Is it on the front side or back side (posterior)? Can you describe the exact location? Does it change in size or disappear with the knee flexion (when you fold it)?
Following possibilities exist for this kind of swelling but all of them are benign so you should not worry about it;
1) There are about 13 bursae around the knee, out of which four are present anteriorly/in front. So this can be one of them. Bursa provide a sort of cushion between bones and tendons. They can get enlarged with excessive synovial fluid in them.
2) Ganglion cysts, which is mucoid degeneration of collagen and connective tissues or is filled with mucin formation.
3) Bony exostosis which is a benign growth of the bone.
Ultrasound is likely to diagnose the nature and origin of the swelling.
Please feel free for your follow up questions.
I tell all my patients to be aware before using internet for their ailments. There is lot of junk all around and to pick the right thing, even for professionals sometimes is difficult. It can unnecessarily cause of lot of anxiety. Please don't worry and bone cancer is unlikely cause for your swelling/lump.
You are welcome.