Can retrograde ejaculation be caused by prostate radiation?

It is important to first recall the pathophysiology of normal ejaculation. The initial step in semen emission is closure of the bladder neck, which is mediated by innervation from the sympathetic nervous system. Retrograde ejaculation occurs from an incompletely closed bladder neck, which most frequently results from either neurologic dysautonomia (e.g. diabetes mellitus, Parkinson's disease), medication use (e.g. α-receptor antagonists), or mechanical surgical disruption (e.g. TURP, GreenLight laser) often from treatment for BPH more so than RP or cystoprostatectomy for prostate cancer.

Interestingly, most of the available data on post-RT sexual and ejaculatory dysfunction in prostate cancer patients does not report on the incidence of retrograde ejaculation per se, although that's not to say that it cannot or does not happen, as follows: 

The first and impressively comprehensive paper by Huyghe et al. analyzed the side effects of I-125 interstitial brachytherapy on subsequent ejaculatory function. Data was collected from ~250 men who completed a mailed questionnaire derived from the Male Sexual Health Questionnaire regarding ejaculatory frequency, volume, dry ejaculation, pleasure, and pain after brachytherapy. Findings from this retrospective review suggest that 18.7% of patients experience dry ejaculation post-RT, but there are no mentions of instances of retrograde ejaculation.

Another salient study by Sullivan et al. reviewed 364 prostate cancer patients in total, 250 of which had treatment with EBRT and 112 with brachytherapy, specifically looking at rates of anejaculation following radiation treatment. The authors presented data that 16%, 69%, and 89% will experience anejaculation at 1,3, and 5 year intervals after RT, respectively. Predictive factors for this side effect include older age, ADT, RT dose > 100 Gy, and smaller prostate size at the time of treatment. 

Obviously, one of the major issues with assessing for retrograde ejaculation post-RT is that it is not necessarily as apparent and physically manifests other side effects; it can be hard for the patient to distinguish anejaculation from retrograde ejaculation, unless a copious amount of semen refluxes into the bladder which is noticed with change in the appearance of urine. As such, the only definitive way to assess for retro vs anejaculation is urinalysis, which was not done in either review.

Some of the aforementioned patients may have had retrograde ejaculation without being aware of such, but were reported and categorized instead as anejaculation or diminished volume in the Huyghe paper. Nevertheless, since this is a specific issue of the bladder neck, the radiation in question would either have to compromise the innervation of the bladder neck or muscular integrity of the sphincter above all other potential causes. 

Sympathomimetic drugs (ephedrine sulfate, imipramine hydrochloride, and pseudoephedrine hydrochloride) have had the most success for treatment of this condition by improving bladder neck contraction during the expulsive phase. Low dose imipramine (25 mg BID) has shown the highest rates of success rates in patients who have undergone retroperitoneal sympathetic denervation.