Proton Therapy: Behind the Scenes
Proton therapy systems are huge in size, some the length of a football field and 3 stories tall. But most of the machinery is built behind walls and not visible to the patient entering the treatment room. These pictures will give you a look inside the proton system.
It all begins with the cyclotron, which can also be called the particle accelerator. Hydrogen atoms are separated from water, using a process called electrolysis. A positively charged proton is extracted from each hydrogen atom and injected into the cyclotron.
Using electromagnetic fields, the cyclotron accelerates the protons up to 2/3 the speed of light, all within fractions of a second.
The proton beam is then taken from the cyclotron and passed through an energy selection system, which makes the beam's energy variable for use in each of the treatment rooms served by this beam. This allows each room to use the energy needed for that patient.
The beam transport system then transports the accelerated protons through the beam transport line into each treatment room. Electromagnets are positioned along the line to route the proton beams around corners and into each treatment room.
The beam transport line can be as long as a football field and links the cyclotron to each treatment room. Some treatment rooms are built without a gantry. The gantry is a large, sphere shaped structure that houses the equipment used to actually give the protons to the patient. The gantry is three stories tall and built into a large concrete casing. The patient enters the treatment area on the second floor. The gantry allows the beam to spin 360 degrees around the patient. A fixed-beam treatment room (see below) does not require the gantry because the beam does not move around the patient.
Like traditional radiation therapy, the patient lies on a "couch" to receive the treatment. In some machines, the couch actually moves the patient around to deliver the precise dose.
Types of Beams
In order to achieve the prescribed dose to the prescribed area, different types of beams and tools are used to direct and shape the beam. Currently, three types of proton delivery systems are in use, passive scattering, uniform scanning, and pencil beam scanning.
In passive scattering, equipment called a modulator (or range shifter wheel) and scatter foil are used to take the thin beam line and widen it to fit the tumor.
To further shape the beam, a collimator and compensator can be used. A collimator is used to shape the beam coming out of the nozzle and is usually made of brass. A compensator, made of wax or acrylic, shapes the far edge or end of the beam, making some areas more or less deep to contour to the tumor. These pieces are made specifically for each patient's tumor treatment plan.
Some cons to passive scatter include the creation of custom pieces for each patient, the disposal of these pieces (as they become radioactive after use) and the shifting of dose towards the front end of the beam, towards the skin, which can result in unwanted dose to the patient (this is demonstrated by the pink area outside the tumor- outlined in black in figure 6, above).
The second type of beam is called a scanning beam. There are actually two different types of scanned beams, uniform scanning and pencil beam scanning. Uniform scanning uses magnets to scan a broad beam across a treatment field. This type of scanning still requires the use of collimators to shape the beam. The second type of scanning was only recently approved for use in the U.S. and you may hear it called pencil beam scanning. The easiest way to describe the beam is to think of the tumor being colored in with a pencil using a back and forth motion. The beam can "draw" different depths and the nozzle contains magnets that steer the beam, thus eliminating the need for the collimator and compensator. The major disadvantage is that the treatment takes longer than with passive scatter. Because scanning beams are so precise, there are challenges to dealing with organ motion. With any movement (breathing, bowel contraction, bladder filling, etc), your organs move ever so slightly, even during the short time it takes to receive a radiation treatment. This movement effects where the radiation dose is given and the radiation team goes to great lengths to account for this and be sure the tumor receives the dose it is prescribed. Centers are also incorporating the same technology behind IMRT, in this case called IMPT (intensity modulated proton therapy), which will allow more precise dose distribution.
A fixed beam (a type of scanning beam or scattering beam) does not move and is often used to treat cancers of the eye or skull. Instead of moving the beam around the patient with a huge gantry, the beam comes out of a pipe stuck in the wall and the patient is moved around the fixed beam. The patient sits on a chair in many centers. However, there are also fixed beam systems that utilize a couch to treat a variety of cancers including brain and prostate tumors.