Purpose: To evaluate interobserver variability in clinical target volume (CTV) delineation in gastric cancer performed with the help of a delineation guide.
Patients and methods: Ten radiotherapy centers that participate in the CRITICS Phase III trial were provided with a delineation atlas, preoperative CT scans, a postoperative planning CT scan, and clinical information for a gastric cancer case and were asked to construct a CTV and create a dosimetric plan according to departmental policy.
Results: The volumes of the CTVs and planning target volumes (PTVs) differed greatly, with a mean (SD) CTV volume of 392 (176) cm(3) (range, 240-821 cm(3)) and PTV volume of 915 (312) cm(3) (range, 634-1677 cm(3)). The overlapping volume was 376 cm(3) for the CTV and 890 cm(3) for the PTV. The greatest differences in the CTV were seen at the cranial and caudal parts. After planning, dose coverage of the overlapping PTV volume showed less variability than the CTV.
Conclusion: In this series of 10 plans, variability of the CTV in postoperative chemoradiotherapy for gastric cancer is large. Strict and clear delineation guidelines should be provided, especially in Phase III multicenter studies. Adaptations of these guidelines should be evaluated in clinical studies.
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