Abstract
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 (5WK). 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 0.8 mm (± 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.PACS number: 87.55.kh
Highlights
Current regimens of highly conformal dose constraints with intensity-modulated radiation therapy (IMRT) and dose escalation demand precision in patient setup and localization of the prostate gland
Many investigators have shown that the movement of the prostate is common and highly varied in magnitude and direction.[1,2,3,4,5] Motion of the prostate both between and during fractions of external beam radiotherapy (EBRT) is well documented,(1,4-6) as organ motion due to rectal and bladder filling makes the prostate a dynamic target.[4,5,7] traditional portal images are often used to match to the pelvic bones, which does not guarantee that the prostate itself is on target as the prostate moves independently from the pelvic bones
Fiducial markers within the prostate gland are often used for image-guided radiotherapy of prostate cancer.[1,2,3] As the prostate gland is not visible on portal images, implanted fiducials in the gland allow portal image matching to these markers, resulting in the higher accuracy necessary to gain the full therapeutic advantage of highly conformal treatment plans
Summary
Current regimens of highly conformal dose constraints with intensity-modulated radiation therapy (IMRT) and dose escalation demand precision in patient setup and localization of the prostate gland. Van der Heide et al[15] evaluated the absolute distance between gold markers for each fraction of radiotherapy using megavoltage portal images This study compared these distances to those found on the planning CT scan, and demonstrated a gradual decrease in distance over the course of radiotherapy, ending with an average decrease of 0.9 mm after 35 fractions. Kitamura et al[6] used a 2 mm diameter gold sphere as a single fiducial marker They measured its migration through the prostate by comparing its position on sequential CT scans relative to the center of mass of the contoured prostate, demonstrating an average absolute migration of 3.0 ± 3.4 mm, with the greatest motion in the craniocaudal dimension. Over the course of 374 fractions evaluated in 10 patients, there were changes in distances between seed positions with the standard deviations of these distances varying from 0.69 to 1.68 mm
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