The findings and location of the lesion in this case are typical of calcific myonecrosis, with radiographic features similar to but not reminiscent of post-traumatic heterotopic ossification (less accurately termed myositis ossificans) which generally shows a more mature pattern of ossification; however, this is the next most likely differential diagnosis.
This disease process is found almost exclusively in the leg. The most common aetiology is prior trauma and/or compartment syndrome, however infection and common peroneal nerve injury have been suggested. This process has been associated with epileptic seizures and dermatomyositis as well. These calcified masses often appear decades after the inciting trauma.