My 8 year old son developed a lump to the side of his left knee 2 days ago. The pediatrician thinks it is either a ganglion cyst or baker's cyst. However I am taking him to an orthopedic surgeon to verify this with further testing. I want to make sure 100% that this is what it is, and not some type of tumor. It is some what moveable, but from what I've read so are some type of soft cell sarcoma's. Any information you can provide would be helpful. The lump goes Up and down, it is more long than wide, not a perfect circle, by any means. More oblong. Thank you.