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 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.

Figure 1

Visual depiction of the selected points of interest (POIs) with the red circle at the most superior aspect of the anterior C5 vertebra (3D reconstructed contour in green) representing the fixed isocenteric reference and the violet circles represent selected POIs of the thyroid cartilage on Day 1 (3D reconstructed contour in blue), while the pink circles represent the same POIs on Day 2 (not all POIs are visible because of the overlap) but in different spatial location caused by laryngeal inter‐fraction motion. White arrows show example of vector displacements of two POIs relative to their original position in relation to the fixed isocenter of Day 1 (solid red arrow) in Day two (dashed red arrow).

Figure 2

Shadowgram showing the difference in distribution probability of points of interest vector displacement over treatment time between the studied cohort and its bootstrap resampling.

Figure 3

Distributional boxplot of geometric vector displacement of cohort POIs and its bootstrap validation. Pale line within the box indicates median value, while the box limits indicat the 25th and 75th percentiles. The lines represent the 10th and 90th percentiles, and the horizontal dotted lines represent the 95% TI.

Figure Fig. A1.

Three‐dimensional scatterplot illustrating the difference between the systemic errors of representative points of interest distribution from a starting point (black circle) presented as a red cloud and the random errors represented as a light blue cloud. The appropriate CTV‐PTV and OAR‐PRV margins accounts for both the systematic and random error component.