Measuring the similarity of target volume delineations independent of the number of observers

The variability of target delineations is a topic of interest in radiotherapy. The similarity of delineations is often quantified by use of a conformity index (CI) defined as the ratio of common to encompassing volume. Several forms of CI are in use, but no consensus exists on how to calculate the CI for more than two delineations. This study proposes a generalization of the CI applicable to any number of delineations. The generalization of the CI was developed, unbiased with respect to the number of delineations. Numerical values were calculated for clinical and theoretical cases, and differences with other forms of CI were considered. A simple expression could be derived, applicable to any number of delineations, and is equivalent to the known CI for two delineations. The use of this index is advised, although another frequently used index obtained from averaging the CI between all possible pairs of delineations results in minor differences. The use of the third generalization for the CI which is based upon the volume common to all delineations shows a clear dependence upon the number of delineations and is discouraged.