Effect of a urinary catheter on seed position and rectal and bladder doses in CT-based post-implant dosimetry for prostate cancer brachytherapy

Abstract

Purpose: To assess the variability in rectal and bladder dosimetric parameters determined according to post-implant computed tomography (CT) images in patients with or without a urethral catheter.

Material and methods: Patients with prostate cancer who were scheduled to undergo CT after brachytherapy between October 2012 and January 2014 were included. We obtained CT series with and without a urinary catheter in each patient. We compared the rectal and bladder doses in 18 patients on each CT series.

Results: The shifts in the seed positions between with and without a catheter in place were 1.3 ± 0.3 mm (mean ± standard deviation). The radiation doses to the rectum, as determined on the CT series, with a urethral catheter were higher than those on CT without a catheter (p < 0.001). Radiation doses to the bladder with a catheter were significantly lower than those without a catheter (p = 0.027).

Conclusions: Post-implant dosimetry (PID) with no catheter showed significantly lower rectal doses and higher bladder doses than those of PID with a catheter. We recommend the PID procedure for CT images in patients without a catheter. Use of CT with a catheter is limited to identifying urethral position.

Keywords: CT-based; brachytherapy; prostate cancer; rectal dose; urinary catheter.

Fig. 1

Example of isodose distributions in the sagittal plane (left) and transverse plane (right) on each PID. In the two upper images, the prostate (red), urethra (green), bladder (yellow), and rectum (dark blue) are contoured on CT images with a catheter. On the two lower images, the prostate (blue), urethra (lemon), bladder (light green), and rectum (light yellow) are contoured on CT images without a catheter. Each red source image indicates each seed source position (the bright image) on each CT image. The seeds move with the target, and a shift in the dose distribution is apparent. The overlay of the 100% isodose area and rectum volume on PID without a catheter is less than that on PID with a catheter

Fig. 2

Projection image of the source in the transverse dimension. In the upper image, the red source images are seed source positions without a catheter, and the bright images are source positions with the catheter. The shifts on the ventrodorsal and crosswise directions are apparent in this image. The lower image is the reference image as a comparison with the upper image to understand the source shift. In the lower image, the red source images and bright images are seed source positions with a catheter

Fig. 3

RD_2cc (A) and BD_1cc (B) values in each patient with or without a catheter. The RD_2cc value (93.4 Gy) on CT with a catheter was significantly higher than that (85.1 Gy) without a catheter (p < 0.001). The BD_1cc value (75.3 Gy) on CT with a catheter was significantly lower than that (81.2 Gy) without a catheter (p = 0.027)