BEGIN:VCALENDAR VERSION:2.0 PRODID:-//Talks.cam//talks.cam.ac.uk// X-WR-CALNAME:Talks.cam BEGIN:VEVENT SUMMARY:"\;Image guidance – benefits and risks"\; - Professor Ma rcel van Herk\, Imaging Physicist\, Project Leader\, The Netherlands Cance r Institute DTSTART:20130625T110000Z DTEND:20130625T120000Z UID:TALK45626@talks.cam.ac.uk CONTACT:Mala Jayasundera DESCRIPTION:In the past 25 years\, the technology of external beam radioth erapy has improved tremendously. Due to the development of advanced diagno stic imaging\, tumor localization has greatly improved. The recognition of the presence of internal organ motion has lead to development of image gu idance 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 brai n\, for instance\, localization of the skull with 3D image guidance is acc urate to well within one mm. This unprecedented precision\, however\, lead to the risk of overconfidence in the accuracy of the total treatment chai n. While for a brain metastasis tumor delineation based on MRI is highly a ccurate\; target volume definition in general\, however\, remains difficul t and is now by far the limiting factor in the accuracy. These limits we m ay have to learn from our clinical mistakes. For instance\, a group in Bru ssels found an almost 50% recurrence rate after introducing marker-based i mage guidance for low risk prostate cancer with a too small margin. Apart from this clear example of geometrical miss (unfortunately very few are pu blished)\, there is little or no evidence that the precision of radiothera py affects outcome. The only indirect evidence is by retrospective analysi s of clinical trials\, correlating outcome to secondary factors that in re trospect are known to affect precision. \n\nThe main factors that affect t he accuracy of treatment after image guidance has been implemented are: un certainties in target volume definition\, the quality of the surrogate (if any) used to localize the tumor\, intrafraction movement\, and movement t hat 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 i mprove this situation is to make sure that target volume definition is con sistent and according to protocol. Then of course\, pathology studies can help to improve knowledge of GTV and CTV delineation. Unfortunately\, pat hology investigations are generally impossible for the actual patient grou ps undergoing radiotherapy. For this reason\, data mining approaches may h elp to correlate e.g.\, local dose variations with outcome. \n\nA 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 eas ily out rated by the benefits of image guidance.\n\nI conclude that\, in s pite of modern IGRT\, there are still uncertainties that need to be covere d by safety margins. The most important uncertainties relate to imaging an d biology that are not corrected by IGRT. Even though PTV margins are desi gned to cover geometrical uncertainties\, they also cover microscopic dise ase. Reducing margins after introducing IGRT may therefore lead to poorer outcome and should be done with utmost care (especially in higher stage di sease).\n LOCATION:CRI Lecture Theatre\, Cambridge Research Institute END:VEVENT END:VCALENDAR