• Francesco Sala, Annarita Dapoto, C Morzenti, Maria Cristina Firetto, Clarissa Valle, A Tomasoni, and Sandro Sironi.
• Radiologist, ASST Papa Giovanni XXIII, Bergamo.
• Br J Radiol. 2019 Nov 1; 92 (1103): 20190249.

ObjectiveThe frequency of enostosis incidentally found on CT and CT attenuation value to distinguish them from untreated osteoblastic metastases (UOM).MethodsEnostosis group: 46 polytrauma patients underwent thoracoabdominal CT. Inclusion criteria: age range 14-35 years. Exclusion criteria: cancer, previous fractures. UOM group: 20 patients with radiological diagnosis of UOM. Analyzed data: number, size, location and density of enostoses and metastases. The density was measured with the broadest possible region of interest at the center of the lesion by two radiologists independently. Receiver operatingcharacteristic analysis to determine the sensitivity and specificity, area under the curve 95% confidence intervals and cutoff values of CT density to differentiate metastases from enostoses.ResultsPatients were 28 ± 7 years old (72% males). 41 (89%) patients had 124 enostoses (2-15 mm) with an average density of 1007 ± 122 Hounsfiled unit (HU, observer1) and 1052 ± 107 (observer2). The most common sites of occurrence were the proximal femur (34%), the pelvis (22%), the acetabulum (20%), the proximal humerus (11%), the vertebrae (11%) and the rib (2%). 13 patients had 1 bone island, 8 patients had 2, 9 cases had 3 and 11 cases had more than 3 enostoses. Overall, 114 UOM were evaluated, their average density was 728 ± 163 HU (observer1) and 712 ± 178 HU (observer2). The area under the curve value of mean density to distinguish enostoses from UOM was 0,982. Using a cut-off of 881 HU for mean density, sensitivity was 98% and specificity 95%.ConclusionThe frequency of enostosis in this study is 89%. The average density identified can help to distinguish enostoses from UOM.Advances In KnowledgeWe report the exact frequency of enostosis.

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