Prostate delineation using CT and MRI for radiotherapy patients with bilateral hip prostheses.

Background And Purpose: Prostate cancer patients with bilateral hip prostheses (b-HP) are rare, but pose unique challenges for high-dose radiotherapy. The purpose of this study was to determine the effect of b-HP on prostate delineation using CT and MR.

Methods And Materials: The planning CT and MR of 7 patients with b-HP were co-registered. Four observers delineated the prostate on both data sets, blinded to the volumes delineated by their peers and those on the alternate image set. Inter-modality differences and inter-observer variability were quantified by calculating 3D volume overlaps. The effect of prostheses physical extent on these variables was evaluated. MR geometric distortion was explored using the differences in vector distances between three intra-prostatic markers (IPMs) on CT and MR.

Results: Prostate volumes on CT were consistently larger than MR volumes in all patients when averaged over the 4 observers (+10% to +46%, p<0.001). The mean inter-modality volume overlap was 1.59, which varied significantly between patients (1.35 to 1.82, p=0.04). There was a trend toward smaller inter-observer variability in the MR volumes (1.95 vs. 1.71, p=0.08). No differences could be identified between the IPM vectors on CT and MR.

Conclusions: Although limited by a small sample size, this study found that MR prostate volumes were significantly smaller than those from CT for b-HP patients. The largest differences between CT and MR prostate volumes and the greatest inter-observer variability on CT tended to be seen in patients with the largest prostheses. b-HP do not appear to cause measurable MR geometric distortions in the sequences used.