Doctors at Dana-Farber/Brigham and Women's Cancer Center (DF/BWCC) recorded patient complaints of shrinkage in 948 men who'd been treated and suffered a recurrence of the disease.
None of the men treated with radiation mentioned decreased penis size, but 3.73% of those who'd had surgery (radical prostatectomy) and 2.67% of those who'd had radiation plus androgen deprivation therapy said they noticed a difference.
Some of them said it interfered with their sexual function and made them regret the treatment they chose.
In this study, actual measurements were not taken. But previous studies have found that prostatectomy in particular can result in penis shortening.
The potential for this side effect -- real or perceived -- is known among physicians and surgeons, said Dr. Paul Nguyen, who led the study, "but it's almost never discussed with patients, so it can be very upsetting to some men when it occurs."
The problem is likely even more widespread than the study shows, because the data relied on patients raising the issue with their doctors unsolicited, rather than being asked, the researchers said.
Their findings appear in the January issue of the journal Urology.
"Patients can deal with almost any side effect if they have some inkling ahead of time that they may happen," Nguyen said.