Inter and intraobserver variation in gross tumor delineation on megavoltage CT images in patients undergoing tomotherapy-based image-guided radiotherapy for postoperative vault recurrences.

Purpose: This study was designed to evaluate inter and intraobserver concordance in gross tumor volume (GTV) delineation on megavoltage CT (MVCT) images in patients with postoperative vault recurrences. Materials and Methods: Three observers delineated GTV on MVCT and CECT and two observers recontoured on MVCT images. Tumor volumes were calculated and correlated using Spearman correlation. The standard deviation of centre of mass was averaged on per patient basis. The ratio of common volume and encompassing volume was used to determine intra and interobserver spatial concordance. Lack of difference in spatial concordance ratio between MVCT and CECT images was used as an index of usability of MVCT images. Results: Thirty six datasets were available for seven patients. High intraobserver GTV correlation was recorded for observer 1 and 2 (r = 0.93 and r = 0.98; P=0.03 and 0.0001). The average intraobserver spatial concordance ratio was 0.57 and 0.62 respectively. The mean GTV of observers 1, 2 and 3 were 31.6 (18.7-52.2); 28.2 (16.7-51.8) and 46.3 cc (29.1-90.5) respectively. Average standard deviation of centre of mass of all observers was less than 5 mm in either direction. Largest interobserver discordance was observed in anterior, inferior and lateral direction. The interobserver spatial concordance of GTV on MVCT and CECT images was 0.34 and 0.36 (P=0.24) respectively. Conclusion: Moderate to good inter and intraobserver GTV correlation was observed on MVCT images, however, was associated with low interobserver spatial concordance on both MVCT and CECT images. Strategies to improve contouring reproducibility on MVCT and KVCT images are desirable.