Radiation treatment modalities

There are many different ways of delivering radiation therapy (known as modalities), and in this article we are going to look at two main types: external beam radiation therapy and brachytherapy.

External beam radiation therapy

As we have seen earlier on in the course, one method of delivering radiation therapy is by using machines called linear accelerators.

^{Linear accelerator}

These machines deliver what is known as external beam radiation therapy, and this is the most common method of delivering radiation therapy. External beam radiation therapy is delivered external to the patient, i.e. through the patient’s skin.

Types of radiation used in external beam radiation therapy

External beam radiation therapy can be delivered using different radiation types, such as photons, electrons, protons and carbon ions. The difference between using these radiation types is based on their beam characteristics. A beam is the high energy x-rays that are generated by a treatment machine.

This graph is known as a percentage depth dose curve. What we can see from this curve is the depth that the beam can penetrate to in millimeters (mm) on the horizontal axis, and the relative percentage dose on the vertical axis.

• Looking at electrons (green line) we can see that they deposit the majority of their dose at the surface of the skin. This makes them useful in treating tumours that involve the skin.

• Protons (red line) and carbon ions are together known as particle therapy. They can be manipulated to deposit their dose in a very specific area and are very useful in treating specific cancers, particularly in children. There are far fewer proton and carbon ion treatment units internationally than traditional linear accelerators. These units are extremely expensive to build as they require a complex magnet deflection system to guide the beam into the treatment room from large machines known as cyclotrons or synchrotrons.

^{Cyclotrons machine. Copyright: Dominika Zarzycka/Shutterstock}

• Photons (blue line) are most commonly used for external beam radiation therapy. Photons deposit the majority of their dose (known as the maximum dose) a few centimetres from the surface of the skin depending on their energy. Different photon energies can be selected to treat different areas of the body. For example, a higher energy photon beam is required to treat tumours in the pelvis compared to the breast. External beam radiation therapy is also known as photon therapy.

External beam photon therapy treatment techniques

External beam photon therapy can be delivered using different methods, or “treatment techniques”.

3D Conformal Radiation Therapy (3DCRT)

3DCRT is planned using 3D images, such as CT or MRI, and the radiation beams are shaped (conformed) to the tumour as much as possible.

^{3D Conformal Radiation Therapy (3DCRT) plan of a lung}

Intensity modulated radiation therapy (IMRT)

IMRT is another treatment technique and also a form of conformal radiation therapy. This means that the intensity of the beams used to treat the tumour can be varied in order to shape the dose very precisely to the tumour. IMRT can be delivered in a number of different ways on the linear accelerator.

• For some IMRT delivery, the linear accelerator moves from one angle to another and delivers its dose while it is stopped or static.
• In other delivery methods, the linear accelerator can be constantly moving or rotating around the patient while it delivers treatment. This is known as volumetric modulated arc therapy or VMAT.

^{Volumetric modulated arc therapy (VMAT) plan of a prostate}

Stereotactic radiation therapy

Stereotactic radiation therapy can be used to treat brain, early stage lung, pancreatic, and prostate tumours, among others. It conforms extremely precisely to the tumour and requires millimetre accuracy. Therefore patients will be positioned and immobilised using specialist devices as well as using advanced imaging methods while lying on the treatment couch.

• Patients typically receive fewer treatments with higher doses of radiation delivered at each treatment (known as hypofractionated radiation therapy).
• From evidence-based practice, we know that some tumours respond well to larger doses of radiation therapy at each treatment session, such as those listed above (Shibamoto et al, 2016).
• When it is used to treat tumours that are not in the brain, it can also be referred to as stereotactic body radiation therapy (SBRT) or stereotactic ablative radiation therapy (SABR).

^{Stereotactic radiation therapy. Copyright: Elekta AB, Stockholm, Sweden}

Brachytherapy

Brachytherapy is another common method of delivering radiation therapy. However, this therapy is delivered internally in the body. Very small sources are implanted directly into or very close to the tumour. These sources deliver gamma rays, which have the same effect on the tumour as x-rays delivered in external beam radiation therapy. Brachytherapy can be delivered by high dose rate (HDR) or by low dose rate (LDR).

LDR brachytherapy is most often used in the treatment of prostate cancer using ‘seeds’. These seeds are implanted in the prostate gland and treat the prostate cancer over time. Men who have this treatment can continue with their typical activities within a few days of seed implantation.

^{Prostate seeds (Prostate brachytherapy). Copyright: Elekta AB, Stockholm, Sweden}

Gynaecological cancers are typically treated with HDR brachytherapy. Here the sources are removed from the patient after each treatment and the dose delivered at each treatment is higher than what is given for LDR brachytherapy. Most gynaecological cancer patients have a combination of external beam radiation therapy and HDR brachytherapy.

^{HDR Brachytherapy. Copyright: Varian Medical Systems, Inc.}