To assess the safety and efficacy of MR guided Focused Ultrasound (MRgFUS) as a non‐invasive treatment modality for osteoid osteoma.Written informed consent was obtained from all subjects. Ablations were performed on the focused ultrasound MR table. Sonication planning was managed by the treating physician. Thermal dose maps were monitored to confirm heating in the lesion nidus. Validated pain and quality of life scales (VAS, SDS, BPI‐QOL, TESS, and PEDS‐QL) were obtained prior to the procedure on the day of treatment as well as one month post procedure.Four patients were treated, two males and two females, with a median age of 20 years (range 18 to 28 years). Two lesions were located in the left tibia, one in the left femur, and one in the fifth metatarsal of the right foot. Lesions were either cortical or subperiosteal in location; no intramedullary lesions were targeted (Fig. 1). The average maximal nidus diameter was 7.25 mm (range 6 to 8 mm). Patients demonstrated periostitis, bone marrow edema, and osseous sclerosis on pre‐procedure imaging (Fig. 2a-c). The median thickness of the sclerotic rim around the nidus was 3.4 mm (range 3 - 5.5 mm). The ExAblate 2100 transducer was operated at 0.95‐1 MHz. Treatment time was 50.5 minutes (range 35 ‐ 68 minutes). An average of 12.75 ± 3.5 sonications were used for each treatment (range 12 - 23). Mean sonication spot energy was 1625.5 J (range from 498 – 3222 J). Sonication duration varied between 20 or 40 s. The averages of the values of temperatures achieved in the soft tissue immediately adjacent to the targeted bone during individual sonications were 61^·C (average) and 77^·C (maximum). MR thermometry demonstrated thermal dose in the nidus (Fig. 3). The nidus appeared devascularized immediately after the procedure (Fig. 2d). All patients had resolution of pain by one month. Pain scores before treatment averaged 7.25 ± 2.2 out of 10 (range 5 - 10), significantly improving after treatment to 0 ± 0 (p = 0.007). Patients terminated use of analgesic medications. Quality of life scores all improved to normal values. No serious adverse events occurred.

MR guided focused ultrasound is used for precise osseous tumor ablation, performed using focused ultrasound energy, combined with MRI for tumor targeting, real-time intra-operative temperature monitoring, and post-procedure treatment verification. MRgFUS has been approved by the United States Food and Drug Administration for the treatment of painful osseous metastases¹. The mechanism of pain relief in this application is through periosteal denervation².

MRgFUS has been used to treat benign bone tumors as well, including osteoid osteomas^3,4. In the larger of these two prospective studies⁴, MRgFUS was shown to achieve complete pain relief in 90% (26 of 29) patients treated. This compares favorably with the clinical success rate of 85-98% for CT guided radiofrequency ablation (RFA), the current standard treatment for osteoid osteomas. The target for MRgFUS treatment of osteoid osteomas is devascularization of the nidus; this allows the use of a few sonications targeting the nidus, decreasing the treatment time and increasing the safety of the treatment. Overall, the safety profile of MRgFUS in the treatment of bone tumors has been excellent^1,3,4; therefore, MRgFUS offers an advantage over RFA for the treatment of osteoid osteomas, as MRgFUS avoids the use of radiation and the drilling required for percutaneous positioning of the RFA probe. A trial directly comparing MRgFUS and RFA is needed to establish MRgFUS as an alternative to RFA for treatment of osteoid osteomas.

MRgFUS is a promising non‐invasive treatment modality that involves no radiation exposure, which may be of particular benefit in the younger patient population afflicted by osteoid osteoma. This series is the largest in North America demonstrating the potential of MRgFUS for treating osteoid osteoma. This preliminary data forms the basis for a proposed Phase 3 clinical trial comparing MRgFUS to RFA, which will establish relative efficacy and safety.