A CT-based evaluation method for permanent implants: application to prostate.

A computerized tomography-based 3-dimensional evaluation scheme, using semi-automatic seed localization from transverse computerized tomography-images, has been developed for permanent implants. The treatment isodose contour is generated at each scan plane and overlaid on the corresponding target contour for coverage visualization. Volume and surface dose histograms are generated for the target and neighboring normal structures. Dose inhomogeneity within the target is also estimated from the full-width at half maximum of the differential volume dose histogram. The matched peripheral dose is calculated from the ellipsoidal volume approximation for the target. The present evaluation method has been applied here to 10 actual permanent I-125 prostatic implantations. Coverages by 150 Gy and 100 Gy levels are found to range from 78-96% and 91-99% of the target volume, respectively. The average matched peripheral dose is found to be about two times the average peripheral dose (the dose encompassing 99% target) and about three times the average minimum peripheral dose (the dose encompassing 100% target). The dose inhomogeneities within the target volume, in these 10 cases, range from 220-420 Gy.