Likelihood of Diagnosing PE in SLE Decreases with Increasing Number of Scans

The likelihood of a positive result from computed tomographic pulmonary angiography (CTPA) scans performed to rule out pulmonary embolism (PE) decreases in patients with systemic lupus erythematosus (SLE) who have had 3 or more previous scans, according to a study published in Arthritis, Care & Research.¹

Patients with SLE have an increased risk of venous thromboembolic disease, in part due to antiphospholipid syndrome (APS), an autoimmune disorder characterized by arterial, venous, or small vessel thrombosis.²

Given this increased risk, patients presenting for evaluation of chest pain, pleuritis, or dyspnea often require diagnostic workups to rule out PE, including obtaining arterial blood gases, serum D-dimer, and brain natriuretic peptide levels, and, depending on patient presentation, CTPA. The modified Wells or Geneva score can be used to determine the pretest probability of PE prior to making the decision to order CTPA,³ given the increased radiation exposure associated with these scans.

Ionizing radiation from medical imaging has risen in the United States.⁴ The cumulative effect of individual doses of radiation in patients undergoing repeated imaging presents a public health concern. The linear no-threshold model is a dose-response model that correlates radiation exposure with the risk of cancer development. It assumes that any exposure to ionizing radiation can increase future risk of malignancy.⁵

To assess the utility of repeated CTPA scans in diagnosing PE in patients with SLE, Ruba Kado, MD, and colleagues from the University of Michigan, Ann Arbor, reviewed patient records from the University of Michigan Lupus Cohort from 1996 to 2014 and analyzed the rate of scan positivity as a function of the number of scans performed in a single patient.¹

The approximate radiation absorption in breast and lung tissue of patients in the cohort was then calculated using the Radimetrics Enterprise Platform, which estimates the effective radiation dose associated with specific radiographic imaging.¹

The researchers then used data from the the BEIR VII report to estimate the lifetime risk of breast and lung cancer per age per 100 mGy of radiation exposure.⁶ Men were excluded from breast cancer risk analysis because data on effects of radiation on breast cancer rates in men are still sparse.

The estimated effective radiation dose was calculated for 854 study participants in the cohort. All participants had received a diagnosis of SLE in accordance with the American College of Rheumatology criteria and had received CTPA to rule out PE. The overall rate of diagnosis of a PE on CTPA was 7.5%.

The likelihood of diagnosing a PE on CTPA decreased proportionately to the number of scans previously performed. Whereas the rate of PE diagnosis on CTPA was 8.8% in patients who had undergone CTPA 1 to 3 time, it was 1.6% in patients who had had 4 or more CTPA scans.

The authors note that this diagnostic trend “could be related to clinical characteristics that tend to improve with longer disease duration in SLE patients, such as proteinuria, disease activity, lipid profile, and smoking cessation . . . or successful anticoagulation” in patients with positive scans. They also commented that the risk of PE in SLE has been suggested to have a bimodal distribution.

Summary and Clinical Applicability

Retrospective data analysis of this SLE cohort showed that the likelihood of diagnosing PE on CTPA drastically decreased in patients receiving 3 or more scans.¹

The results of this study are limited by the estimations of malignancy risk associated with radiation using the linear no-threshold model. Additionally, use of the Radimetric system may overestimate organ-effective doses received,¹ thereby magnifying the risks associated with radiation.

The authors note that despite these findings, the absolute increase in risk of cancer from a single computed tomography scan performed to rule out PE probably is of an order of magnitude less than that of the mortality associated with failure to detect an acute PE. They advise against deferring CTPA if clinical indications point to the diagnosis of PE, even if the patient has a history of 4 or more prior scans.

Reference

1. Kado R, Siegwald E, Lewis E, et al. Utility and Associated Risk of Pulmonary Embolism Computed Tomography Scans in the Michigan Lupus Cohort. Arthritis Care Res (Hoboken). 2016;68(3):406-11.

2. Kinuya K, Kakuda K, Matano S, et al. Prevalence of deep venous thrombosis in the lower limbs and the pelvis and pulmonary embolism in patients with positive antiphospholipid antibodies. Ann Nucl Med. 2001;15(6):495-7.

3. Gruettner J, Walter T, Lang S, et al. Importance of Wells score and Geneva score for the evaluation of patients suspected of pulmonary embolism. In Vivo. 2015;29(2):269-72.

4. Smith-Bindman R, Miglioretti DL, Johnson E, et al. Use of diagnostic imaging studies and associated radiation exposure for patients enrolled in large integrated health care systems, 1996-2010. JAMA. 2012;307:2400-2409.

5. Evaluation of the linear nonthreshold dose-response model for ionizing radiation. Report No 136. National Council on Radiation Protection and Measurements, Bethesda, MD. Source Code.

6. National Research Council of the National Academies of Sciences, Engineering, and Medicine. Health risks from exposure to low levels of ionizing radiation: BEIR VII Phase 2. Washington DC; National Academies Press: 2006.