Reducing interobserver variation of boost-CTV delineation in breast conserving radiation therapy using a pre-operative CT and delineation guidelines

Aims: To investigate whether using a pre-operative CT scan (Preop-CT) (1) decreases interobserver variation of boost-CTV delineation in breast conserving therapy (BCT), and (2) influences the size of the delineated volumes. Patients and methods: Thirty cT1-2N0-1 breast cancer patients underwent a CT-scan in radiation treatment position, prior to and after lumpectomy. Five observers delineated a boost-CTV, both with and without access to the Preop-CT. For each patient and for each observer pair, the conformity index (Cl) and the distance between the centres of mass (COMd) for both boost volumes were calculated. In addition, all delineated volumes including the standard deviation (SD) with respect to the median delineation were calculated. Results: Using a Preop-CT reduced the mean COMd of the boost-CTV from 1.1 cm to 1.0 cm (p <0.001). No effect was seen on the Cl. but the boost-CTV volume reduced from 42 cc to 36 cc (p = 0.005), implying a reduction of interobserver variation. We saw no significant change in the SD. Conclusion: Use of a Preop-CT in BCT results in a modest but statistically significant reduction in interobserver variation of the boost-CTV delineations and in a significant reduction in the boost-CTV volume.