Is there a preferred strength for regularly spaced 125I seeds in inverse-planned prostate implants?

To determine whether a preferred seed strength exists for 125I prostate implants preplanned using a fixed intraneedle seed spacing of 1 cm and an objective needle placement strategy within the planning target volume (PTV), and incorporating explicit dose-volume constraints for the PTV and tissues at risk.

Prostate, urethra, and rectum contours for 10 patients were obtained from transrectal ultrasound studies. The PTV was defined in accordance with Radiation Therapy Oncology Group (RTOG) 0019 protocol. Inverse planning software was used to optimally arrange seeds of strength 0.3-0.8 U to cover the PTV to D(Rx) = 145 Gy, and limit urethra and rectum doses to 150% and 100% of D(Rx), respectively. Isodose distributions and dosimetric indices were calculated: V(200), V(150), V(100), V(90), D(100), D(90) for PTV; V(150) for urethra; and V(100) for rectum. For seeds of strength 0.414 and 0.6 U and three prostate sizes, the sensitivity of V(90) and D(90) to elementary perturbations of the optimal seed arrangement were examined.

For our planning scenario, 125I seeds of strength 0.5-0.6 U provided the best possible PTV coverage while maintaining V(200) at approximately 25%. The source arrangement for 0.6-U seeds was only modestly more sensitive to perturbations than that for 0.414-U seeds. These findings may not be applicable to implants planned manually or that involve needle placement outside the PTV.

Given a particular source arrangement, inverse planning aimed at maximizing dosimetric coverage of the prostate while limiting doses to the urethra and rectum can be used to search for a preferred seed strength. For regularly spaced sources within the PTV, higher strength seeds can provide better dose coverage and better urethral protection than lower strength seeds.