Judging from the above MR images, what seems to be the main tissue constituent of the tumour and why?
Cartilage, because of high T2 signal and contrast-enhancement on T1-weighted sequences with a rings and arcs pattern.
What diagnosis is thus most likely?
Chondromyxoid fibroma. Other cartilage containing tumours (enchondroma and low grade chondrosarcoma) are also possible.
This lesion was resected and proven to be a chondromyxoid fibroma. Does it appear in the typical location?
Yes. The upper third of the tibia is the classical site, accounting for 25% of cases.
MRI demonstrates the lesion to have very high T2 signal, low T1 signal with diffuse somewhat heterogeneous contrast enhancement. There is no surrounding oedema (not in bone nor in the adjacent soft tissues).