Correlation between real-time intraoperative and postoperative dosimetry and its implications on intraoperative planning.

Abstract

PURPOSE\nThe purpose of this study was to study the correlation between intraoperative and postimplant dosimetry. We investigated the correlation between prostate (V150) and urethra (D30, D5) dose limits, and whether it is possible to increase prostate D90 and V100 in intraoperative planning without violating postimplant urethra and rectum dose limits.\n\n\nMETHODS AND MATERIALS\nSeventy-nine patients who underwent real-time ultrasound-guided prostate implants using intraoperative planning from 2013 to 2017 were analyzed. Forty-one of the 79 implants were 125I as monotherapy and the remainder was 103Pd as boost to external beam radiation therapy or external beam radiation therapy plus androgen deprivation therapy. Prescriptions followed the guidelines of AAPM TG-137. The urethra was catheterized during intraoperative implantation and postimplant imaging to facilitate the urethra identification. T2-cubed MRI and CT were acquired on the same day and about 1\xa0month after the low-dose-rate procedure, and MRI was later fused with the CT scan for accurate delineation of the prostate and postimplant dosimetry evaluation. An institutionally based peer-review process and document procedure were established based on national recommendations. Correlation of dose parameters: D90, V150, V100 of prostate, D30, D5 of urethra, and V100 of rectum between intraoperative and postimplant plans were evaluated.\n\n\nRESULTS\nD90 and V100 declined for all implants between intraoperative and postimplant dosimetry. On average, D90 declined by 17.5% and 21.7% for 125I and 103Pb implants, respectively. V100 declined for all implants between intraoperative and postimplant dosimetry but less pronounced. Prostate V150 and urethra D30 and D5 also showed different tendency of decline. Of the 79 implants, 60 did not meet the postimplant dosimetry target for prostate (V150\xa0≤\xa050%), and 46 of the 60 implants met the optimal dosimetry targets for both D30 (<125%) and D5 (<150%), and the other 14 of the 60 implants failed to meet either the D30 or the D5 limits. All the implants met the postimplant target dose for rectum: V100≤ 1.3\xa0cc.\n\n\nCONCLUSION\nIntraoperative implant dosimetry could not accurately predict postimplant dosimetry; however, to avoid underdosage of prostate, intraoperative D90 should be close to 120% of prescribed dose and V100 needs to be close to 100% of prescribed dose. Prostate V150> 50% does not necessarily indicate the violation of urethra D30 and D5 dose limits. For most of the implants, target intraoperative D90 and V100 could be raised without violating urethra D30 and D5 limits recommended by American Brachytherapy Society in postimplant evaluation.