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"Image guidance – benefits and risks"

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If you have a question about this talk, please contact Mala Jayasundera.

In the past 25 years, the technology of external beam radiotherapy has improved tremendously. Due to the development of advanced diagnostic imaging, tumor localization has greatly improved. The recognition of the presence of internal organ motion has lead to development of image guidance systems based on 2D, 3D or 4D imaging systems integrated with the treatment machines. The achievable localization accuracy is so high, that invasive fixation is no longer needed. For treatment of tumor in the brain, for instance, localization of the skull with 3D image guidance is accurate to well within one mm. This unprecedented precision, however, lead to the risk of overconfidence in the accuracy of the total treatment chain. While for a brain metastasis tumor delineation based on MRI is highly accurate; target volume definition in general, however, remains difficult and is now by far the limiting factor in the accuracy. These limits we may have to learn from our clinical mistakes. For instance, a group in Brussels found an almost 50% recurrence rate after introducing marker-based image guidance for low risk prostate cancer with a too small margin. Apart from this clear example of geometrical miss (unfortunately very few are published), there is little or no evidence that the precision of radiotherapy affects outcome. The only indirect evidence is by retrospective analysis of clinical trials, correlating outcome to secondary factors that in retrospect are known to affect precision.

The main factors that affect the accuracy of treatment after image guidance has been implemented are: uncertainties in target volume definition, the quality of the surrogate (if any) used to localize the tumor, intrafraction movement, and movement that is too complex to be corrected by the image guidance solution (e.g., deformations). Based on this overview, one can see that a major step to improve this situation is to make sure that target volume definition is consistent and according to protocol. Then of course, pathology studies can help to improve knowledge of GTV and CTV delineation. Unfortunately, pathology investigations are generally impossible for the actual patient groups undergoing radiotherapy. For this reason, data mining approaches may help to correlate e.g., local dose variations with outcome.

A totally different risk factor in image guided radiotherapy is the dose applied for imaging. I will argue that the risk of imaging dose is very small and easily out rated by the benefits of image guidance.

I conclude that, in spite of modern IGRT , there are still uncertainties that need to be covered by safety margins. The most important uncertainties relate to imaging and biology that are not corrected by IGRT . Even though PTV margins are designed to cover geometrical uncertainties, they also cover microscopic disease. Reducing margins after introducing IGRT may therefore lead to poorer outcome and should be done with utmost care (especially in higher stage disease).

This talk is part of the Cambridge Oncology Seminar Series series.

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