The differential diagnosis should be constructed from the plain radiographs and the patient's history. Once a list of possibilities is assembled, the CT and MRI results are examined and the probability of each diagnosis is increased or decreased. The final result should be a list of two or three possibilities. Beware of the temptation to start listing possible diagnoses based on the MRI, the bone scan, or the CT, since these modalities may "overcall" the lesion. The lesion has no aggressive features by radiographic criteria. The lesion does have a small round lucent area seen on the CT that might be interpreted as a nidus, leading to the possibility of osteoid osteoma, but the patient does not have the expected pain pattern. The reformatted CT and the lateral MRI are very helpful, since they show the how the lateral head of the gastrocnemius is inserting on the posterior femur right at the site of the lesion. This, the location, and the patient's age make the diagnosis clear.