Particle therapy was, as expected, one of the hot topics at this week's American Association of Physicists in Medicine (AAPM) annual meeting. Visitors to the Anaheim, CA-based meeting found out about the latest developments in a wide variety of areas: from new ways of producing proton beams to methods of monitoring deposited dose, and even the introduction of proton-based tomographic imaging. Here's a taster of just some of the novel research being discussed at this year's event.
Compact accelerators
The drive to develop compact, cost-effective alternatives to conventional accelerator-based particle therapy has prompted many research groups to investigate the application of laser-accelerated proton and ion beams. Speaking in a session dedicated to "Novel Proton Accelerators for Radiotherapy", Charlie Ma from Fox Chase Cancer Center (Philadelphia, PA) presented a review of recent work in the field. Ma also reported on the progress of Fox Chase's laser-ion acceleration facility (abstract 11988).
Ma explained that the Fox Chase acceleration system comprises a commercial 150 TW laser, custom-made laser-pulse compression and target chambers, particle selection and beam collimating devices, dosimetry monitoring systems and shielding constructions. Initial testing with a 1018 W/cm2 laser (at 20 TW) produced up to 4 MeV protons with a broad energy spectrum. The team has also investigated a compact shielding design that enables installation of the particle-therapy head on a small rotating gantry. Ma concluded that laser-accelerated particle beams have great potential, though he noted that many technical and engineering issues still need to be solved before building a clinical prototype.
Dose verification
PET imaging offers a non-invasive means for in-vivo verification of the dose delivered during particle therapy, by monitoring the production of positron-emitting isotopes such as 11C and 15O. To date, the Massachusetts General Hospital (MGH) in Boston, MA, the National Cancer Center (NCC) of Kashiwa, Japan, and the University of Florida, Jacksonville, have reported clinical trials results for about 50 patients scanned with PET/CT after treatment. Katia Parodi - previously at MGH and now at the Heidelberg Ion Beam Therapy Center in Germany - told AAPM delegates about the initial clinical experience at MGH (abstract 11887).
Parodi explained that post-radiation imaging suffers from limitations, such as the degradation of activity between irradiation and imaging, and the need to reposition patients prior to imaging. These drawbacks have prompted the development of in-beam and in-room PET detectors, at NCC and MGH, respectively. In addition, while clinical experience so far has been restricted to passive beam delivery, PET verification is also applicable to emerging scanned ion-beam techniques. Parodi described the potential benefits of time-resolved imaging, as supported by phantom experiments for 4D in-beam PET verification of carbon-ion beam tracking at GSI (the heavy-ion research centre in Darmstadt, Germany).
Imaging with protons
Researchers at the Loma Linda University Medical Center (Loma Linda, CA) are investigating the use of protons for tomographic imaging. Proton CT works in a similar way to standard CT, but while X-ray CT measures the attenuation of multiple photons, proton CT detects energy loss from single protons, enabling imaging at a lower dose (between two and 10 times less dose, according to simulations). Using proton-based images to create proton-therapy treatment plans, as opposed to using X-ray CT data, would eliminate the need to convert CT Hounsfield units into relative proton stopping power values, a process that can add uncertainties into treatment planning calculations.
Delegates at the AAPM meeting heard Reinhard Schulte present a status update on Loma Linda's proton CT project. Schulte revealed both simulated images and experimental data obtained with a small prototype installed on the centre's proton research beamline. He also discussed decisions regarding the next-generation proton CT scanner, which will permit scanning of head-sized objects (abstract 10533).
Pencil-beam scanning
Ben Clasie of Massachusetts General Hospital and Harvard Medical School (Boston, MA) described the technical and practical considerations associated with proton pencil-beam scanning. To deliver a pencil-beam field, the MGH system converts the treatment-plan beam parameters to equipment settings, sorted in energy layers and sent to various measurement and control devices that check the parameters every 0.25 ms. The on-line dosimetry system was adapted to support this speed and characterized over a range of beam positions and dose rates. The researchers calibrated these data against measurements at the isocenter, identified any sources of potential error and noise, and reduced these to a level suitable for accurate clinical treatment.
The scanning system produces pencil beams smaller than 6-8 mm wide with focusing magnets, and 9-14 mm without. The system proved to be accurate, safe and able to achieve clinically useful dose distributions, with a dose accuracy of ±0.2 cGy in the Bragg peak for a single pencil-beam. Clasie noted that the research team (working in collaboration with IBA of Belgium and Pyramid Technical Consultants of Lexington, MA) has developed the capability to treat large tumours, an application not usually considered for proton-beam scanning but which they see as an important opportunity (abstract 11450).