images, medical physicists have to convert the X-ray attenuation to relative stopping power for protons. Using protons instead avoids this uncertainty. pCT is also much less radiation-intensive for the patient: the planning pCT protocol used by Schulte and colleagues delivers less than 15% of the dose of the equivalent X-ray procedure.
As proton range in tissue depends on the initial momentum of the particles, applications for pCT are limited by accelerator energies. “Head, neck and thorax imaging is possible with existing therapeutic proton accelerators, but energies higher than 250 MeV are necessary to penetrate a lateral pelvis or the abdomen of an obese patient,” Schulte explains. Such energetic beams are only just becoming clinically available.
More distant but still possible is the prospect of replacing protons with heavier particles that are less susceptible to scattering from nuclei. “We have done experiments at the Heidelberg Ion Therapy (HIT) Center and our system also works well with helium,” says Schulte. “There is great interest at ion facilities that usually treat with carbon ions but also provide helium ions as well.”