S. van Beek

Validation of deformable registration in head and neck cancer using analysis of variance.

PURPOSE Deformable image registration (DIR) is often validated based on a distance-to-agreement (DTA) criterion of automatically propagated anatomical landmarks that were manually identified. Due to human observer variability, however, the performance of the registration method is diluted. The purpose of this study was to evaluate an analysis of variance (ANOVA) based validation to account for such observer variation. METHODS Weekly cone beam CTs (CBCTs) of ten head and neck cancer patients undergoing five weeks of radiotherapy were used. An expert identified 23 anatomical features (landmarks) on the planning CT. The landmarks were automatically propagated to the CBCT using multiregion-of-interest (mROI) registration. Additionally, two human observers independently localized these landmarks on the CBCTs. Subsequently, ANOVA was used to compute the variance of each observer on the pairwise distance (PWD). RESULTS ANOVA based analysis demonstrated that a classical DTA approach underestimated the precision for the mROI due to human observer variation by about 25%. The systematic error (accuracy) of mROI ranged from 0.13 to 0.17 mm; the variability (1 SD) (precision) ranged from 1.3 to 1.5 mm demonstrating that its performance is dominated by the precision. CONCLUSIONS The PWD-ANOVA method accounts for human observer variation allowing a better estimation of the of DIR errors.

OC-0470: Library of plans in radiotherapy of rectal cancer: feasible and inter-observer consistent?

S223 ______________________________________________________________________________________________________ nearest whole number. The median DVH value of new genitalia contours denotes the optimal constraint and the 75th centile denotes the mandatory constraint. Horizontal lines represent current genitalia dose constraints. It can be observed that new recommended dose constraints contrast the current dose constraints highlighting the need for gender and tumour stage specific genitalia dose constraints.