Proton beams constitute a very desirable form of radiotherapy; physicians and physicists worldwide are researching and testing ways to make this cancer treatment even more efficient and effective. One of the delivery mechanisms under active development at many proton therapy centers is Pencil Beam Scanning (PBS), a treatment mode that allows for precise movement and flexible dose delivery of a small proton pencil beam.
One well-known research and testing center for PBS is the Francis H. Burr Proton Therapy Center at Massachusetts General Hospital (MGH) in Boston, Massachusetts. Hanne Kooy, Ph.D., Associate Director of the Burr Proton Therapy Center, Associate Professor of radiation oncology at Harvard Medical School and a recognized expert in the field of proton therapy, sees PBS as having a number of benefits over conventional passive-scattering techniques. Chief among these benefits are more efficacious treatment and reduced neutron dose.
It should also be possible to realize greater procedural efficiencies in treatment design and delivery, which could result in shorter treatment times and increased patient throughput. “Right now, except for treating smaller tumor volumes, the current way of delivering proton therapy is cumbersome,” says Kooy. “We need to achieve better automation. Just like in photon radiotherapy, a great benefit of automation was achieved with multileaf collimators. Now, Pencil Beam Scanning should allow us to achieve greater process efficiency as well.”
The hardware currently used to accurately shape the proton beam — apertures and range compensators, for example — are often heavy pieces of material that proton therapy staff members need to physically manipulate for each patient treatment, Kooy explains. The larger the treatment field, the more awkward this manipulation becomes. With PBS, scanning magnets manipulate the proton beam more efficiently, more quickly, and with far less physical effort by the professional staff overseeing the treatment — all of which contributing to less time on the treatment table for the patient.
Jay Flanz, Ph.D., Technical Director of the Burr Proton Therapy Center, notes that he doesn’t believe PBS will totally replace the need for apertures or compensators. “However, there may be a significant number of cases in which these efficiencies can be realized,” says Flanz. PBS will allow a more complex dose distribution to be delivered, which can result in a more conformal overall dose. “With Pencil Beam Scanning, you can control the dose and the position of the dose on the x/y plane, perpendicular to the depth. You can control the intensity of the beam and put the dose you want just where you want it within the treatment field,” says Hassan Bentefour, Ph.D., an IBA physicist working onsite at MGH. This works particularly well with irregularly shaped tumors or ones that surround an organ.