RTY2003 – Radiation Therapy Physics – Linac (Part 2)

Because there’s just too much to talk about the linac, right?

This short chapter will focus on the gantry head components for photon beam generation, the various types of beam modification, and compare photon and electron generation gantry head configurations.

T C F I J M

These 6 letters come in this order, and it is important to memorise this as it is. These represent the components of the gantry head, from target to the output. It is important that you find your own way to remember these letters. I’ve settled with: This Cheerful Feeling Is Just Meaningless.

The target is where the electrons slam into to produce photons through Compton interaction.

The primary collimator is a block with a conically shaped bore, and it only permits the transmission of forward scattered photons. As such, probability would dictate that the photons in the center of the field have a higher energy than those around it, and the energy decreases in a Gaussian distribution away from the center of the field.

This uneven distribution of energy necessitates the flattening filter, which rids the forward pointing, bullet shaped dose distribution into a flat, clinically useful beam.

Next, it is important to ensure that the dose delivered to the patient is accurately measured and accounted for, and hence we have not one, but two ionisation chambers to precisely monitor the dose produced. An interesting mechanism here is that the interlock that is wired to the ionisation chamber does not detect the malfunction of any of the chambers directly. Rather, it detects if the readings from both the chambers are within 10% of each other. This way, in the event of a malfunction, one of the chambers will read 0 output, ensuring a more reliable method to trigger the interlock.

The jaws, also known as the secondary collimators, are the next component, and come in pairs. These jaws are responsible for controlling the field size, and are mobile and adjustable, as opposed to the primary collimators, which is just a fixed cone void.

Finally, the last beam modifying component of the gantry head is the MLC, multi-leaf collimators. This part is responsible for shaping and conforming the beam to a field shape that does not necessarily have to be a square / rectangle, sparing the organs at risk around the tumour, where no dose is necessary.

After the gantry head, there are more components that can be added in if necessary. The two modifiers are the compensator and the wedge.

Both are used to modify the beam, although the compensator is largely obsolete, as it is a device that has to be manufactured for a single patient, and it cannot be reused once the patient has completed their treatment.

A wedge is used to create a slope in the treatment field. It is worth noting that the nomenclature of the wedge is not reflective of the actual angle on the physical wedge, but the slope that it creates on the isodose line at the isocentre.

The four types of beam modifiers are hence:

1. Shielding (MLC)

2. Compensation (compensators)

3. Wedge (wedge)

4. Flattening (flattening filters)

The primary and secondary collimators are not technically considered beam modifiers as they simply filter and cut off parts of the beam that aren’t useful, as opposed to the actual modification components, which photons have to pass through directly.

Here is a comparison of the gantry head components during photon mode and electron mode: