Dosimetric impact of interfraction catheter movement in high-dose rate prostate brachytherapy

To evaluate the impact of interfraction catheter movement on dosimetry in prostate high-dose-rate (HDR) brachytherapy.

Fifteen patients were treated with fractionated HDR brachytherapy. Implants were performed on day 1 under transrectal ultrasound guidance. A computed tomography (CT) scan was performed. Inverse planning simulated annealing was used for treatment planning. The first fraction was delivered on day 1. A cone beam CT (CBCT) was performed on day 2 before the second fraction was given. A fusion of the CBCT and CT was performed using intraprostatic gold markers as landmarks. Initial prostate and urethra contours were transferred to the CBCT images. Bladder and rectum contours were drawn, and catheters were digitized on the CBCT. The planned treatment was applied to the CBCT dataset, and dosimetry was analyzed and compared to the initial dose distribution. This process was repeated after a reoptimization was performed, using the same constraints used on day 1.

Mean interfraction catheter displacement was 5.1 mm. When we used the initial plan on day 2, the mean prostate V100 (volume receiving 100 Gy or more) decreased from 93.8% to 76.2% (p < 0.01). Rectal V75 went from 0.75 cm(3) to 1.49 cm(3) (p < 0.01). A reoptimization resulted in a mean prostate V100 of 88.1%, closer to the initial plan (p = 0.05). Mean rectal V75 was also improved with a value of 0.59 cm(3). There was no significant change in bladder and urethra dose on day 2.

A mean interfraction catheter displacement of 5.1 mm results in a significant decrease in prostate V100 and an increase in rectum dose. A reoptimization before the second treatment improves dose distribution.