From “Long-Term Functional Outcomes after Treatment for Localized Prostate Cancer,” by Matthew Resnick and colleagues (NEJM, 2013):
At 15 years, the prevalence of erectile dysfunction was nearly universal, affecting 87.0% of men in the prostatectomy group and 93.9% of those in the radiotherapy group. Nonetheless, only 43.5% of men in the prostatectomy group and 37.7% of those in the radiotherapy group reported being bothered with respect to sexual symptoms. The possible reasons for the second finding include declining sexual interest with age, acceptance of sexual dysfunction over time, or both. Despite some evidence of stabilization or improvement of urinary and sexual symptoms from 2 to 5 years, long-term follow-up reveals consistent functional declines after 5 years. It remains unknown whether this continued decline is due to prostate cancer and its treatment, the normal aging process, or a combination of factors.
For what it’s worth, by age 70, the low end for the study quoted above, one published estimate is that half of men experience moderate to complete erectile dysfunction (other studies cited therein report similar rates for older cohorts, though other estimates are higher). So, my best guess — and it is only that — is that prostatectomy or external-beam radiation therapy is associated with an absolute increase in risk of erectile dysfunction of, ballpark, 30 percentage points. Also relevant, to save one life after 11 years of follow-up, 37 cancers need to be detected and 23 of them* to be treated for prostate cancer.
Thus, blithely combining the information above (which requires a few hidden assumptions**), about 7 men are made impotent by a treatment that does not extend their life. (I emphasize, this is a guesstimate based on a quick look at the erectile dysfunction article to which I link above.) We can each make our own decision about the risk trade-offs in situations like these. Just saying, these are the sorts of numbers treatment candidates might like to know.
* I have not yet confirmed from source documents what is in the comment to which I’ve linked.
** The mortality study is after 11 years of follow-up. The erectile dysfunction one is after 15. And I’m approximating a guess of the increase in erectile dysfunction risk due to treatment.