Visualisation of Delineation Structure Variability in Radiotherapy

It is well known that observer variability during delineation and organ motion relative to set-up markers heavily contribute to errors accrued during the radiotherapy treatment planning process. Margins are placed around the delineated tumour volumes to account for this uncertainty, inevitably resulting in healthy tissue exposure with the associated radiotoxicity risks. The variability of such delineations can be analysed via corresponding points established on their surfaces. We establish correspondences via the spherical parameterisation and subsequent canonical alignment of rectal delineations taken from serial time-of-treatment cone beam CT volumetric datasets. The variability is analysed and displayed in two ways: as a colourwash over a mean surface, where each corresponding point is treated as a (Bonferroni corrected) one dimensional statistical problem, and as a Point Distribution Model with appropriate bony anatomical scale markers.