Seed Displacements After Permanent Brachytherapy For Prostate Cancer In Dependence On The Prostate Level

Main Category: Prostate / Prostate Cancer
Also Included In: Urology / Nephrology
Article Date: 24 Dec 2008 - 0:00 PDT

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UroToday.com - Interstitial brachytherapy became an established treatment for localized prostate cancer in the last decade with excellent long-term results. CT-based dosimetry is currently recommended as the standard for postimplant dosimetry, with a suggested interval of four weeks after I-125 brachytherapy. For practical and logistic reasons many brachytherapists prefer to rely on early scanning. With increasing post-planning intervals, anatomic and dosimetric changes coincide with decreasing prostate oedema and a seed displacement.

To further improve the knowledge of these changes during the first post-implant month, this study evaluated seed displacements in different areas of the prostate relative to constant coordinates (defined by the position of pelvic bones) and the associated dosimetric changes in a group of 61 patients. Post-implant CT scans were performed one day and one month after brachytherapy with stranded seeds as monotherapy.

Post-implant dosimetry results have shown significantly increasing doses to the prostate and rectal wall between day 1 and day 30 (mean prostate D90 of 138Gy vs. 155Gy, p<0.001; mean rectal wall V100 of 0.3cc vs. 1.2cc; p<0.001). With decreasing prostate oedema, seeds moved predominantly along the needle track in the inferior and posterior directions. Due to anatomic limitations, tissue expansions are more limited at the apex (no significant displacement) anterior-posteriorly or laterally (mean 1.2-1.6mm inwards displacement). Bladder filling can be easily displaced. A consequence of larger inferior seed displacements at the base (mean 5.2mm), as compared to the apex (mean 2.2mm), is an implant compression (mean 1.7mm; p<0.001). With a decreasing oedema between day 1 and 30 (mean prostate volume of 51cc vs. 41cc; p<0.001), the largest mean inward prostate surface displacement resulted at the base (3.9mm), but only 1.2 - 1.6mm in the remaining directions. Posterior displacements predominated both at the base and the central region. The smallest displacements were found for single seeds, the largest for the strands including five seeds.

As a result of the prostate surface and seed displacements, the distance of the prescription isodose to the respective prostate surface (treatment margin) changed. The largest differences were found at the apex (mean increase 3.7mm; p<0.001 - vs. 1.8mm at the base; p<0.001) and the posterior border (mean increase 2.5mm; p<0.001 - vs. 0.0mm at the anterior border). Prostate tissue displacements correlated well with seed displacements. Seed displacements were associated with larger prostate volume changes between day 1 and day 30. Patients with larger prostate volume decreases were found to have greater seed displacements, greater treatment margin changes and a higher dose to the prostate on day 30 (comparing a volume decrease >24% vs. <24%: mean 4.0mm vs. 2.7mm inferior and 1.5mm vs. 0.5mm posterior seed displacements; mean prostate D90 of 167Gy vs. 143Gy on day 30; p<0.01 respectively).

Predominantly inferior and posterior displacements implicate consequential smaller preplanning margins at the apex and the posterior prostate. As a consequence of larger displacements of longer strands, this study supports the application of single seeds or shorter strands particularly at the prostate apex.

Written by Michael Pinkawa, MD as part of Beyond the Abstract on UroToday.com

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