PMC6180815_01
CT image shows (a) a well-limited lytic lesion of the posterior elements of L2 with sclerosis that indicates an inflammatory reaction in the vertebral body. MRI shows the peritumoral oedema, (b) sagittal T
2 weighted, (c) axial T
1 weighted and (d) T
2 weighted images, with hypointense signal on T
1 weighted images and hyperintense signal on T
2 weighted images.
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T 1 with a mildly increased T 2 signal (Figure 1). The patient was treated with curettage and the histological diagnosis was OBL.
Microscopic examination shows an osteoblastoma: irregular woven bone, ectatic blood vessels, osteoblasts and giant cells are evident (haematoxylin and eosin, magnification 250x).
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At the time of our examination, we reviewed the histological slides of the previous diagnosis and the diagnosis of an OBL was confirmed (Figure 2).
CT image of the knee shows the osteoid osteoma with a very small and calcified nidus.
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The CT scan showed a small nidus without sclerosis and the MRI confirmed the nidus with an inflammation of the surrounding tissue (Figure 3).
The final position of the radiofrequency electrode is shown, with the active tip inside the nidus.
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Then, we placed the needle electrode inside the nidus (monopolar, non-refrigerated; SMK, Radionics, Burlington, MA), and RFA was performed through the same tract with a 5-mm radiofrequency probe heated to 90  C for 5 min, using the radiofrequency generator (RFG-3C; Radionics) (Figure 4).
The sample removed with the biopsy and examined under a microscope shows an osteoid osteoma. The haematoxylin and eosin staining demonstrates a very sharp circumscription of the nidus (a), magnification 100x. Trabeculae are thin and form an inter-anastomosing network with abundant cement lines and a single layer of osteoblasts. The intertrabecular space is filled with fibrovascular stroma (b), magnification 200x.
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The diagnosis was an OO (Figure 5).