Ewing sarcoma
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Presentation
Knee pain
Patient Data
MRI performed for knee pain demonstrates hyperintensity within the proximal tibial plateau and an irregular serpentine hypointense line within the metaphysis. There is a periosteal reaction seen at the lateral tibial cortex - not appreciated on initial interpretation.
The initial interpretation of the MRI was a stress fracture.
No plain radiograph was performed at this time.
Two months after the initial MRI with persisting symptoms the patient was reimaged - the plain film shows patchy sclerotic lesion within the tibial metaphysis with a very wide zone of transition. There is a multilayered immature periosteal reaction seen most notably at the lateral cortex.
Axial T1 MRI shows marrow infiltration of the tumor replacing the normal hyperintense fat. Coronal T2FS covering the whole Tibia shows an extraosseous soft tissue mass as well as an intraosseous tumor. The focus of high signal in the opposite tibia was shown on a subsequent PET scan to represent metastasis.
Case Discussion
This case illustrates the difficulty of ortho-oncology cases and the need for multi-modality imaging.
The case was initially interpreted as a stress fracture, the final histological diagnosis was Ewing sarcoma.
The initial MRI demonstrates a few key points, firstly all of the sequences are PD or PDFS, which is common in musculoskeletal MRI imaging but can be deceptive when the problem is not of an injury but rather an infiltration of the marrow by either tumor or infection. In both cases, the marrow fat signal will be decreased on a T1-weighted scan but can be masked by the increased PD/fluid signal on PD imaging. The axial T1 scan from the second MRI clearly shows the extent of marrow infiltration. Secondly, the imaging on the initial scan does not extend back to normal bone - and as can be seen on the subsequent scan (performed 2 months later) there is a better appreciation of the extent of the disease when the imaging (despite being lower resolution) covers the whole region of abnormal signal.
Thirdly, the plain film correlation is critical in bone tumor cases, the plain film in this case was also performed two months after the first MRI and shows the periosteal reaction and zone of transition better than the MRI. The main teaching points of this case are:
always include a T1 in the MSK protocol
always extend the scan back to normal bone
always correlate with plain film
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