Idealized line source configuration for permanent 125I prostate implants.

BACKGROUND AND PURPOSE: To validate the use of idealized seed orientations in conjunction with the line source formalism for post-implant dosimetry of permanent 125I prostate implants.

PATIENTS AND METHODS: Post-implant, a CT scan and three fluoroscopic images were obtained for 32 patients having undergone permanent implants. From these images, the seed positions and orientations (phi,theta) were determined (1625 individual seeds). Two different dosimetric calculations were done: one using real orientations and one using idealized orientations (seeds along the axis of implantation). Dose volume histograms (DVHs) and key dosimetric parameters were compiled for the prostate, urethra, rectum, bladder and penile bulb, to evaluate the difference between the two approximations.

RESULTS: The phi angle distribution (phi = 1.1 degrees; sigma phi = 22.9 degrees) and the theta angle distribution (theta= -4.29 degrees; sigma theta = 27.1 degrees) were found to be similar to the first order except for the pronounced peak of the phi angle distribution. The DVHs comparison and dosimetric parameters study reveal no significant difference between the two approximations. The difference in D90 for the prostate was only 0.02% (sigma = 0.91%) The differences were slightly higher in the case of the organs at risk, as expected from the dosimetric characteristics of the seed model used.

CONCLUSIONS: The angular distributions (phi,theta) of individual seeds were determined. The dosimetric evaluation shows that line source formalism can be used in conjunction with an idealized seed configuration presented here to report prostate and organs at risk dose coverage.