Inter-observer variation in delineation of bladder and rectum contours for brachytherapy of cervical cancer

In 3D treatment planning of low dose rate brachytherapy of cervical carcinoma the dose in bladder and rectum can be estimated from dose-volume histograms (DVHs). In this study, the influence of inter-observer variation in delineation of bladder and rectum on DVHs and dose at specific bladder and rectum points was investigated.

Three observers delineated bladder and rectum on axial CT images of ten patients. The highest minimum dose in bladder and rectum was determined for, respectively, 2 cm(3) (D(2)) and 5 cm(3) (D(5)), as well as the dose at specific points placed on the bladder and rectum wall.

The inter-observer variation in D(2) was 10% (1 average relative SD) in bladder and 11% (1 SD) in rectum. In D(5) the variation was 8% (1 SD) in bladder and 11% in rectum. The variation in the bladder point was 13% (1 SD) and in the rectum point 11% (1 SD). Differences in delineation among the observers were caused by unclear organ boundaries on the CT images.

Taking the inter-observer variation caused by delineation differences into account, dose in bladder and rectum can be determined within an accuracy of about 10% (1 SD).