Estimation of daily interfractional larynx residual setup error after isocentric alignment for head and neck radiotherapy: quality assurance implications for target volume and organs‐at‐risk margination using daily CT on‐rails imaging

Abstract

Larynx may alternatively serve as a target or organ-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 6 points were calculated post-isocentric alignment. Subsequently, using the first scan as a reference, the magnitude of vector differences for all 6 points for all scans over the course of treatment were 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 5.07mm, with mean systematic error of 1.1mm and mean random setup error of 2.63mm, while bootstrapped POIs grand mean displacement was 5.09mm, with mean systematic error of 1.23mm and mean random setup error of 2.61mm. Required margin for CTV-PTV expansion was 4.6mm for all cohort points, while the bootstrap estimator of the equivalent margin was 4.9mm. The calculated OAR-to-PRV expansion for the observed residual set-up error was 2.7mm, and bootstrap estimated expansion of 2.9mm. We conclude that the interfractional larynx setup error is a significant source of RT set-up/delivery error in HNC both when the larynx is considered as a CTV or OAR. We estimate the need for a uniform expansion of 5mm to compensate for set up error if the larynx is a target or 3mm if the larynx is an OAR when using a non-laryngeal bony isocenter.

Outcomes of carotid‐sparing IMRT for T1 glottic cancer: Comparison with conventional radiation

“…We previously reported that small volume IMRT for T1 glottic cancer requires rigorous patient evaluation, radiation planning, delivery, monitoring, and quality assurance . This includes laryngoscopic examination, multidisciplinary surgical/radiotherapist evaluation of tumor extent, high‐resolution angled axial computed tomography examination for evaluation of occult extension, careful target delineation, evaluation of glottic motion, rigorous clinical and physics quality assurance, and in‐treatment image guidance …”

Dose-volume correlates of mandibular osteoradionecrosis in Oropharynx cancer patients receiving intensity-modulated radiotherapy: Results from a case-matched comparison

“…In a programmatic effort to define normal tissue[18, 33–35] and tumor response [36] profiles that include gradient-dose representation, future efforts are focused on determining not only DVH-based (2D) volumetric representations of dose, but include developing efforts at 3D-dose deposition differentials across mandibular sub-volumes using validated image-registration platforms[37]. Likewise, while this dataset, in contrast to previously presented salivary gland patient series[38], focused exclusively on oropharyngeal cancer patients in order to isolate a comparatively uniform treatment paradigm (e.g., avoiding cases with therapeutic oral cavity surgery as a confounder); future efforts will be directed at construction of comprehensive dose response models, including more elaborate classifiers of potential risk strata.…”

Carotid sparing intensity modulated radiotherapy on early glottic cancer: preliminary study

“…Otherwise, in the aspect of interfraction set-up error, we used 3-mm PTV expansions margins with weekly portal films. Some studies reported that 5-mm margin is needed to compensate the setup error if the larynx is a target, and usually verified the larynx motion by daily image such as portal films or in room CT [ 14 18 22 ]. However, by analyzing the log files of the couch movement after the acquisition of weekly portal films, we observed that weekly couch variation was within 3 mm (mean, 2.3 ± 0.8).…”