The history, radiograph, and MRI scan are consistent with a diagnosis of myositis ossificans. This benign condition is common after trauma, burns, or surgery secondary to a circulating factor which activates mesenchymal stem cells residing within muscle to form extra-osseous bone. Treatment is conservative in nature and if required, it is recommended treatment be delayed until complete ossification.
The radiograph shows a large area of bone formation in the patient's right groin which on advanced imaging is seen to be well ossified and adjacent to but not stuck on the bone with peripheral enhancement. There are no malignant features suggestive of a process like osteosarcoma on either of these two imaging studies.
Answers 1 and 2: while many lipomas and synovial sarcomas can have ossification, this pattern of dense ossification is classic for myositis ossificans
Answer 4: Periosteal chondrosarcoma - the location of the lesion isn't on the bone but adjacent to the bone. In addition, a periosteal chondrosarcoma would have mineralization which looked like cartilage, not bone as in this case.
Answer 5: Inguinal hernia - there is nothing in the stem nor images which suggest an inguinal hernia.
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