Proton therapy primarily uses pencil beam scanning (PBS) to deliver precise dose distribution. However, this method is limited by the number of beam directions that can be delivered in an acceptable treatment time. Proton arc therapy (PAT) addresses this issue by using an arc trajectory. PAT differs from PBS in that it has a larger number of beam angles and the ability to optimize the energy for each direction, reducing treatment time.
Researchers from the Trento Proton Therapy Center, together with colleagues from RaySearch Laboratories, compared the dosimetric parameters of proton arc therapy plans with modern multi-field optimized pencil beam scanning (MFO PBS) plans. The study involved 10 patients with head and neck cancer. This localization was chosen because of the large number of important organs located near the tumor that can be protected using the new technique.
In the future, PAT treatments are planned to be performed with the beam turned on during gantry movement (dynamic mode). This means that all system parameters will need to be automatically adjusted while the structure is rotating. Due to the complexity of the equipment and software, such treatments are not yet used in hospitals. Instead of using dynamic PAT, the scientists used a simpler, static option. They divided the arc into several PBS beams and used a conventional method of delivering the radiation. Using the RayStation software, MFO plans (with six beam directions) and PAT plans (with 30 beam directions) were created, optimized to minimize errors.
The new PAT plans were found to be much better at targeting the radiation to the tumor than the older MFO methods. Although both were equally good at covering the diseased tissue, PAT reduced the dose to important organs.
This was particularly noticeable in the brainstem, where the dose was reduced by 19.6 units (Gy (RBE)) at maximum and by 9.5 on average. For other nearby organs, the difference was insignificant, but PAT significantly reduced the dose for those organs that are far from the tumor.
The researchers also looked at how differences in radiation exposure affected the likelihood of side effects. They found that PAT reduced the risk of dry mouth by 8.5%, and slightly reduced the likelihood of other problems, such as difficulty swallowing, the need for tube feeding, and increased salivation.
To test whether PAT could be used in a real hospital, the team irradiated one patient using both MFO and PAT plans on a clinical gantry. They found that the irradiation times were nearly identical: 36 minutes for PAT (30 directions) and 31 minutes for MFO. If they reduced the number of directions to 20, the time would drop to 25 minutes, with little change in the quality of the irradiation.
Nine patients have already received or are receiving PAT treatment: five with head and neck tumors, three with brain tumors, and one with breast cancer. The first two patients with head and neck tumors were given half-arc (180°) PAT plans with 10 beam directions, with a mean treatment time of 12 minutes. The next two patients were given full-arc (360°) plans with 20 directions, with a mean treatment time of 24 minutes. Quality control showed a high compliance rate (99.6%), with only one patient requiring a plan adjustment. The researchers plan to investigate whether this method reduces toxicity and are working to reduce delivery time.