Waveguide replacement leads to large change in machine output

I am currently going through a two year medical physics residency program at Oncologics, Inc, based in Lafayette, Louisiana. Medical physicists work primarily in radiation oncology, and one of our primary responsibilities is to calibrate the radiation treatment machines and test them for proper function. One of these days I mean to write a better explanation of what it means to be a medical physicist, but today is not that day. However, I do have a tale of physics I'd like to tell that may serve as instruction for me or others in the future.

Varian Clinac 600C

A linear accelerator (or linac) is the primary method used to treat cancer with radiation. To the right is a picture of the model we have in service at our Morgan City facility, the Varian Clinac 600C. Last week, one of the main components, called the waveguide, broke down, and needed to be replaced. When our service engineers opened up the machine, they found radioactive material scattered inside, and the physics team determined that, in addition to replacing the waveguide, we needed replace the primary collimator as well. This sets the stage for my story.

A linac does not produce radiation by using radioactive source materials, but instead uses electric fields to accelerate electrons to near light speeds. When the beam is off, the machine is not radioactive. So how did our engineers find radioactive sources inside the machine? In some older machines, like this one, the primary collimator is made of depleted uranium, which is radioactive. Generally this is not a problem, as the radiation level is low, and contained inside the machine. However, in this case, we found that the primary collimator had degraded in places, and some radioactive material was free to spread out of the machine.

I tried to find a good diagram of the parts of a linac relevant to this story, but was disappointed in the internet's offerings. So I made my own rough diagram, shown here. This diagram is obviously simplistic, but it will fulfill my purposes. This diagram is applicable to a Clinac 600C, which only treats with 6 MV photons, and does not use a bending magnet.

Electrons are first accelerated in the waveguide to near light speeds. Since the Clinac 600C is a 6 MV accelerator, electrons are accelerated to 6 MeV, or roughly 0.94 c. At the end of the waveguide sits the target, which is just a piece of tungsten. The electrons slam into the target and produce X-rays via bremsstrahlung, or 'braking" radiation (meaning as they slow down or "brake"). These X-rays have a range of energies, with the max being 6 MeV. (To distinguish photon beams composed of mixed energies from electron beams composed of (relatively) uniform energy, we use the term MV (megavolt) for photons, instead of MeV (megaelectronvolt).)

Once photons are produced, they travel through the primary collimator. The purpose of this collimator is to shape the beam so that it is expanding in a cone the focuses back at the target. As stated above, depleted uranium used to be used for this part, but nowadays we use tungsten, which is not radioactive. After the primary collimator comes the lead flattening filter, which makes the beam flat (i.e. so that the amount of radiation delivered in the center of the beam is approximately equal to the amount near the edges).

The ionization chambers play an important role. Ion chambers are devices that measure the amount of radiation passing through them. The ion chambers in a linac constantly monitor the radiation beam, and turn the beam off when the correct amount of radiation has been delivered (the radiation oncologist determines what is the "correct amount"). There are two chambers, one primary chamber, and one backup chamber that will also terminate the beam in case the primary chamber fails.

Now, back to our story. After much effort on Monday, the service engineers replaced the waveguide and also the primary collimator with one made of tungsten. I and another physicist began our tests of the radiation beam to verify that the beam characteristics were correct and adequate for patient treatment. After several tests, we began the calibration of the radiation output, where ensure that the machine delivers the amount of radiation it is supposed to. This is important because, while a linac has an internal system (called monitor units) to keep track of how much radiation it produces, it takes an outside observer to measure the amount of radiation that is actually delivered to a patient. The machine doesn't know how many monitor units equals how many gray (gray is a unit of radiation). So we adjust the machine output until it is where we want it to be.

When we first measured the machine output, we found it to be 15% higher than before the part replacements. This is a rather large jump, and the parts we replaced really shouldn't cause such a jump. We decreased the output by adjusting the gain on the primary ion chamber (thus increasing the ion chamber signal, which makes the machine terminate the radiation sooner), but ran into problems with the backup ion chamber. The two chambers should agree, so if you adjust one you must adjust the other. However, we ran out of room on the adjustment scale for the backup chamber, and could no longer change its readings. This left us in quite a pickle, because if we can't calibrate the machine properly, we can't treat patients.

We tried several different things, including measuring the beam with different instruments to make sure our readings were correct, and adjusting some different machine parameters and settings. Despite our efforts, the problem remained unsolved.

Then one of the service engineers remembered something he had done when installing the waveguide: Thinking that the previous waveguide had failed because the target located at the end of the waveguide was too close to the primary collimator, he moved the new waveguide and target assembly up approximately 4 millimeters from their previous position. Our initial reaction to this was that 4 mm is too small to make such a large difference, because we usually are concerned about distances of around 1 meter (the distance from the target to the patient), and a 4 mm change would only produce a difference of maybe 1%. However, we realized that in this case we shouldn't be looking at the distance from the target to the patient, but the distance from the target to the ion chambers, which is much smaller. A 4 mm change relative to that distance is significant, especially after taking into account the 1/r² falloff (r is the radius, or distance in this case).

So the engineer adjusted the target location, and we were able to calibrate the machine successfully. The point of the story is that sometimes small changes can have big consequences. When making small changes, we need to be sure that they are small on the appropriate scale. What is small on one scale may actually be large on another scale, and if that other scale is what really matters, then the effect may be large.