SU‐E‐T‐692: Validation of Permanent Interstitial I‐125 Implant Pre‐Plans Against An Experimentally Derived Nomogram Can Potentially Reduce Variability of Day 0 Post‐Implant Dosimetry

Abstract

Purpose: To evaluate whether standardization of total pre‐plan activity against a nomogram reduces variability of post‐implant dosimetry for I‐125 permanent interstitial prostate implants (PIPI). Methods: Between January 2007 and December 2011, 843 patients underwent TRUS guided PIPI (MPD 108Gy) in combination with external irradiation. The implants and all contouring for pre‐and post‐plans were performed by a single physician. Post‐implant dosimetry was determined based on CTs taken on the day of implant. Implants were grouped into the following categories based on their post‐implant dosimetry: 1.Excellent (90%< D90 <120% and V150>70%); 2.Hot (D90>120% and V150>70%); 3. Cold (D90<80%); and 4.Other. A known‐effective activity versus volume nomogram was derived from a fit of the data from the excellent implants. The actual activities used in the hot and cold implant groups were evaluated for deviations from the predictions of the known‐effective nomogram. The potential impact of imposing a tolerance window was investigated. Results: Of all implants, 347, 81, 37 and 378 were excellent, hot, cold and other, respectively. The activity versus volume data of the excellent implants were well fit with a power law (R^2=0.972). The average and standard deviation of the differences between actual implanted activity and the prediction of the known‐effective nomogram was 3.8% +/− 4.0 and −0.6% +/− 5.0 for the hot and cold groups, respectively. The implanted activity agreed with the known‐effective nomogram prediction within +/−4% for 514 plans. For these plans, 44%, 7.0% and 3.9% were excellent, hot and cold, respectively. Outside this tolerance window, the corresponding values were 36%, 14% and 5.2%. Conclusion: Imposing a quality assurance requirement that total pre‐plan activity agree with the prediction of a known‐effective nomogram should improve homogeneity of post‐implant dosimetric parameters by increasing the frequency of excellent implants and reducing the frequency of hot implants.