Stability of serrated gold coil markers in prostate localization

We investigated the stability of serrated gold coils (Visicoil) implanted within the prostate glands of patients undergoing definitive external beam radiotherapy for prostate cancer. Radiopaque Visicoils of diameter 0.75 mm and median length 3 cm (range 2-4 cm) were implanted, one into each lobe of the prostate glands of 30 patients planned for external beam treatment. The coils were visualized on CT simulation and again after 25 fractions of treatment (5 WK). Data from 30 patients were studied, of whom 19 also received androgen ablation therapy. The average change in the distance between the two coils over five weeks of treatment was 0.8mm (± 0.6 mm), with a maximum of 2.5 mm in one patient. Average residual errors (standard deviations) for the positions of individual coil segments after five weeks of therapy were only 0.7 mm LAT, 0.6 mm AP, and 0.4 mm SI. The average change in distance between the coils over five weeks compared favorably with published data regarding marker seed stability. Overall, less than a 2 mm margin (i.e., 2 standard deviations) would adequately compensate for positioning uncertainty of the coils in more than 95% of cases.