Michael Douglass - 2023 ACPSEM Ion Beam Fellow

Lessons from Switzerland

Michael Douglass — Principal Medical Physicist for the Central Adelaide Local Health Network — was awarded the 2023 ACPSEM Ion Beam Fellowship to travel to Switzerland for three weeks and learn about proton therapy. From patient care to patient follow-up, quality assurance and treatment planning, Michael spent time at the Paul Scherrer Institut (PSI) to inform the development of Australian proton therapy delivery as the opening of the Australian Bragg Centre for Proton Therapy and Research in Adelaide where Michael is a research fellow, approached. The Bragg Centre will be the first dedicated Proton Therapy Centre in the Southern Hemisphere.

Michael explained the science behind proton therapy and its significant contribution to the treatment of cancer.

In principle, the major benefit of proton therapy comes from the different way protons interact in a patient compared with x-rays. When x-rays enter a patient, they deposit most of their dose close to the surface of the patient and then exponentially decrease with depth inside the patient. In practice, for this to be usable for patient treatments, you need to deliver lots of x-ray beams from lots of different angles around the patient so that the maximum dose is in the tumour and not in the normal tissue.

Protons are very exciting in the radiation oncology community because they deliver their energy in a much more favourable way. They deliver very little energy when they first enter the patient and then gradually increase to a maximum value (called the “Bragg Peak”) in the tumour. Then the protons just stop. So, there is little dose delivered beyond the tumour. In principle, this means you can spare the healthy tissue behind the tumour compared with protons where they just keep depositing dose. This is particularly useful for cranial sites, for example, where the tumour is abutting the brainstem, and you don’t want to deliver any dose to the brainstem.

In practice, however, it’s a little bit more complicated.

A single sharp “Bragg peak” of dose (the green graph below) is not useful for uniformly irradiating a large tumour. You need to deliver multiple proton beams of different energy to get a nice uniform dose of radiation over the tumour (the blue curve below). Protons also have some extra uncertainties you need to consider when coming up with a clinical proton plan for a patient, so it ends up being a lot more complicated than is shown in the graph below.

The upshot is that protons are very good at reducing the total, integral dose delivered to a patient compared with x-rays.

This is like the total radiation energy delivered to a patient. As a result, protons may result in fewer radiation induced cancers in patients after their primary tumour has been treated.

The most important thing that I came to appreciate during my time in Switzerland was that our conventional x-ray treatments are exceedingly good at delivering a very conformal high dose of radiation to the patient. It is often hard to beat these conventional treatments with protons due to this fact. Where protons really benefit patients is the intermediate to low dose sparing. This often means fewer radiation induced toxicities to healthy organs around the tumour. This is why proton therapy is widely accepted to be superior to conventional x-ray treatments around the world for sites such as paediatric brain cancers or other adult cranial tumours where there are a lot of sensitive organs around the tumour which are important to spare.

Michael's expertise and dedication have also been recognised through several prestigious awards, including ACPSEM’s Boyce Worthley Early Career Award for his contributions to proton therapy and medical physics in 2021 and ACPSEM’s Kenneth Clarke Journal Award in 2021 for a paper on cost-effectiveness modelling in proton therapy.

The Australasian College of Physical Scientists and Engineers in Medicine
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