When oncologists run through the list of possible side effects of surgery and other treatments for prostate cancer, rarely mentioned is one that leaves some men to regret their treatment choice: shrinkage.

Yet a significant percentage of men who undergo surgery to remove their prostate gland or hormone therapy combined with radiation report that they experience a decrease in their penis size after treatment, Dana-Farber Cancer Institute researchers found in a new study. Radiation treatments alone did not lead to a reduction in penis size, according to the study published Wednesday in the journal Urology.