Interobserver variation in clinical target volume and organs at risk segmentation in post-parotidectomy radiotherapy: can segmentation protocols help?

Objective: A study of interobserver variation in the segmentation of the post-operative clinical target volume (CTV) and organs at risk (OARs) for parotid tumours was undertaken. The segmentation exercise was performed as a baseline, and repeated after 3 months using a segmentation protocol to assess whether CTV conformity improved. Methods: Four head and neck oncologists independently segmented CTVs and OARs (contralateral parotid, spinal cord and brain stem) on CT data sets of five patients post parotidectomy. For each CTV or OAR delineation, total volume was calculated. The conformity level (CL) between different clinicians' outlines was measured using a validated outline analysis tool. The data for CTVs were reaanalysed after using the cochlear sparing therapy and conventional radiation segmentation protocol. Results: Significant differences in CTV morphology were observed at baseline, yielding a mean CL of 30% (range 25-39%). The CL improved after using the segmentation protocol with a mean CL of 54% (range 50-65%). For OARs, the mean CL was 60% (range 53-68%) for the contralateral parotid gland, 23% (range 13-27%) for the brain stem and 25% (range 22-31%) for the spinal cord. Conclusions: There was low conformity for CTVs and OARs between different clinicians. The CL for CTVs improved with use of a segmentation protocol, but the CLs remained lower than expected. This study supports the need for clear guidelines for segmentation of target and OARs to compare and interpret the results of head and neck cancer radiation studies.