What Causes A Spot In The X Ray Over The Surgical Site On The Ankle?

Our 26 year old son had a tumor develop on his ankle. It was removed 1 1/2 yrs ago. It was a long surgery, bone removed and replaced with artificial bone material. He goes for regular check ups. At todays visit, xray showed a "spot". He is scheduled for mri next week. can you give us some additional information as to what"spot" might indicate?