Magnetic resonance imaging for prostate bed radiotherapy planning: An inter‐ and intra‐observer variability study

We assessed the inter‐ and intra‐observer variability in contouring the prostate bed for radiation therapy planning using MRI compared with computed tomography ( CT ). Methods We selected 15 patients with prior radical prostatectomy. All had CT and MRI simulation for planning purposes. Image fusions were done between CT and MRI . Three radiation oncologists with several years of experience in treating prostate cancer contoured the prostate bed first on CT and then on MRI . Before contouring, each radiation oncologist had to review the R adiation T herapy O ncology G roup guidelines for postoperative external beam radiotherapy. The agreement between volumes was calculated using the D ice similarity coefficient ( DSC ). Analysis was done using the Matlab software. The DSC was compared using non‐parametric statistical tests. Results Contouring on CT alone showed a statistically significant ( P  = 0.001) higher similarity between observers with a mean DSC of 0.76 (standard deviation ± 0.05) compared with contouring on MRI with a mean of 0.66 (standard deviation ± 0.05). Mean intra‐observer variability between CT and MRI was 0.68, 0.75 and 0.78 for the three observers. The clinical target volume was 19–74% larger on CT than on MRI . The intra‐observer difference in clinical target volume between CT and MRI was statistically significant in two observers and non‐significant in the third one ( P  = 0.09). Conclusions We found less inter‐observer variability when contouring on CT than on MRI . R adiation T herapy O ncology G roup contouring guidelines are based on anatomical landmarks readily visible on CT . These landmarks are more inter‐observer dependent on MRI . Therefore, present contouring guidelines might not be applicable to MRI planning.