Extra: Pencil Beam Scanning

Physics for the 21st Century
Unit 9 Extra #2: Pencil Beam Scanning

Ethan Cascio and Harald Paganetti

ETHAN CASCIO: The way proton therapy has been done for most of its history and is still predominately done is you take a small proton beam and you spread it out into a large uniform beam. Then you mask the part of the beam you don’t want to use with a brass aperture. And then you use a plastic piece to adjust the depth of penetration across the field. It’s what is known as passive scattering.

HARALD PAGANETTI: There’s a lot of hardware involved in shaping these fields. And the disadvantage of a lot of hardware is for example that you get a lot of scattered radiation. Currently we’re doing a lot of research in proton therapy to change the way we deliver the beam.

ETHAN CASCIO: The next step would be instead of using a large uniform beam, use a very thin pencil beam—so called pencil beam because it’s very thin. And scan that through the treatment region both in lateral position and in depth.

HARALD PAGANETTI: We have a pencil beam coming in from here that would go through the magnets and the magnets make sure that this pencil beam is moved in x or y directions. So we can pinpoint every certain point in the patient and by doing so we can fill the whole volume of the tumor with these little Bragg peaks. And if we treat with pencil beam scanning we would not need aperture and compensator. So we don’t need any patient specific devices.

So it’s like on your old TV screen where you would scan an image by going through the different levels. And so you would basically scan the particular depth of your tumor then you would use the energy. You scan the next layer. You use the energy. You scan the next layer and so forth. So in that way, you can fill this tumor with small little dose spots.

But beam scanning offers another degree of freedom in that you can really assemble your dose distribution in different ways. So you can place your pencils in a way that the intensity is modulated. So from each direction you have an inhomogeneous dose distribution and this is called intensity motivated proton therapy.

So what you would do for a particular slice of the tumor you deliver certain spots of dose. And of course for each spot you can choose a particular intensity. So these spots can have a different dose level.

Each of these spots basically resembles a Bragg peak that you stop in the slice of the tumor at different colors and different intensities. So this slice of the tumor would get an inhomogeneous dose distribution but since we’re coming in from different angles of different fields the total dose distribution would still be

homogeneous.

So the main reason for doing beam scanning is really to give us one more degree of freedom to save critical structures or to give less dose to all those at risk.

ETHAN CASCIO: It is far more complex to do well than passive scattering. And far more complex, you have to make sure that you’re doing it very, very carefully because of the capabilities of delivering very high doses to very small areas. As I often say it’s not new science, it’s a purely technological problem and one that people are addressing and I think will be solved probably very soon.