What are the odds of a teenager (18 years old) developing a cyst in his testicle ?

Unknown, but common: Cystic structures can develop in or around the testicle. Cystic enlargements of the epididymis (spermatocele), the space around the testicle (hydrocele), or arising from the cells on or inside the testicle can occur and are easily visible on ultrasound. The prevalence is unknown, but ultrasound screening studies show cysts in about 10% of scans. Fortunately, these are benign.