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The purpose of this study was to determine the prevalence, distribution, risk factors, and clinical outcome associated with the detection of unsuspected pulmonary emboli (PE) in routine thoracic MDCT examinations of pediatric oncology patients.We used our hospital information system to retrospectively identify all consecutively registered pediatric oncology patients 18 years old and younger who underwent contrast-enhanced thoracic MDCT for indications other than PE from July 2004 to May 2008. Two pediatric radiologists in consensus reviewed diagnostic-quality images from consecutive routine thoracic MDCT examinations for the presence and anatomic distribution of PE. The distribution of PE was assessed according to pulmonary arterial level and lobe. Clinical and radiology reports were reviewed for demographic data, type of underlying neoplasm, prospective embolus detection, risk factors, treatment, and outcome. Subgroups of patients with and without PE were compared with respect to type of neoplasm and risk factors.The study sample consisted of 468 children (249 boys, 219 girls; mean age, 9.5 +/- 5.7 years) who underwent a total of 1,002 thoracic MDCT examinations. Nine of the 468 children (1.9%) had PE, including seven with venous thromboembolism and two with tumor emboli. In these nine patients, the pulmonary arterial locations of 17 emboli were nine (53%) segmental, five (29%) lobar, two (12%) central, and one (6%) subsegmental. Classified by lobar location, six of 11 PE (55%) were in the left lower lobe, four (36%) were in the right lower lobe, and one (9%) was in the right upper lobe. PE were not detected prospectively in five of the nine patients (56%). All PE in this subgroup were solitary and located either within the segmental pulmonary arteries (four PE) or a lobar pulmonary artery (one PE). Underlying coagulation disorder (p0.001) and history of deep venous thrombosis or PE (p0.01) were risk factors for unsuspected PE. Two of nine patients (22%) with unsuspected PE died of causes not directly related to PE. The other seven patients survived, four of whom were not treated for PE.PE are unsuspected findings at nearly 2% of routine thoracic MDCT examinations of pediatric oncology patients. More than one half of PE were not detected prospectively at initial CT image interpretation, but lack of diagnosis and treatment did not appear to have a negative influence on patient outcome. Underlying coagulation disorder and a history of deep venous thrombosis or PE were risk factors for unsuspected PE.