Inter- and intra-observer variability in contouring of the prostate gland on planning computed tomography and cone beam computed tomography

Purpose. To investigate inter-/intra-observer variability in defining the prostate by use of planning computed tomography (PCT) and cone beam CT (CBCT) with magnetic resonance image (MRI) as guidance prior to the introduction of an adaptive radiotherapy for prostate cancer. Material and methods. We reviewed PCT and firstly acquired CBCT datasets of each ten patients with prostate cancer. Three physicians independently delineated the prostate based on PCT and CBCT with MRI as guidance, allowing determination of inter-physician variability. Two physicians repeated prostate contouring three times in total to investigate intra-physician variability. We compared delineated prostate volumes in terms of the generalized conformity index (CIgen), maximum variation ratio (MVR), and center of mass (COM). Results. There were no significant inter-/intra-observer differences in the estimation of prostate volume on both PCT and CBCT. For both inter-and intra-observer variability in contouring the prostate gland, there were no significant differences in MVR between PCT and CBCT. The CIgen for inter-observer variability was 0.74 by PCT and 0.69 by CBCT. The CIgen for intra-observer variability on PCT and CBCT was 0.84 and 0.81 for observer 2 and 0.76 and 0.73 for observer 3. COM analyses showed that the greatest inter-/intra-observer variability was in the measurement of the prostate apex and base. With respect to CIgen and COM analysis for the inter-observer variability, more precise delineation of the prostate was possible on PCT than CBCT. More precise contouring in terms of both CIgen and COM was demonstrated by observer 2 than observer 3. Conclusions. Despite some ambiguity in apex and base level, there was a good consistency in delineating the gland on CBCT plus MRI-guided modification both among/within observer(s), without any significant difference from the consistency in defining the prostate on PCT. This study provides a framework for future studies of CBCT imaging of the prostate.