Quantitative comparison of delineated structure shape in radiotherapy

There has been an influx of imaging and treatment technologies into cancer radiotherapy over the past fifteen years. The result is that radiation fields can now be accurately shaped to target disease delineated on pre-treatment planning scans whilst sparing critical healthy structures. Two well known problems remain causes for concern. The first is inter- and intra-observer variability in planning scan delineations, the second is the motion and deformation of a tumour and interacting adjacent organs during the course of radiotherapy which compromise the planned targeting regime. To be able to properly address these problems, and hence accurately shape the margins of error used to account for them, an intuitive and quantitative system of describing this variability must be used. This paper discusses a method of automatically creating correspondence points over similar non-polar delineation volumes, via spherical parameterisation, so that their shape variability can be analysed as a set of independent one dimensional statistical problems. The importance of 'pole' selection to initial parameterisation and hence ease of optimisation is highlighted, the use of sparse anatomical landmarks rather than spherical harmonic expansion for establishing point correspondence discussed, and point variability mapping introduced. A case study is presented to illustrate the method. A group of observers were asked to delineate a rectum on a series of time-of-treatment Cone Beam CT scans over a patient's fractionation schedule. The overall observer variability was calculated using the above method and the significance of the organ motion over time evaluated.