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Validation of Permanent Interstitial I-125 Implant Pre-Plans Against An Experimentally Derived Nomogram Can Potentially Reduce Variability of Day 0 Post-Implant Dosimetry

B Wolthuis

B Wolthuis*, J Dolan, D Shasha, J Santoro, R Ambrose, E Furhang, Continuum Cancer Centers of New York, NY

SU-E-T-692 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

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.

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