This study quantified plan quality differences across the four cancer centers in XXX(blinded for review) for plans that followed the PROstate Fractionated Irradiation Trial (PROFIT) protocol.
235 prostate plans were retrospectively reviewed. Interinstitutional plan quality comparisons were made based on distributions of protocol-specified parameters using one-way ANOVA with Games-Howell post-hoc analysis. Dosimetrically representative cases were selected from each center using k-medoid clustering, enabling side-by-side comparison of DVHs and dose distributions. 14 anatomical features were investigated to explore inter-institutional 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 impact of the differences in plan quality.
Comparing the mean value of each center to the other three, statistically significant differences were observed for: bladder wall D30% and D50%, left and right femoral heads D5%, PTV D99% and D1cc, and CTV D99%. Dosimetrically representative cases demonstrated consistent results. Although anatomical 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 anatomical differences. Minimal differences (<1%) were noticed for TCP and NTCPs between the four centers, suggesting the reported differences in plan quality may not have any clinical significance.
Despite common guidelines, statistically significant differences in plan quality metrics occurred between the four 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 between centers that can be accepted when a common protocol is adopted.

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