Planning based on postneedle volume with early dosimetric assessment is beneficial for Cesium-131 permanent prostate seed implantation

Purpose This study reports on prostate edema after prostate brachytherapy using Cesium-131 (131Cs) and describes our method to compensate. Methods and materials Thirty-one patients underwent brachytherapy using an afterloading technique. Volume measurements of the prostate were taken at various time intervals relative to the date of implant. Real-time operating room dosimetry was used for seed placement on the postneedle prostate volume. The prostate volumes at the various time points were used to determine the effect of prostate edema on dosimetry. Results Increase in prostate volume occurred immediately after needle placement, as measured by both ultrasound (mean increase of 17.7% (0–75.0%) from 36.8 to 46.9 cc) and Day 0 CT (mean increase of 15.3% (0–54.8%) to 45.9 cc). Day 0 assessment of dosimetry revealed a median D90 of 102.7% (86.7–133.4%), median V100 of 91.8% (75.9–98.4%), median V150 of 44.4% (23.8–81.3%), and median V200 of 16.3% (7.8–36.9%). This edema dissipated over the next 4 weeks, with resultant changes in dosimetric parameters. By 4 weeks, prostate volume had returned to the preimplant volume (37.7 cc) with increased D90 (118.2%), V100 (95.6%), V150(63.9%), and V200 (28.4%). Conclusions There is significant immediate edema with prostate brachytherapy. This affects the dosimetry of the implant substantially. Because of this edema, our planning for brachytherapy is done on the postneedle implant volume. Quality assurance studies should be done on the same day as the implant to avoid substantial overestimation of dosimetric parameters.