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

PURPOSE

The purpose of this study was to study the correlation between intraoperative and postimplant dosimetry. We investigated the correlation between prostate (V) and urethra (D, D) dose limits, and whether it is possible to increase prostate D and V in intraoperative planning without violating postimplant urethra and rectum dose limits.

METHODS AND MATERIALS

Seventy-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 I as monotherapy and the remainder was Pd 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 month 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: D, V, V of prostate, D, D of urethra, and V of rectum between intraoperative and postimplant plans were evaluated.

RESULTS

D and V declined for all implants between intraoperative and postimplant dosimetry. On average, D declined by 17.5% and 21.7% for I and Pb implants, respectively. V declined for all implants between intraoperative and postimplant dosimetry but less pronounced. Prostate V and urethra D and D also showed different tendency of decline. Of the 79 implants, 60 did not meet the postimplant dosimetry target for prostate (V ≤ 50%), and 46 of the 60 implants met the optimal dosimetry targets for both D (<125%) and D (<150%), and the other 14 of the 60 implants failed to meet either the D or the D limits. All the implants met the postimplant target dose for rectum: V≤ 1.3 cc.

CONCLUSION

Intraoperative implant dosimetry could not accurately predict postimplant dosimetry; however, to avoid underdosage of prostate, intraoperative D should be close to 120% of prescribed dose and V needs to be close to 100% of prescribed dose. Prostate V> 50% does not necessarily indicate the violation of urethra D and D dose limits. For most of the implants, target intraoperative D and V could be raised without violating urethra D and D limits recommended by American Brachytherapy Society in postimplant evaluation.