And yet, it moves!
8 March 2019
Galileo is said to have muttered ‘E pur, si muove!’ – ‘And yet, it moves’ as he left the courtroom in which he had been forced to concede that the sun moved round a fixed earth. Today, these words could be used to under-pin the value of MR-Linacs in radiotherapy.
First, let us define the problem. The goal of radiation therapy is to destroy tumour cells, but we cannot target tumour cells alone. Radiotherapy therefore damage normal cells as well as cancer cells, which causes side effects, which reduce quality of life for our patients. We go to extraordinary lengths to minimise the amount of normal tissue that is irradiated, and we are pretty good at it.
Modern radiotherapy concentrates radiation coming from different angles to shape the dose to match the shape of the tumour. We do this by getting as close as we can to solving insoluble multi-parametric equations. The results of solving these equations can be coupled to large machines, which generate beams containing protons or other heavy charged particles, which make these solutions more forgiving.
All very tidy in theory, but this is real life. We have to deliver these beams, be it X-rays, protons, Helium nuclei, Carbon ions, or Higgs bosons (I wish) to a living breathing patient. Real patients are soft. They move (voluntarily and involuntarily), they breathe, their hearts beat and their digestive system works away (Don’t ask!). So we solve our equations and deliver our beam: E pur si muove – And yet, it moves!
Since our target moves, we create a double jeopardy. The intended target will always receive less dose (compromising the curability) and whatever we did not aim for gets a higher than intended dose (increasing treatment complications). We respond to this by choosing the lesser of two evils. We treat a volume larger than intended and manage any increase in complications, the larger the volume treated the worse and more likely the complications. We therefore want to increase the volume but not too much; ideally we do not need to have margins at all.
Enter in-treatment imaging, i.e. devices that allow us to observe the patient externally and internally and make sure that we aim at the correct thing. This allows smaller margins, but there is more to imaging than just making visible the internal anatomy. Great imaging tells us what is going on as well. With great imaging, we can see better, aim better, treat better and monitor biological changes.
Before MR-Linacs, images were taken using X-rays or MegaVoltage beams. X-ray based computed tomography (CT) is used to provide geometrically and physically (electron density) accurate information. But, X-rays are limited because only bony anatomy can be visualised reliably and soft tissue only in some rare cases. So most of the time when treating a patient we can't easily see what we are treating. The image of hitting a pinata blindfolded comes to mind, but we are slightly better than that, and making the pinata bigger (i.e. using margins) also helps. In addition, the information obtained is not real-time, which in a constantly moving patient provides you with outdated information as soon as you see it.
Recently, some clever people have managed to combine a radiotherapy treatment device (a Linac) with a Magnetic Resonance imaging device (MR). This is more difficult than it seems as the strong magnetic fields affect the workings of a Linac and the Linac uses Radio Frequency technology (RF) which in turn messes up the imaging device.
MRI is extremely good at visualising soft tissue. It is really good at generating the gold standard images which the treating physician uses to tell his team: Shoot here! This is great, but MR attached to the treating Linac also provides a way to see our target move in real time. Being able to follow our target as it moves allows us to start and stop the treatment in time with the movement of the target.
As you can tell, I am really excited by this new toy! It has real scientific value. However, this is not just a research toy for excited physicists. We will be working with Genesis Care and the Oxford University Hospitals to use this new technology to treat patients. By working with real patients, we will be able to prove the technology and show how much benefit it brings in terms of successful treatments with minimal side effects.
We will be using the MR-Linac to explore the benefits: for escalated treatment for Pancreas, in fully guided prostate cancer therapy, with biologically informed treatment planning and adaptation, in the impact of radiation delivered in strong magnetic fields, and many more.