Pencil Beam Scanning: A New Type of Proton Delivery

A lot has changed since I trained at the Indiana University Proton Center. Specifically, the development of a new type of proton delivery called Pencil Beam Scanning (PBS).  The University of Pennsylvania has a well –established proton center and provides an excellent educational course. The slide below is from a Proton Physics lecture given by Alejandro Carabe, Ph.D.  from UPenn and shows how the PBS technique provides an even more conformal radiation plan for the treatment of a brain tumor.

Note: If you are not familiar with proton radiation or the Bragg Peak, you may want to review this. If you really have to know more about the mechanism of protons, or if you just love reading about physics, here’s a presentation from a guy at University of Florida that will give you all the details (You’ll also understand why radiation oncology is a 5 year residency….).

Why is PBS so Important?

For the same reason many patients consider proton therapy in the first place, PBS allows for another level of precision. Meaning the radiation can be more targeted and there is less low dose scatter. When I trained at a proton center, it was necessary for our physics team to create multiple heavy brass apertures to conform the proton beam to the desired target. Now, with PBS there are no physical devices to be created, just large magnets guiding the beam toward the tumor target.

Even with PBS, it still takes longer to plan a proton radiation plan then a conventional radiation plan (using photons). It may take your radiation oncologist anywhere from 2-3 weeks to create a good proton plan. One of the many reasons is that protons are, in effect, about 10% stronger then photons. Consequently the dose distribution through the tissues of your body, are very important and must be considered.

Because of the intensity of proton radiation, you will find that there is much time spent ensuring you are in the same exact position, every single day for treatment. Not only externally, but internally as well.  If you are receiving proton radiation to your prostate, you will most likely need an endorectal balloon to immobilize your rectum (yup, you read that right). You will likely also need to drink water to fill your bladder prior to treatment. This helps push your intestines up, out of the radiation field.

Why do you have to be so still and in the exact same position every day? Take a look at another slide from Dr. Carabe’s lecture.

Compare the top photo to the bottom photo. If your hip bone (blue arrow) is elevated by just by a few millimeters, the top part of your prostate (red circle) gets too little radiation dose (red arrow).

Now that I have the ability to use PBS for my proton plans, it does give me another level of precision in targeting tumors that I was unable to have with prior proton plans. It’s worthwhile asking your proton center if they are using PBS for your proton plan and what benefits it may provide for your radiation plan.