Osteoblastoma


AP view showing lucent geographic lesion surrounded by sclerotic bone and buttressing of the medical cortex of the femur.	Frog lateral view showing compact benign periosteal reaction.

*Coronal T1 showing the nidus with surrounding mild decrease in signal secondary to the peritumoral edema. However, the marrow is not replaced* by tumor around the nidus.**	Coronal STIR shows the marked amount of perilesional edema characteristic of osteoid osteomas / osteoblastomas.

Axial T1 showing a faint calcified rim around the central nidus.	Axial T2

Early uptake / blood pool	Delayed

Comments:

There are three traits in this lesion that help arrive at the right diagnosis:

1. The lesion is geographic and lytic surrounded by a marked amount of sclerosis. These features are pathognomonic of a benign lesion.

2. There is a faint low-signal ring-like structure within the lucency, which strongly suggest a calcified nidus. This should have been proven by obtaining a CT-scan.

3. There is a marked amount of edema surrounding the lesion. The key is not to confuse the surrounding reactive edema with tumoral infiltration. Familiarity with MSK MR makes this differentiation somewhat straightforward, but sometimes it may be challenging. In case of doubt, an in-phase and out-of-phase sequence would probably show some signal drop off when the marrow is not infiltrated or replaced, but simply edematous.

There are only a handful of lesions with so much edema. It has been postulated that PG E, PG F1 alpha and thromboxane B may have a role in producing this edema, which is commonly seen in osteoid osteomas / osteoblastomas, chondroblastomas and, ucommonly, giant-cell tumors. Evidently Brodie's abscesses are to be included in the differential, and in young individuals, eosinophilic granuloma.