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t’s been called the final indignity: after men have their prostate removed to treat cancer there, at least two-thirds find that their penis has shrunk, typically by nearly an inch. But in a much-needed glimmer of hope, a new study finds that, after the penis hits a nadir 10 days after surgery, it usually recovers to its pre-surgery length after a year.

The study of 102 prostate cancer patients in Japan “can help men to better understand the risks of a radical prostatectomy and make an informed choice when selecting a treatment option,” said Dr. George Suarez, a urologist in Miami who was not involved in the research, which is published in the journal BJU International (formerly British Journal of Urology). “At the end of the day, no man wants to risk sexual potency or have their penis shortened, the latter even if [it’s] only temporary.”

The shrinkage hits men especially hard because it’s a side effect that surgeons usually don’t warn patients about. While one small study (of just six men) found that the most common reaction to the loss of length was resignation — prostate cancer patients are often simply glad to be alive — on social media and in private discussions, many patients have a less sanguine view. They say in one internet chat room they were unprepared for the shrinkage, experienced adverse psychological effects, and resented having “to find this out for ourselves after the operation.”

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There is a continuing debate among prostate cancer experts over whether men are more likely to die (from prostate cancer or any other cause) if they are treated with surgery or radiation, but a 2016 analysis of 19 observational studies gave surgery an edge. That conclusion comes with an asterisk, however, because men who opted for surgery could be different — healthier, wealthier, under better medical care — than those who chose radiation, in which case it would be something about the patients and not the treatment that led to higher survival.

For their study, physicians at Kanazawa University School of Medicine in Japan followed 102 men who had undergone radical prostatectomy — surgery to remove the prostate — as part of cancer treatment. Using a ruler, they measured the length of each patient’s stretched penis (considered a proxy for an erect one) at room temperature while the men were lying down, before the operation and again eight times over the next two years.

Penis length was shortest 10 days after surgery, having lost an average 0.8 inch from before the operation. The larger the size of the prostate, the greater the shrinkage. But penis length “gradually recovered,” Dr. Atsushi Mizokami, a prostate cancer specialist, and his colleagues reported, until it was “not significantly different from [the] preoperative” length.

Their scrutiny of the anatomy, they wrote, offers the first detailed explanation of both shrinkage and recovery. Although animal studies had implicated nerve damage, the researchers found something else.

After the prostate is removed, the part of the urethra at the very top of the penis (it’s called the membranous urethra) retracts into the pelvis, MRIs showed. Anatomically, the tissue surrounding the urethra as it travels from the bladder through the (now-removed) prostate to the tip of the penis, the membranous urethra, and the penis itself act as a single integrated structure. As a result, the shaft of the penis gets pulled up into the pelvis, too. “Slight vertical repositioning of the membranous urethra after [prostate surgery] causes changes in [penis length] over time,” the researchers wrote.

But over the next 12 months the tension in the connective tissues that hold the pelvic organs in place loosens, they explained, allowing the membranous urethra to return to its original, lower position. The penis follows.

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