SU‐E‐T‐697: Correlation Between Day 0 Post‐Implant Dosimetry Parameters and Differences in Pre‐and Post‐Implant Prostate Volume‐How Much Can Post‐Plan Quality Be Improved Through Better Assessment of Volume for Permanent Interstitial I‐125 Prostate Implants?

Abstract

Purpose: To evaluate the relationship between the difference in pre‐and post‐implant prostate volume and post‐implant dosimetric parameters for pre‐planned permanent interstitial prostate implants. Methods: Between January 2007 and December 2011, 964 patients underwent TRUS guided interstitial prostate I‐125 brachytherapy either as monotherapy (MPD 144Gy) or in combination with external irradiation (MPD 108Gy). All procedures and contouring for pre‐and post‐planning was performed by a single physician. Post‐implant dosimetric analysis was performed on CTs taken on the day of implant. Differences in prostate volume as determined by TRUS and CT were evaluated by computing a parameter, δvol, defined as the TRUS prostate volume (VTrus) minus the CT prostate volume (VCT) divided by VTrus. Implants were grouped into the following categories based on their post‐implant dosimetry: 1.Hot (D90>120% and V150>70%); 2. Cold (D90<80%) and; 3.) Standard. Differences in δvol across groups were evaluated. D90, V100 and V150 were plotted against their corresponding δvol values. Results: Overall, the average and standard deviation of δvol, D90, V100 and V150 was 0.0%+/−17.6, 101%+/−12.9, 89.3%+/− 7.0, and 47%+/−12.8, respectively. For the standard, hot, and cold groups, the average and standard deviation of δvol was 0.3%+/−15.5, − 18.0%+/−9.3 and 26.6%+/−24.0, respectively. Plots of the dosimetric parameters versus δvol show wide dispersion, for example R^2 = 0.422 for the D90 plot, but they strongly suggest a downward slope, highlighting the fact that cases with a large difference between VTrus and VCT are more likely to have nonstandard dosimetry than cases with a small difference in these volumes. It is noted that there are cases with δvol close to 0 that have nonstandard post‐plan dosimetry. Conclusion: Our work demonstrates that limiting variation in δvol may reduce but not eliminate the occurrence of nonstandard post‐plans.