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Objective: To report on the particular imaging features and high success rate of cold mode radio-frequency thermal ablation (RFTA) as the treatment of choice for intramedullary osteoid osteoma., Materials and Methods: The study population consisted of 51 patients (39 males, 12 females; mean age 7.2 years; 11 patients under 6 years of age, including 7 males and 4 females) who underwent RFTA for osteoid osteoma and were retrospectively observed. The affected sites were the tibia (n = 22, 43%), femur (n = 13, 25%), pelvis (n = 5, 10%), anklebone (n = 3, 6%), humerus (n = 2, 4%), sacrum (n = 2, 4%), heel, radium, patella ,and rib (n = 1, 2%), respectively. Three patients had tibial intramedullary osteoid osteoma (14% of the tibial lesions, 6% of all cases). Cold mode RFTA was performed for these three patients to obtain a large ablation area without positioning two probes. The noncooled mode was used to treat cortical and subperiosteal lesions., Results: Following RFTA, all patients were pain-free and in good clinical condition. In the intramedullary osteoid osteoma group, no recurrences were observed during the 24-month follow-up period, but one patient, who was affected by cortical osteoid osteoma, required two RFTA treatments to heal completely., Conclusion: Children less than 6 years of age with recurrent nocturnal pain and limb swelling should be investigated for intramedullary osteoid osteoma. Once confirmed, CT-guided RFTA should be the first treatment for intramedullary osteoid osteomas because of the high success rate and reduced invasivity, especially with cold mode RFTA. The outcome is related to the disappearance of pain, and the efficacy may be checked shortly after treatment with MR imaging to evaluate the absence of lesion in the ablation area.

Objective: To compare the healing pattern of osteoid osteomas on computed tomography (CT) and magnetic resonance imaging (MRI) after successful and unsuccessful thermocoagulation., Materials and Methods: Eighty-six patients were examined by CT and 18 patients by dynamic gadolinium-enhanced MRI before and after thermocoagulation for osteoid osteoma. Thermocoagulation was successful in 73% (63/86) and unsuccessful in 27% (23/86) of patients followed by CT. Thermocoagulation was successful in 72% (13/18) of patients followed by MRI. After treatment, the healing of the nidus on CT was evaluated using different healing patterns (complete ossification, minimal nidus rest, decreased size, unchanged size or thermonecrosis). On MRI the presence of reactive changes (joint effusion, "oedema-like" changes of bone marrow and soft tissue oedema) and the delay time (between arterial and nidus enhancement) were assessed and compared before and after thermocoagulation., Results: Complete ossification or a minimal nidus rest was observed on CT in 58% (16/28) of treatment successes (with > 12 months follow-up), but not in treatment failures. "Oedema-like" changes of bone marrow and/or soft tissue oedema were seen on MR in all patients before thermocoagulation and in all treatment failures. However, residual "oedema-like" changes of bone marrow were also found in 69% (9/13) of treatment successes. An increased delay time was observed in 62% (8/13) of treatment successes and in 1/5 of treatment failures., Conclusion: Complete, or almost complete, ossification of the treated nidus on CT correlated with successful treatment. Absence of this ossification pattern, however, did not correlate with treatment failure. CT could not be used to identify the activity of the nidus following treatment. The value of MR parameters to assess residual activity of the nidus was limited in this study.

The benign bone lesions--osteoma, osteoid osteoma, and osteoblastoma--are characterized as bone-forming because tumor cells produce osteoid or mature bone. Osteoma is a slow-growing lesion most commonly seen in the paranasal sinuses and in the calvaria. When it occurs in the long bones, it is invariably juxtacortical and may need to be differentiated from, among others, parosteal osteosarcoma, sessile osteochondroma, and a matured juxtacortical focus of myositis ossificans. Osteoid osteoma and osteoblastoma appear histologically very similar. Their clinical presentations and distribution in the skeleton, however, are distinct: osteoid osteoma is usually accompanied by nocturnal pain promptly relieved by salicylates; osteoblastoma arises predominantly in the axial skeleton, spinal lesions constituting one-third of reported cases. This review focuses on the application of the various imaging modalities in the diagnosis, differential diagnosis, and evaluation of these lesions. Their histopathology also is discussed, and their treatment briefly outlined.