Calypso radiotherapy device could help reduce damage to surrounding tissue

A technique used to map breathing patterns could track movement of organs during radiotherapy, improving cancer treatment.

Radiotherapy is a clinical technique that uses high-energy X-ray radiation to kill cancer cells and hence tumours.

Inevitably, however, during the treatment process, some X-rays are absorbed by healthy tissue as well. For this reason, many researchers are working to improve the performance of radiotherapy systems to maximise the dose delivered to tumours while minimising the dose to structures surrounding them.

To spare normal tissue from unnecessary exposure to radiation, modern radiotherapy systems use shaped radiation beams aimed at the body from several angles that intersect at the tumour, providing a larger absorbed dose there than in the surrounding healthy tissue.

But according to Prof Phil Evans, team leader of the Radiotherapy Physics Research Team at the Institute of Cancer Research (ICR), while these external beam radiotherapy (EBRT) techniques are used to more precisely deliver radiation, they do not adequately account for motion of the organs during radiation therapy.

Prof Evans said that many companies are now designing systems to address that issue. One such system, developed by Seattle-based Calypso Medical, can track the motion of an organ while X-rays are being delivered to it, sparing the surrounding tissues from exposure.

This system, which has already been used in the treatment of prostate cancer, comprises an electromagnetic array that excites three passive electromagnetic transponders implanted within the prostate prior to treatment. When activated, the transponders transmit a radio-frequency signal to the electromagnetic array that generates position and motion information about the target.

Using that data, the system is able to track the tumour’s location during treatment, enabling the delivery of radiation to be more accurately targeted and even stopped each time the target tissue moves outside a preset threshold.

Complementing the Calypso approach, Dr Anthony Kavanagh, a physicist in the Radiotherapy Physics Research Team, has developed an algorithm to recreate a moving image of the lungs from a series of sequential images taken by a scanner prior to radiotherapy treatment, which also provides information on how the soft tissue was moving.

’The scan takes a few minutes, in which the patient will take several breaths. Then, by analysing the variation in pixel values between several images, the patient’s breathing signal is determined. This is then used to reconstruct a four-dimensional moving image of the organs scanned,’ said Dr Kavanagh.

Now, the researchers at the ICR are examining ways they might incorporate this breathing-pattern model into an X-ray treatment system.

The researchers built a phantom: a model of a human with properties similar to that of human tissue. This was placed on a bespoke platform that could be moved in three orthogonal directions, simulating the breathing motions of a patient.

While mimicking several breathing profiles, the researchers measured the delivered radiation to specific targets in the phantom and then correlated this data with data expected from a computational model of the breathing patient. The result was then used to target specific doses of radiation to areas of the moving phantom with a radiotherapy system. The test results proved the technique would improve the accuracy of the targeted X-ray dose.

The effect of deploying such software would complement the existing Calypso approach by allowing extra information on the tumour’s shape and size to be taken into account during the radiotherapy treatment.

’This could lead to substantially improving the five-year survival rate of those patients with lung cancer,’ added Prof Evans.