So now that we know the why of radiation therapy, it’s time to move on to the how. Therapeutic radiation comes from one of two types of sources: X rays from a particle accelerator or gamma rays from radioactive material. Radioactive material is mostly used for brachytherapy, in which the radioactive material is placed inside the body to treat the tumor. For external beam treatment, where a device beams radiation into the body from outside, a linear accelerator is most often used. There are of course exceptions to this. There are X ray brachytherapy sources and some older therapy machines (and current ones such as the Gamma Knife) use Cobalt-60 to treat the patient with gamma rays.
A linear accelerator produces X rays by slamming a beam of high energy particles (usually electrons) into a metal target. This gives off X ray photons in a process called bremsstrahlung, literally “braking radiation”, that occurs when a charged particle undergoes acceleration (or deceleration in this case). The energy of the photons can be as high as the energy of the incoming particles, but the most probable energy is some fraction of that. Most of the photons are given off in the direction of the incident beam, so this results in a beam of photons with a spectrum of energies somewhat less than the incident beam energy. (There is also a second source of X ray photons called characteristic radiation that is important for diagnostic X rays, but usually negligible at the higher energies of therapy machines.)
The simplest device for creating X rays is the X ray tube. An X ray tube is simply a vacuum tube that has a filament on one end that emits electrons and an anode on the other end to collect them. A high voltage is placed between the filament or cathode and the anode to accelerate the electrons. The energy of the electrons is measured in electron volts. One electron volt is the energy an electron gets when it is accelerated by one volt. If you were to place a voltage of 1 kilovolt across an X ray tube, the electrons would have an energy of 1 kilo electron volt (or 1 keV). When the electrons hit the anode, bremsstrahlung photons are given off. These photons can have an energy up to the energy of the incident particles. Therefore, if 100 kilovolts were placed across the tube, the energy of the electrons would be 100 keV and the photons could have an energy of up to 100 keV. Usually, though, the peak of the spectrum is at an energy of about 1/3 of the incident energy.
Systems like X ray tubes work well for energies in the kilovolt range, but become large and cumbersome if used to generate photons in the megavolt range. Since photons with a higher energy can penetrate further into the patient to treat a deep seated tumor, other methods of generating radiation that could reach megavolt energies were developed. There are several types of accelerators that can generate photons with that high of an energy, but the one that is most often used is the linear accelerator. This figure shows a block diagram of a typical linear accelerator or linac.
A block diagram of a typical linac.
The linear accelerators in use today use microwaves to accelerate electrons. The waves are generated by a device called a magnetron (you have a smaller version of one in your microwave oven) or by a microwave amplifier called a klystron. The microwaves then travel through a waveguide into an accelerator tube. When electrons are injected into the accelerator tube from an electron gun, they can pick up energy from the waves and be accelerated. The accelerator tube can be manipulated to give different amounts of energy to the electrons and therefore different final energies for the beam. Very large accelerators like the Stanford Linear Accelerator Center can accelerate particles to energies of Giga electron volts. Medical accelerators are more compact and generate electron beams with energies in the Mega electron volt (MeV) range. A bending magnet then steers the beam into a metal target to generate X-rays. The bremsstrahlung process is very inefficient. Only about one tenth of a percent of the electron beam energy is converted into usable X rays. Most of the rest is dissipated in heat in the target. Therefore, the target must be made of something that is very heat resistant like tungsten and is usually water cooled. Once the X rays are generated, the beam is collimated and shaped in the treatment head.
Whether the X rays are generated by a linear accelerator, an X ray tube or another type of source, the energy of the beam is described by the energy of the incident electron beam. So the X rays resulting from a beam of electrons with an energy of 1 MeV will be described as a 1 megavolt (MV) beam. This is even though as discussed above, very few of the X ray photons will have an energy close to that of the incident electrons. It just provides a convenient shorthand for describing the shape of the X ray spectrum. In future posts I will describe X ray beams using terminology like 6 MV or 18 MV. When I talk about a 6 MV beam, I mean the beam of photons with an X ray spectrum that results from accelerating electrons with a voltage of 6 MV.
You can also get rid of the target and allow the electron beam to treat the patient directly. Electrons have advantages over photons for some types of treatments. Electrons have a much shorter range and are therefore more desirable for treatments that are very close to the skin. It’s important to remember though that by removing the target you get rid of the inefficiency of the bremsstrahlung process. Therefore the electron beam current will have to be lowered by a factor of 1000 (since the X-ray production of the target is 0.1% efficient) to have roughly the same dose as an X-ray beam. Electron beams are described in units of MeV, as in 6 MeV. They can also be described with just the letter E as in 6E (as opposed to X for photons as in 6X). The 6 in 6E or 6X is understood to mean 6 MeV or 6 MV.
Once the X rays have been generated, they have to be shaped to conform to the area that needs to be treated. In the next post I will describe the various devices we use to shape the beam to provide the optimal treatment for an individual patient.
