My 7 years old son was diagnosed with osteochondroma. we did only x-ray on the lump on right knee. should we make mri or biopsy ? want to make sure.

Easy call: Osteochondromas on the knee are very familiar and commonplace. If the radiologist and orthopedist are satisfied on plain film, then leave it alone unless there's a good reason to excise it. Get a 2nd radiologist before a $$ scan. Mri isn't going to add more information. The decision to excise would rest upon whether it is growing rapidly, has very thick cartilage, or hurts.

Not necessary: An osteochondroma is a very common benign tumor involving the bone. The tumors have a very characteristic appearance and location. The chances of this developing into something else, like cancer, is extremely small. Mri is unlikely to provide any further information. A biopsy can be used to confirm the diagnosis, if there is any question. See a bird certified orthopaedic surgeon for evaluation.

Basic choices: The investigations should never be worse than the condition under study. You wouldn't want to put a kid through general anesthesia for a biopsy when the standard (passive) study finds a benign osteochondroma. MRI would add nothing. If your fear of cancer is enough to offset the risks/pain for the kid, push ahead. Otherwise find docs you can trust & monitor the situation until you are comfortable