The Australian MRI-linac program is a 16 million dollar project at Liverpool hospital to build a Magnetic Resonance Imaging (MRI) device combined with a Linear Accelerator, which is used to treat cancer using radiotherapy. The goal of the project is to develop next generation cancer therapy equipment, in which changing patient anatomy and physiology can be visualized during treatment. This will dramatically improve the accuracy of radiotherapy, directly improving outcomes for cancer patients. This project is one of only four similar projects in the world, and the only one existing in the southern hemisphere. A number of groups are collaborating on this project, both nationally and internationally. The Australian MRI-linac program is led by Professor Paul Keall.
Cancer is one of the leading causes of death, both in Australia and worldwide. Radiotherapy is one of the primary modalities used to treat cancer, with 40% of patients receiving treatment in Australia (Baume, 2002). Radiotherapy involves identifying cancerous tissue and irradiating it with high energy particle beams in such a way that maximum sparing of the healthy tissue surrounding the tumour is achieved.
One of the main problems in conventional radiotherapy is that tumours and their surrounding organs move during and between treatments. This can result in the tumour receiving less radiation than was intended, or the healthy tissue receiving more. This in turn can decrease the efficacy of radiotherapy, whilst increasing the detrimental side effects .
In order to rectify this effect, image guidance in radiotherapy is becoming increasingly common. Image guidance refers to imaging the patient between or during treatments in order to obtain information about any anatomical changes. There has been a steadily increasing uptake of image guidance techniques in the clinic (Simpson et al, cancer 2010), demonstrating the efficacy of such techniques. However, current techniques suffer from a number of shortcomings, such as:
- Poor soft tissue contrast – this can make it difficult to visualise the tumour from its surroundings
- Secondary imaging dose – Imaging dose can result in non negligible increase in secondary cancer risk (Smith-Bindman 2009). In turn, this limits how often some techniques can be utilised.
- Reliance on fiducial markers – Some techniques require the surgical insertion of a number of small metallic markers
- Assumption of correlation between surface anatomy and internal anatomy – Some techniques use information about movement of the patient surface to infer information about the internal anatomy. This assumes a correlation which may or may not exist.
An MRI/linear accelerator has a number of benefits compared to existing image guidance technology.
- Excellent soft tissue contrast
- No imaging dose
- 3D Volumetric data
- Potential for physiological imaging
- No fiducial markers
However, combining an MRI with a Linear accelerator is a non trivial engineering task. MRI utilises strong magnetic fields to form images; these fields will influence many aspects of normal Linac operation. In turn, the presence of the linear accelerator interferes with the MRI image acquisition process.
Through the Australian MRI-Linac Program, we plan to overcome these challenges and develop a new cancer radiotherapy system that can image tumours as they move within the body, and target the radiation beam to follow this motion. Ultimately, we expect that the MRI-Linac will improve tumour control and cancer survival, whilst reducing the side effects from radiation therapy, leading to an improved quality of life for cancer patients.