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V/Q Scan Is as Accurate as CTA, With Less Radiation Exposure

February 2, 2008
Written by: , Filed in: Breast Imaging, Safety Issues
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In a recent study, V/Q scanning has been found to be as accurate as CT angiography with a fraction of radiation to the breast.

Discussion
4- and 64-slice CT angiography (CTA) deliver almost 100 times the radiation to the breast than ventilation-perfusion (V/Q) scanning, which is of particular concern in young women. It is up to the radiologist to present the clinician with diagnostically equivalent options.

While CTA may help diagnose other abnormalities not diagnosable on V/Q scanning, only some of these may be related to the patient’s symptoms, such as dissecting aneurysms or pneumothorax. PIOPED II study results do not clearly support the superiority of CTA over V/Q scanning.

In PIOPED II, overall sensitivity of CTA for the diagnosis of pulmonary embolism (PE) was 83% and the specificity was 96%, with a positive-predictive value (PPV) and a negative-predictive value (NPV) of 86% and 95%, respectively.

These are comparable to V/Q statistics, where a high probability scan has a >85% likelihood of PE and a low probability result has <20% probability. A "very low probability" category correlates with <10% PPV. Best predictive values are obtained when the diagnostic test is consistent with the pretest clinical probability. Furthermore, D-dimer assays can also be used to triage subsequent studies. A negative D-dimer in conjunction with a low-probability clinical assessment effectively rules out PE. At Montefiore, chest x-ray is used as the initial exam for PE. If normal, it is followed by V/Q exam. If abnormal, then multi-detector CTA (MDCTA) is performed. This approach relies on collaborating with chest radiologists to educate clinicians, including the fact that a negative CTA still has a 17% false-negative rate, very comparable to V/Q scanning. Following an aggressive educational blitz, the number of CTA studies for PE dropped by 27%, while V/Q studies increased by 82%. At Montefiore, CTA is available 24 hours a day. Dr Freeman is able to call in technologists for V/Q in <1 hour's time. Patients could also theoretically be observed following a single dose of low-molecular-weight heparin, which will temporize for 6 to 8 hours. Even if MDCTA is the primary method, special cases exist to perform V/Q scanning for contrast allergy, nephrotoxicity, claustrophobia, and obesity. For pregnancy, often a "rule out" scenario, Dr Freeman uses a low-dose perfusion-only study with 37 MBq of Tc-99m MAA. Baseline studies are obtained several weeks following positive CT to establish a new baseline. This is also helpful in patients with documented DVT so that any subsequent positive lung study can be determined as acute or chronic in origin. Reviewer’s Comments
It is our obligation to periodically take stock and become involved in counseling our referring physicians regarding the most appropriate studies for particular diagnoses and, in particular, patient populations.

We shouldn’t give up on the V/Q scan, as it achieves similar accuracy to MDCTA with approximately 100-fold less radiation to the breast.

Author: Lionel S. Zuckier, MD

Reference:
Freeman LM. Bury the V/Q Scan: It’s as Good as Multidetector CT Angiograms With a Lot Less Radiation Exposure. J Nucl Med; 2008; 49 (January): 5-8

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