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Lesson: Tumor Treatment Volumes IM, ITV ICRU 62 recommends that an additional margin around the CTV is needed to account for internal organ motion. This expanded margin is not necessarily uniform around the CTV in that it may be due to different factors, such as movement due to respiration, heartbeat, or swallowing, or to daily volume differences in the bladder or rectum, etc. This expansion is termed the Internal Margin (IM).
In this image, the IM expansion (dotted line) is added to the CTV, creating a new volume called the Internal Target Volume (ITV). That is CTV + IM = ITV.
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