These sorts of solitary, fibrous tumors are not unheard of, but are also not terribly common. They can appear after trauma (which didn't happen with you), but can also appear without any sort of stimuli or trigger. They can also be painful and tender as you report, some of which is due to the pressure that these can place on the surrounding tissues.
Sometimes fibrous tumors can be malignant, but since there was no local invasion or invasion into the nearby bone noticed on these scans, and since your orthopedist was not concerned, then this is reassuring. When it is removed, it will probably be sent to a pathologist who will examine it thoroughly and classify it. So it just sounds potentially like a benign fibrous tumor.
I hope this helps. Please let me know if you have any further questions - I'm more than happy to help. Take care - Dr. Fay
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