Radiotherapy is used to treat ~50% of all cancers, by firing cancer-killing radiation at the tumour. However, some tumours, or parts of the tumour, are radiation-proof, and are barely wounded after treatment. To improve the attack strategy, intelligence must be gathered about the state of the tumour after radiation. Then, tactics can be adapted, such as boosting the dose to more aggressive areas or adding drugs to increase the effectiveness of radiotherapy.

Breast tumours may grow or shrink following treatment, but this takes many months, whereas the insides of the tumour may be changing much sooner. New, non-invasive techniques are required to see these biological changes. Teasing out this microscopic information will allow resistance to radiotherapy and/or relapse to be spotted earlier.

One method to collect intel is by imaging the patient in a magnetic resonance imaging (MRI) machine. Tumours are jam-packed with cells, and a technique to capture this information involves molecular water. Water molecules are in constant random motion, moving in and out of cells. If there are many cells, the molecules will hit more barriers, restricting their movement. When there are less cells, the water molecules move more freely and can travel further before hitting a barrier. This information is gathered by a technique called diffusion-weighted (DW-) MRI.

Tumours also have many blood vessels, which branch chaotically throughout the tumour. In normal tissue, all parts will receive the same supply of blood; however, in tumours, different areas will receive more blood than others. Furthermore, tumour blood vessels are very leaky. This information is collected by a technique called dynamic contrast-enhanced (DCE-) MRI, which involves injecting a small contrast molecule which travels through the blood vessels and leaks out into the tissue. This area is then seen to light up quickly in an MRI scan.

Although we can get information about these small structures, MRI cannot physically see them. However, super-resolution ultrasound (SRUS) is a new technique which can zoom in on the blood vessels. Small bubbles of an inert gas are injected into the body and travel through the tumour blood vessels. By locating these bubbles and calculating their speeds and directions, a picture of the blood vessels and how the blood moves can be formed.

The information from DW-MRI, DCE-MRI and SRUS is being combined for the first time in clinic to monitor how breast tumours respond to radiotherapy, and to develop a much better understanding at hitherto unprecedented resolution of how tumour cellularity and the blood vessel system changes over time.

By Megan Morris

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