Over a week ago I had some slight tenderness coming from my left testicle. When I checked it I noticed a firm round mass directly on top of the testicle. It is perfectly round, smooth, and about the size of a marble. It feels like it is attached to the epididymis and separate from the testicle, and is mobile. My GP felt it and said it is probably an epididymal cyst, but has sent me for an ultrasound to confirm. I have been reading about it on the internet which is a bad habit of mine, and I read somewhere that if you shine a light behind the mass (transillumination) and the light shines through it is most likely a cyst. I tried this with a small flashlight and the mass seemed to be somewhat dark with some light shining through at times, but not clearly. Is this a reliable test? Could I be doing it wrong? I am worried now that I have a solid tumor attached to my epididymis. My ultrasound is not for 2 weeks. Does anyone else have a similar problem? Please help.