The use of a thin guide‐wire for urethral definition in prostate SBRT treatments with Cyberknife

PURPOSE: To study and analyze the effect of the use of a thin guide‐wire instead of a Foley catheter for urethral definition in prostate stereotactic body radiation therapy (SBRT) treatments and to compare treatment parameters in both situations. MATERIAL AND METHODS: Thirty‐seven prostate SBRT patients were employed in this study. A Foley catheter was employed in nine of them, and a guide‐wire was employed in the other 28 patients. For each of the 28 patients in which the guide‐wire was employed, a comparison between urethral positions in both situations was performed, allowing for a margin definition of the urethra when a Foley catheter was employed. Displacements of the prostate during treatment were obtained, allowing for an analysis of prostate positions in both situations. Also, different treatment parameters such as the number of treatment interruptions, couch movements performed, and x‐rays needed were gathered. RESULTS: Large differences between urethral positions can be found in the anterior‐posterior (AP) directions compared to those in the lateral (LAT) direction. Differences are also larger in areas closer to the base of the prostate, where margins applied in the case of using a Foley catheter are 16 mm with a mean displacement of 6 mm in the posterior direction. No differences in the treatment parameters were found during treatment in both situations. The difference found in absolute prostate pitch rotations suggests that the Foley catheter provokes a shift of the prostate position, which does not occur when employing the guide‐wire. CONCLUSIONS: Foley catheters shift the urethral position, making them a wrong surrogate of the urethra when no catheters are present. The margins needed to assess uncertainties introduced by the use of a Foley catheter are larger than those usually applied. The use of a Foley catheter did not present any additional difficulty during treatment delivery in terms of images employed or interruptions produced.