From the case:
Chordoma - clivus
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A high T2 signal mass arises from the posterior surface of the clivus and indents the pons. This has been likened to a thumb and the term "thumb sign" has even been proposed. It is said to be relatively specific for a chordoma in the correct clinical and imaging setting.
Case Discussion
This patient went on to have the mass resected which confirmed the diagnosis of chordoma.