Synovial sarcoma is a rare type of cancer that tends to arise near large joints, particularly the knee, in young adults. Despite its name, it typically doesn’t affect the interior of joints, where synovial tissue and fluid are located. The first sign of trouble is usually a deep-seated lump that may be tender or painful.
https://www.mayoclinic.org/diseases-conditions/synovial-sarcoma/cdc-20387747