Urethra

The prostatic urethra runs through the center of the prostate from the bladder neck to the apex, where it continues as the membranous urethra.

The urethra carries urine from the bladder out of the body; its first part runs through the prostate.

Because the urethra passes through the target, it cannot be avoided — only protected from hotspots. This matters most in prostate SBRT and when boosting a dominant intraprostatic lesion, where high doses per fraction make urinary toxicity more closely tied to how hot the urethra gets. Plans therefore cap the highest dose to the urethra (or a PRV around it) rather than trying to keep it cool.

In Richardson 2021 the prostatic urethra was contoured inside the prostate CTV from the bladder neck to the prostate apex, on the sagittal view with a 2 mm brush. Large TURP defects inside the CTV were included. A further 3 mm expansion made a cylindrical urethra PRV.

The study found a thin (0.9 mm), 3D T2-weighted MRI cut variation between observers markedly compared with planning CT or a standard 3 mm T2 scan. The urethra can still need some estimation, so use the method your protocol describes.