In this exclusive video interview, Uwe Oelfke from the German Cancer Research Center in Heidelberg, explains how image-guided radiation therapy (IGRT) can address this challenge. IGRT involves acquiring additional images of the patient in the treatment position, immediately before or during radiation treatment, ensuring that the beam is precisely targeted to the tumour.

Incorrect patient set-up, tumour shrinkage and day-to-day changes in internal anatomy all compromise the accuracy of radotherapy. Oelfke discusses how such inter-fractional motion can be corrected by repositioning the patient prior to treatment using, for example, a cone-beam CT system integrated with the linac to detect deviations from the planned set-up.

Even once a patient is accurately aligned, organ motion can occur during treatment due to respiration or digestion. Oelfke takes a look at the pros and cons of methods such as online X-ray imaging and electromagnetic tracking for monitoring intra-fractional changes, and describes some approaches for real-time compensation of any detected motion.

Ultimately, says Oelfke, the future may lie in biologically guided radiation therapy, in which functional imaging is used to characterize a tissue's biological function or radiosensitivity. Work is still required to understand exactly how the images correlate to dose response, but the technique could one day enable individual dose levels to be prescribed to individual tissues: so-called dose painting by numbers.

• Look out for the final video in this series of reports, produced in association with physicsworld.com, in which we examine advances in small-animal imaging. And if you haven't seen it yet, check out last week's video report: The low down on molecular imaging.