<< Could be a simple case of washer woman's knee, otherwise known as bursitis >>
Recently experienced the same condition except with my left elbow - the fluid sac was the size of a golf ball Bursitis, which is the call from STW, was the diagnosis by my primary care physician who drained it using (several) syringes. The elbow area is still sensitive, but the swelling has all but disappeared.
I was concerned, as you must be, that this was Pca (tumor) related but got a definite NO from the Doc because the excised fluid was clear.
Hope this helps,
Mitch in SC