Unsuspected pulmonary emboli in oncology patients undergoing routine computed tomography imaging.

Pulmonary embolism is an increasingly recognized occurrence in asymptomatic patients.1–4 Although the clinical guidelines for the treatment of symptomatic patients with acute pulmonary emboli, based on several large multicenter trials,5 reduces the risk of fatality to approximately 2%, the details regarding risk, prevalence, usefulness of anticoagulation therapy, and outcome in patients with asymptomatic emboli are less well defined. The study by Browne et al.,6 reported in this issue of the Journal of Thoracic Oncology, adds to the growing body of literature addressing the prevalence and clinical significance of pulmonary emboli in asymptomatic patients. The authors focus on a group of patients at higher risk for pulmonary emboli than the general population. Specifically, the authors report their experience in detecting pulmonary emboli in asymptomatic oncology patients, and their study contributes to a better understanding of the prevalence, predisposing factors, and outcomes of unsuspected pulmonary emboli. Browne et al. prospectively evaluated 407 patients during a 10-month period for the presence of unsuspected/asymptomatic pulmonary embolism and the associated risk factors, and 6-month clinical outcome in those patients with emboli. Browne et al. observed a 4.4% prevalence of unsuspected pulmonary embolism, 6.4% among inpatients, and 3.4% among outpatients. These data correspond well with other reports of a 3% to 6% prevalence of incidental pulmonary emboli among oncology patients.1–4 In addition, the authors identified recent chemotherapy as the only significant predictor of unsuspected pulmonary embolism. The authors also evaluated the effect of image slice thickness on the detection of unsuspected emboli. Browne et al. visualized pulmonary emboli confidentially on 1to 1.5-mm thick images, whereas in 39% of cases these emboli were not detected on 5-mm thick images. This finding is not unexpected as the use of thinner slices is well known to improve the detection of emboli, especially segmental or smaller emboli, in symptomatic patients.7 Most of the reports on the detection of unsuspected pulmonary emboli have evaluated a slice thickness of 3.75 mm and yielded similar prevalence among oncology patients. Interestingly, a report of the prevalence of unsuspected pulmonary embolism by Cronin et al.8 (that includes some of the authors of this study), performed using thick slices, inexplicably (considering that 8-mm thick images were used), reported only a slightly lower prevalence of unsuspected pulmonary embolism of 3.3%. The distribution of pulmonary emboli reported by Browne et al. is similar to our reported experience4 in that approximately 50% of patients had lobar or larger emboli, whereas 50% had only segmental or smaller emboli. This high prevalence of segmental or smaller emboli and their poor visualization on thick images highlights the needs for thin images in the evaluation for unsuspected pulmonary embolism. However, the optimal slice thickness for balancing the detection for small abnormalities, such as segmental and subsegmental pulmonary emboli, with radiation dose and image noise of the thinner images, has not been definitively determined. It is likely that evaluation for small pulmonary emboli on routine computed tomography studies requires a slice thickness of