Since the prostate treatment was 10 years ago, the current symptoms typically would not be related to that treatment.
There are several possible causes of swelling in both testicles.
Infection/inflammation is very common, but these problems usually present with acute swelling, not 6 months of swelling. If you had an exam, then there would usually have been evidence of infection/inflammation.
Anatomic abnormalities are another very common cause. Clear fluid collection in a sac, called a hydrocele, is the most common. There are some other similar structures, called a spermatocele or a varicocele.
It is also possible to get an appearance of testicular swelling related to a hernia and this is the other common cause of swelling. The doctor can usually detect a hernia on exam, so the doctor saying that it appeared to be a fluid collection would usually also mean that a hernia is not present.
Swelling also can occur from a twisting of the testicle, but this also causes acute swelling, not chronic swelling.
So, this is most likely a hydrocele. The definitive test for a hydrocele is an ultrasound of the scrotum.
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