While tumour motion is problematic during X-ray radiotherapy, it can be even more so when it comes to protons. The sharp distal fall-off in the depth-dose distribution makes the technique extremely sensitive to target motion, particularly perpendicular to the beam direction. But how much movement can one expect during a treatment session, and how can margins be employed to account for these changes? Wen Chien Hsi, from the University of Florida Proton Therapy Institute (Jacksonville, FL), described a study examining these questions for the case of prostate cancer.
Hsi and colleagues employed PET imaging to determine typical levels of intrafraction motion during prostate-cancer treatments. The researchers performed 50 PET/CT studies on 10 patients immediately following daily proton therapy. Separating out those cases with prostate movement occurring after treatment (during the PET scan) revealed that in most cases, intrafraction prostate motion was less than 2 mm. However, about 14% of cases exhibited displacements of around 4 mm during proton therapy.
Hsi concluded that a systematic analysis of proton-activated PET distributions can provide patient-specific information as to prostate motion. Such information could be employed to establish patient-specific target margins for use in treatment planning. Cases that exhibit larger motion, for example, could be treated using increased margins. Hsi noted that this technique could also prove valuable during proton therapy of the lung. "In vivo dosimetry is even more important and useful for proton lung treatment," he said.
Image guidance
Of course, accurate pre-treatment alignment is imperative too, a task that's generally achieved via patient imaging. But according to Ajay Kapur, director of medical physics and dosimetry at the ProCure Proton Therapy Center in Oklahoma City, although there are a plethora of image-guidance techniques available in the photon world, the technology is lagging behind somewhat when it comes to proton therapy. "In the proton world, we still rely on 2D or 2.5D imaging," he said. "We need to go volumetric with CT."
As for what he'd really like to see in a proton-therapy treatment room, Kapur cited in-room robotic cone-beam CT (CBCT) coupled with robotic positioning. Such a set-up should ideally offer fast 3D/4D imaging and easy integration with alignment capabilities, as well as the ability to image patients in various positions, with the same position used for pre-treatment imaging and treatment delivery.
Challenges in establishing such a system include ensuring accurate correlation of the in-room CBCT with the planning CT, as well as developing a means of converting Hounsfield numbers to proton stopping power for CBCT. The set-up must also offer reproducibility, and the positioning device must not interfere with beam. Kapur touched on the issue of intrafraction motion, explaining that while the current fix is to use repainting, 4D CBCT imaging is an emerging trend.
Such a CBCT prototype is installed at ProCure's Training and Development Center in Bloomington, IN, and has already produced phantom and preliminary patient images. Kapur noted that current CBCT feasibility studies are promising, with reproducible data and sufficient image quality for treatment planning.
Increasing uptake
Presenting a broader outlook, Hanne Kooy, associate director of the Francis H Burr Proton Therapy Center (Boston, MA), posed the simple question: Why is every patient not treated with proton therapy? A suitable target, he suggested, would be to treat 20% of patients with protons, as opposed to the current 1%. "Protons have superior dose distributions, but we don't treat enough sites," he explained.
The answer to this question is not so straightforward. Polarized opinions abound in an ongoing debate in which some say "there's no convincing evidence for proton therapy", while others claim "it's an ethical conflict not to use proton therapy". According to Kooy, requirements for the future include optimizing image guidance for improved targeting, increasing the therapeutic ratio and reducing the treatment time, as well as reducing the cost of implementing proton therapy.
Kooy cited pencil-beam scanning as a means to facilitate many of these aims, and a necessity for increasing the clinical utilization of proton therapy. Pencil-beam scanning, in which the proton beam is actively scanned throughout the target volume, offers a great opportunity to improve target conformity and improve sparing of nearby normal tissues. It is the ultimate form of intensity-modulated proton therapy, and can be optimized using image-guided deliveries. He noted that image guidance with low-dose protons could prove a powerful option in the future.
"I would argue that protons can outperform photons, though there's a lot of work to be done," said Kooy. "If we believe that photons are good enough, then there's no need for proton therapy; but if we believe that protons give a better plan, then protons are optimal for future. And if we can reduce the cost, proton therapy will achieve market penetration.”
Latest facility
The final speaker on the panel was radiation oncologist James Metz, who provided an insight into the thinking behind the new Roberts Proton Therapy Center in Philadelphia, PA. The centre, which opens in January 2010, houses five proton treatment rooms (four gantries and one fixed-beam), as well as two research beam lines and five linac vaults.
"We thought that proton therapy would be better for some, not all, of our patients," Metz explained. As such, the Roberts Proton Therapy Center integrates conventional X-ray radiotherapy with proton treatment. "We must begin to change the vision of proton therapy as a sole treatment modality, and work out how it fits into multidisciplinary care," he said.
Metz notes that the centre is exploiting many advanced and emerging technologies to help improve its efficiency. For example, the IBA universal nozzle will be used to enable both scanning and scattering delivery in each treatment room, with rapid changeover between the two. Another requirement was fast switching between beam lines. Having achieved switching times of 10–15 s, the researchers are now developing a scheduling system to optimize beam allocation between treatment rooms.
The team decided to use multileaf collimators (MLCs) during proton-beam delivery, in order to more easily treat with multiple fields per treatment session. In collaboration with Varian Medical Systems, IBA and Penn Medicine, the researchers are currently developing a dedicated MLC with 5 mm tungsten leaves (as opposed to brass apertures).
As for patient positioning options, Metz says that the team looked at CBCT but decided it could be complex to implement. He suggested that a number of proton centres should to get together to work on this. Another possibility under evaluation is the implanted-transponder system from Calypso Medical (Seattle, WA).
Summing up the panel session, Sameer Keole, a radiation oncologist at the ProCure Proton Therapy Center in Oklahoma City, took a punt as to where proton therapy may be in 2020. He predicted that proton therapy will expand 25–40% by that time, and that "systems will be downsized and will be less expensive". There will be minimal need for a 230 MeV-capable gantry as we know it, Keole suggests. Instead, we'll see smaller gantries with limited energy, or smaller gantries that use superconducting magnets to offer higher energies. "I truly believe superconducting technology will be a key component to many or all of these systems," he concluded.