The last post in this series showed how a linear accelerator is used to create a beam of radiation. Now we will talk about how we can conform the beam to the shape of the tumor we are trying to treat while blocking the beam from irradiating normal tissue.
Once the beam is generated, it is collimated so that only photons travelling forward are let through. This beam is much more intense in the forward direction, so a filter is put in place to cool the beam down towards the center. After passing through this flattening filter, the beam profile is more uniform across its width. This is important since we do not want one side of the tumor to get less dose than another. Once we have a relatively flat beam to work with, we can begin to shape the field to the patient. Most linear accelerators have a pair of collimating “jaws”. These jaws are made of lead or tungsten, and are movable so that the field can be blocked to make it square or rectangular with sides up to 40 cm long (for a typical linac). These collimators are housed in the “head” of the linear accelerator.

The directions in which a linear accelerator can rotate.
The head is mounted on a “gantry” that is able to rotate 360 degrees around the patient. The jaws can also rotate over a range of about 240 degrees. This allows the beam to enter the body at the angle that minimizes the amount of normal tissue irradiated.
The beam then exits the linac through a window. Further tools to shape the beam can be mounted to the outside of the window between the beam and the patient. One such device is a wedge, a physical wedge of metal that lowers the dose on one side of the field relative to the other. Another is a block, a layer of attenuating material that stops the beam in certain areas to shield normal tissue beneath it. These blocks are usually custom made for each patient to conform to the edges of the treatment volume. Usually the material used is called cerrobend or Wood’s metal. This is a lead alloy that has a very low melting point, only 70 degrees Celsius. This makes it very convenient to melt down and form.
The downside is that forming cerrobend blocks is very labor intensive, and for a busy clinic, making blocks is a full time job. In addition, any change that needs to be made during a patient’s treatment means that you have to start from scratch with a new block. Since lead is a hazardous material, making blocks is complicated from an occupational safety standpoint as well. For these reasons, the multi-leaf collimator or MLC was developed.

The leaves of a multi-leaf collimator
This consists of many (around 80 or 120) leaves of metal with a thickness from 1 cm to 0.5 cm or smaller. These leaves are mounted inside the linac just after the collimating jaws and are attached to motors that can drive them in and out of the field. The leaves of the MLC can be controlled with a computer and adjusted as needed to conform to the physician’s wishes. The downsides of an MLC are that radiation can leak between the leaves, although clever design of the system can minimize this, and that since the leaves have a finite width, the edge of the field is slightly jagged.

Shielding the eye from radiation using a block or mlc.
This image is of a patient that had a tumor (shown in green) in the sinus cavity. This is called a beams eye view and shows what you would see if you could look from the linac head directly down the beam toward the patient. The image on the left shows the position of the collimating jaws, outlining in yellow the treatment field. As you can see, the patient’s eye, outlined in blue, is within the field and will receive a significant amount of radiation unless it is shielded. The middle image shows how a block would shield the eye and only leave the tumor, plus a slight amount of margin, treated. The right image shows how the same thing could be accomplished using an MLC.
Conforming the beam to match the desired field is where the physics staff (physicists and dosimetrists) start to make the physician’s treatment plan a reality. It is where we work closest with the physician to make sure the right balance between tumor dose and normal tissue dose is struck. So what goes into that decision? My next post in this series will explore how the beam of radiation delivers the dose to the patient, and future posts will discuss how we use that information to create a treatment plan.
Image licensed under the Creative Commons Attribution ShareAlike 2.5 license from Vojtěch Hála.