Abstract
Larynx may alternatively serve as a target or organs at risk (OAR) in head and neck cancer (HNC) image‐guided radiotherapy (IGRT). The objective of this study was to estimate IGRT parameters required for larynx positional error independent of isocentric alignment and suggest population‐based compensatory margins. Ten HNC patients receiving radiotherapy (RT) with daily CT on‐rails imaging were assessed. Seven landmark points were placed on each daily scan. Taking the most superior‐anterior point of the C5 vertebra as a reference isocenter for each scan, residual displacement vectors to the other six points were calculated postisocentric alignment. Subsequently, using the first scan as a reference, the magnitude of vector differences for all six points for all scans over the course of treatment was calculated. Residual systematic and random error and the necessary compensatory CTV‐to‐PTV and OAR‐to‐PRV margins were calculated, using both observational cohort data and a bootstrap‐resampled population estimator. The grand mean displacements for all anatomical points was 5.07 mm, with mean systematic error of 1.1 mm and mean random setup error of 2.63 mm, while bootstrapped POIs grand mean displacement was 5.09 mm, with mean systematic error of 1.23 mm and mean random setup error of 2.61 mm. Required margin for CTV‐PTV expansion was 4.6 mm for all cohort points, while the bootstrap estimator of the equivalent margin was 4.9 mm. The calculated OAR‐to‐PRV expansion for the observed residual setup error was 2.7 mm and bootstrap estimated expansion of 2.9 mm. We conclude that the interfractional larynx setup error is a significant source of RT setup/delivery error in HNC, both when the larynx is considered as a CTV or OAR. We estimate the need for a uniform expansion of 5 mm to compensate for setup error if the larynx is a target, or 3 mm if the larynx is an OAR, when using a nonlaryngeal bony isocenter.PACS numbers: 87.55.D‐, 87.55.Qr
Highlights
While intensity-modulated radiotherapy (IMRT) has led to the ability to deliver highly conformal radiotherapy (RT) doses, a major limitation in sparing normal tissues while delivering tumoricidal doses to target volumes, after target delineation, is setup error.[1,2] Conceptually, in ICRU62(3) and ICRU 83,(4) the planning target volume (PTV) and planning organs-at-risk volume (PRV) account for this setup error and ensure that precision target delineation does not result in either a geometric miss nor inadvertent normal tissue overdose.[5,6] a significant limitation of most current image-guided radiation therapy (IGRT) systems is their reliance on a single point reference for corrective setup translations
Our group and others have adopted strategies to minimize laryngeal doses for nonlaryngeal head and neck cancers when a low neck match cannot be utilized practically.[16]. Such approaches are beneficial for organs like larynx where a defined planning organ at risk volume (PRV) margin might be of possible value for plan optimization as it is well documented that laryngeal overdose results in quantifiable toxicity.[17]
The amount of error in daily setup in an immobilized patient has been studied previously[33,34,35] and CTV-PTV corrections necessary have been suggested previously using a variety of image-guided radiotherapy (IGRT) devices.[7,36] Interestingly, while studies have been performed to determine the effect that setup error and the movement of a patient as a whole can have on RT accuracy in head and neck cancer (HNC), there has been a lack of analysis of the effects of independent TV/organs at risk (OAR)/ROI motion, this data is beginning to emerge.[9,21,26,27,29]
Summary
While intensity-modulated radiotherapy (IMRT) has led to the ability to deliver highly conformal radiotherapy (RT) doses, a major limitation in sparing normal tissues while delivering tumoricidal doses to target volumes, after target delineation, is setup error.[1,2] Conceptually, in ICRU62(3) and ICRU 83,(4) the planning target volume (PTV) and planning organs-at-risk volume (PRV) account for this setup error and ensure that precision target delineation does not result in either a geometric miss nor inadvertent normal tissue overdose.[5,6] a significant limitation of most current image-guided radiation therapy (IGRT) systems is their reliance on a single point reference for corrective setup translations. It is imperative that the use of IMRT does not result in inadvertent geometric miss which may in aggregate reduce survival probability For this reason, we sought to ascertain the relative geometric variation in the motion of the larynx relative to a single isocenter (defined as a bony landmark) in order to ensure that our current radiotherapy margins are within evidence-based limits. The specific aims of the current study are: 1) estimation of the relative interfraction setup error of the laryngeal apparatus relative to a fixed isocenter, using both experimentally observed CT on-rails data and robust estimators of population setup error using a bootstrap methodology; 2) determination of the PTV expansions required for laryngeal-targeting radiotherapy for larynx cancers; and
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