Target Definition in Prostate, Head, and Neck
Section snippets
Geometrical Error Sources
Several geometrical uncertainties are involved in the delineation process. First, the imaging modalities have a limited resolution, in particular in the direction perpendicular to the slice planes,6, 7 causing the partial volume effect.8 Second, there is a certain amount of observer “noise” (ie, when the same observer is asked to delineate the target volume twice, the answer will not be the same [intraobserver variation]).8, 9, 10 More important are interpretation differences between different
Prostate Cancer
The definition of the target area in prostate cancer conformal radiotherapy has been fairly straightforward. Apart from inclusion or exclusion of the draining lymph nodes in the radiation fields, either the prostate or the prostate including the seminal vesicles is outlined. We will focus on the delineation of the prostate with or without seminal vesicles. Several attempts have been made to decrease the variability of prostate delineation, mostly aiming at the localization of the prostate apex.
Image Coregistration
The use of image registration to combine multiple-image modalities for radiotherapy treatment planning is currently applied in many clinics.17, 82, 83 Frequently, information from multiple modalities or multiple scans of the same modality is important for the target volume definition of a given case.84 Numerous image registration methods are available (eg,83, 84, 85 each with characteristic strengths and weaknesses). Less attention has been given to the tools used to present these data to the
Conclusion
It is necessary to translate reductions in geometrical uncertainties into a possible reduction of target margins. Various methods have been proposed to define the impact of geometrical deviations on the target margin. For example, Bel and coworkers showed,26, 30 through simulation, that a margin for random variations of 0.7 times the SD of the random variations and 2.5 times the systematic variation is adequate to keep a 95% dose coverage. Aaltonen and coworkers97 described an analysis based on
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2023, Radiotherapy and OncologyCitation Excerpt :However, the number of anatomic structures contoured and independent contouring physicians provided sufficient power to meet the non-inferiority threshold for ASSD analysis. Additionally, physician-based contouring is inherently subjective, with known errors and variability that persist despite introduction of advanced imaging into target delineation [39]. Therefore, while the lack of variability observed here in contouring on specific CT image types is encouraging, it may not necessarily reflect consistent contouring accuracy.
Supported in part by the Netherlands Cancer Society, grant NKI 2000-2247.