Analysis of serial CT scans to assess template and catheter movement in prostate HDR brachytherapy.

Purpose: As prostate high-dose-rate (HDR) brachytherapy becomes more prevalent, varying amounts of catheter displacement have been noted. To investigate the constancy of catheter position and its impact on dose distribution, we analyzed serial dosimetric CT scans.

Methods And Materials: The data from 50 patients were analyzed. During initial CT treatment planning, transverse images of the implant volume were collected, and all structures were digitized into the Nucletron Brachytherapy Planning System. Digitally reconstructed radiographs were generated with rendering of the catheter tips, ischial tuberosity, and perineal template. The distance from each catheter tip to the template and to the ischial tuberosity was measured. The distance between the ischial tuberosity and the template was similarly measured. A second CT set was obtained at different intervals and compared with the first measurement to assess catheter and/or template movement. In 10 patients, the second CT set was obtained before the third fraction in both 2-mm and 5-mm slice sequences, and the latter was used to re-create the HDR plan.

Results: Although no interfraction catheter movement relative to the template was found, the template-catheter unit moved in a caudal direction between HDR fractions. The amount of displacement was time dependent: 2 mm before the second fraction, 8 mm before the third, and 10 mm before the fourth. When comparing the first HDR treatment with the third, median decreases in the following dosimetric parameters were noted: dose to 90% of the prostate volume, 35% (r = 0-60); minimal dose to the base, 35% (r = 17-65); and maximal dose to 1 cm(3), 13% (r = 3-19%).

Conclusion: The interstitial catheters did not slip within the template and were not caudally displaced independently but rather in conjunction with the template. The displacement occurred in a time-dependent fashion, and, without redress, significant dosimetric changes are encountered by the third fraction.