B Medical Physics and The Particles Used

[Theudius]

Hi, I'm trying to think of a better alternative particle to protons for cancer treatment and was wondering if someone could put me in the right direction, do any other particles have properties such as the Bragg peak that would make them a good contender?

 

[Dale]

Well, first you need to determine what would make a particle “better”. What problems to protons have? What do they do well?

 

[Theudius]

Well just like photons when they enter the body they still irradiate human tissue, but what makes them good is the Bragg peak they deliver there maximum dose at once and hence they are stopped and do not further irradiate human tissue on exit. They are much more controlled because they are charged this allows us to modify the beam by altering the energy of protons depending on the depth of the tumor. We can do this using a cyclotron.

Also I would like to ask, how do we alter the energy of photons in radiotherapy?

 

[Choppy]

Just about any kind of charged particle will give you a Bragg peak - the heavier the particle, the sharper the peak tends to be. Even electrons tracks technically have a Bragg peak, it's just that they scatter so much it can't really be taken advantage of the way it is with protons and heavier ions.

Over the years a number of heavier ions have been explored: carbon, neon, silicon, argon, etc. I think there are a few carbon ion facilities up and running at the moment. While there may be an advantage in the basic physics in terms of beam delivery (i.e. using the spread-out Bragg peak to target a specific volume and taking advantage of the higher linear energy transfer properties of such beams), I think there are still questions in terms of clinical outcome and whether any small gains due to the technology outweigh the enormous increase in cost. On top of that we also still struggle with the issue of identifying the precise location of the cancer in the first place (and the fact that it can move around), which means that the even though we may be able to deliver much more conformal doses in principle, we don't necessarily see that translate into major differences in clinical outcomes.

 

[Choppy]

Also I would like to ask, how do we alter the energy of photons in radiotherapy?

Photons in radiotherapy are in most cases generated by medical linear accelerators. Radio frequency power is used to create a standing wave (or in some designs a traveling wave) inside a waveguide. These accelerate electrons from an electron gun and onto a target. As a general rule, the longer the waveguide, the higher the kinetic energy you can achieve. They interact and generate a bremsstrahlung spectrum that is collimated, modulated and ultimately used for treatment. You can adjust the peak energy of the spectrum by controlling the number of cavities that accelerate the electrons.

This spectrum of photons can be modified or filtered, but you still have to work with a spectrum at the end of the day.