Variation in Interinstitutional Plan Quality When Adopting a Hypofractionated Protocol for Prostate Cancer External Beam Radiation Therapy.

Purpose: This study quantified plan quality differences across the 4 cancer centers in Alberta, Canada for plans that followed the PROstate Fractionated Irradiation Trial protocol.

Methods And Materials: Prostate plans of 235 patients were retrospectively reviewed. Interinstitutional plan quality comparisons were made based on distributions of protocol-specified parameters using 1-way analysis of variance with Games-Howell post hoc analysis. Dosimetrically representative cases were selected from each center using k-medoid clustering, enabling side-by-side comparison of dose-volume histograms and dose distributions. Fourteen anatomic features were investigated to explore interinstitutional patient population differences. Anatomically representative cases were selected from each center to explore differences in planning practices. Tumor control probability (TCP), as well as rectal wall and bladder wall normal tissue complication probabilities (NTCPs), were calculated to quantify the clinical effect of the differences in plan quality.

Results: Comparing the mean value of each center to the other 3, statistically significant differences were observed for bladder wall D30% and D50%, left and right femoral heads D5%, planning target volume D99% and D1cc, and clinical target volume D99%. Dosimetrically representative cases demonstrated consistent results. Although anatomic differences were observed between the center-specific populations, an analysis using anatomically similar cases demonstrated consistent trends in the dosimetric differences, suggesting the dosimetric variation is not exclusively due to anatomic differences. Minimal differences (<1%) among the 4 centers were noted for TCP and NTCPs, suggesting the reported differences in plan quality may not have any clinical significance.

Conclusions: Despite common guidelines, statistically significant differences in plan quality metrics occurred among the 4 investigated centers. The differences are due at least in part to variation in local planning practices. TCP and NTCP calculations suggest that the clinical significance of the differences is minimal. These results can serve as a reference for the degree of variation among centers that can be accepted when a common protocol is adopted.