An evaluation of our experience in position verification of catheters used for interstitial high-dose-rate brachytherapy of solitary bladder tumors.

PURPOSE: The goal of this study was to verify the position of catheters used over 4 days for brachytherapy of solitary bladder tumors.

METHODS AND MATERIALS: The study covered three phases. Shifting of catheters was studied using daily position verification CT scans of 20 patients. The possibility to omit the CT scan on Day 2 by adding a loading margin of 4 mm on each side was studied using data of 5 patients. Whether the Day 4 verification CT scan could be omitted if this margin was used, was studied for another group of 10 patients, comparing the Day 3 treatment plan to the Day 4 CT scan.

RESULTS: An average catheter shift on Days 2, 3, and 4 of, respectively, -0.3 mm (-8 to 10 mm), -0.5 mm (-14 to 10 mm), and -0.1 mm (-16 to 28 mm) was found over the measurements at both sites of the catheter. Including only shifts causing underdosing of the clinical target volume (CTV), the average shift on Days 2, 3, and 4 was, respectively, -3.6 mm (-1 to -8 mm), -5.4 mm (-1 to -14 mm), and -5.3 mm (-1 to -16 mm). After adding a loading margin, the CTV was covered on Day 2; however, the margin was not sufficient for Days 3 and 4. On Day 4, in 2/10 patients, the CTV was not completely covered. In 5/10 patients, an increased 200% isodose volume was found.

CONCLUSIONS: Position verification is necessary in bladder brachytherapy. If a 4-mm margin on each side of the loading pattern was added, position verification on Day 2 could be omitted. The verification CT scan of Days 3 and 4 is still necessary.