Why do prostate cancer treatments cause urinary incontinence?

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The prostate sits between the bladder and the urethra and urine actually passes through the prostate on its journey from the bladder to the penis (ie, penile urethra). Think of the prostate (or prostatic part of the urethra) as a tunnel that connects New Jersey and Manhattan. If you remove part of the tunnel, you have to reconnect the two sides and build a new makeshift tunnel. As tissue is healing from this type of surgery and the muscles that control the flow of urine are recovering, men will often suffer from incontinence.   
The prostate sits between the bladder and urethra and helps maintain continence. Since surgery removes the prostate, radiation kills the cancer by damaging the prostate, and hormone therapy causes the prostate to shrink, prostate cancer treatments interfere with this mechanism.
 
The prostate lies in the pelvis and lies between the penile urethra and the urinary bladder. Urinary continence relies on coordination of 2 mechanisms, the involuntary (smooth muscle) and the voluntary (skeletal muscle).
The prostate literally rests on top of the voluntary urinary sphincter and is lies below the involuntary urinary sphincter that derives from the bladder. As you can imagine, any treatment delivered to the prostate can thus lead to urinary incontinence.
The degree, severity and probability of having short or long term urinary incontinence clearly depends on the degree of impact on these 2 continence mechanisms and the patient's underlying bladder physiology.
The close relationship between the prostate and bladder is responsible for the increased rates of incontinence following prostate cancer therapy. Normally urinary continence is maintained by muscles which surround the urethra at its junction with the bladder. In the male, the prostate sits at the base of the bladder where it wraps around the urethra, and as such is in close proximity to the muscles which control urine flow. 
With radiation therapy, ionized wave particles are responsible for killing the tumor cells, and unfortunately neighboring cells, such as those involved in urinary control, are also affected resulting in the loss of continence. In the case of prostate surgery, a portion of the urethra, which is encircled by the prostate, is removed and the urethra must be reconnected. During the dissection of the urethra it is possible to damage the muscles responsible for urinary control.  However, with improving technology, such as the introduction of robotic assisted laparoscopic prostatectomy, the surgeon is able to better visualize these muscles, resulting decreased rates of incontinence.  
It is important to discuss your concerns regarding possible incontinence with your physician, in order to gain a thorough understanding of realistic expectations and the possible treatments available for incontinence. 
The urinary sphincter muscle that provides urinary control is very close to the bottom of the prostate gland and any treatment for prostate cancer can lead to damage or injury of this structure. The consequence of this damage is urinary incontinence which may be temporary or permanent.
 

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.