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

The 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.

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 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 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: D₉₀, V₁₅₀, V₁₀₀ of prostate, D₃₀, D₅ of urethra, and V₁₀₀ of rectum between intraoperative and postimplant plans were evaluated.

D₉₀ 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 ≤ 50%), 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 cc.

Intraoperative 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.