Can benign tumors ever become malignant? how often does this occur? i have osteochondroma on my right knee. it is painful at the moment when i walk.

Usually no: Osteochondroma, also called exostosis is the most common benign bone tumor. Slow growing, painful if impinges on nerve or stalk is broken; usually stops growing and ossifies at puberty. Benign, but 1-2% of solitary tumors and 5-25% of multiple tumors undergo malignant transformation to chondrosarcoma. May be neoplastic, due to mutations in EXT1 and EXT2 polymerases. May visit an orthopedist.