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LETTERS TO THE EDITOR

severely calcified lesions (cross-sectional arc calcium >180°) yield low diagnostic accuracy as well as poor interobserver agreement for quantification. We agree with Dr Du that such patients should be regarded as positive for further analysis if this was a prospective study. However, inclusion of severely calcified lesions, which are currently not feasible for quantification, would no doubt have led to low diagnostic accuracy in our retrospective study. Therefore, we decided to drop those data and outlined the limitation of clinical application in severely calcified lesions accordingly. The application of the Poiseuille equation definitely has limitations, as pointed out by Dr Du. However, the main aim of our study was to preliminarily explore the feasibility of using this relatively simple morphologic index to predict the hemodynamic status of coronary stenosis. This index is easy to calculate and not limited by imaging protocols. Other equations or more sophisticated methods such as CT-derived fractional flow reserve and transluminal attenuation gradient might have better performance but need either complicated calculation or single-heart-beat acquisition (3,4). Finally, we agree with Dr Du that patient-based analysis would strengthen the conclusion of this study. Therefore, we would like to include both patientbased and lesion-based analysis to test the diagnostic performance in our future prospective research. Disclosures of Conflicts of Interest: No relevant conflicts of interest to disclose.

References 1. Vavere AL, Arbab-Zadeh A, Rochitte CE, et al. Coronary artery stenoses: accuracy of 64– detector row CT angiography in segments with mild, moderate, or severe calcification— a subanalysis of the CORE-64 trial. Radiology 2011;261(1):100–108. 2. Li Y, Zhang J, Lu Z, et al. Discrepant findings of computed tomography quantification of minimal lumen area of coronary artery stenosis: correlation with intravascular ultrasound. Eur J Radiol 2012;81(11):3270–3275. 3. Min JK, Berman DS, Budoff MJ, et al. Rationale and design of the DeFACTO (determination of fractional flow reserve by anatomic

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computed tomographic angiography) study. J Cardiovasc Comput Tomogr 2011;5(5):301– 309. 4. Steigner ML, Mitsouras D, Whitmore AG, et al. Iodinated contrast opacification gradients in normal coronary arteries imaged with prospectively ECG-gated single heart beat 320– detector row computed tomography. Circ Cardiovasc Imaging 2010;3(2):179–186.

Rib Fractures after Radiofrequency and Microwave Ablation of Lung Tumors From Koichiro Yamakado, MD, PhD, Haruyuki Takaki, MD, PhD, and Atsuhiro Nakatsuka, MD, PhD Department of Interventional Radiology, Mie University Hospital, 2-174 Edobashi Tsu, Mie 514-8507, Japan e-mail: [email protected] Editor: We read with interest the recent article by Alexander and colleagues in the March 2013 issue of Radiology (1) concerning the incidence and relevance of rib fractures after lung radiofrequency (RF) ablation and microwave ablation. The results showed that the incidence of rib fracture was 13.5% after lung RF and microwave ablation, and it was as high as 15.9% after lung RF ablation. Female sex, tumors closer to the chest wall, and ablative zones closer to the visceral pleura were significant factors for rib fracture. We have concerns about these results. We have already reported adverse events following 1000 lung RF sessions in 462 patients (2), but in that study we did not find rib fractures in any patient. This discrepancy might be attributed to the difference in the way the RF electrodes were placed. We place the RF electrode to avoid injuring (heating up) the pleura or chest wall as much as possible by taking into account the entry site, puncture route, and estimated ablative zone. One possible explanation for the occurrence of rib fractures is bone necrosis caused by high temperature during RF and microwave ablation. However, given that bone fracture is a rare adverse event

even in bone RF ablation (3), the incidence of rib fractures described in this study seems to be very high to just be caused by local heating and bone necrosis. Another explanation for the rib fractures might be mechanical injury caused by the RF electrode or microwave antenna. Although the difference was not significant, the incidence of rib fractures tended to be higher in patients in whom cluster electrodes were used. Was there a difference in the incidence of rib fractures between patients in whom cluster electrodes were used and those in whom single RF electrodes were used? Women might be more vulnerable to mechanical injury owing to narrow intercostal spaces and thin ribs. Disclosures of Conflicts of Interest: K.Y. No relevant conflicts of interest to disclose. H.T. No relevant conflicts of interest to disclose. A.N. No relevant conflicts of interest to disclose.

References 1. Alexander ES, Hankins CA, Machan JT, Healey TT, Dupuy DE. Rib fractures after percutaneous radiofrequency and microwave ablation of lung tumors: incidence and relevance. Radiology 2013;266(3):971–978. 2. Kashima M, Yamakado K, Takaki H, et al. Complications after 1000 lung radiofrequency ablation sessions in 420 patients: a single center’s experiences. AJR Am J Roentgenol 2011;197(4):W576–580. 3. Dupuy DE, Liu D, Hartfeil D, et al. Percutaneous radiofrequency ablation of painful osseous metastases: a multicenter American College of Radiology Imaging Network trial. Cancer 2010;116(4):989–997.

Response From Erica Alexander, BS, and Damian Dupuy, MD Department of Diagnostic Imaging, Warren Alpert Medical School of Brown University, Rhode Island Hospital, 593 Eddy St, Providence, RI 02903 e-mail: [email protected] We appreciate Dr Yamakado and colleagues’ letter in response to our article. Of note, our study provided extensive follow-up for the 163 pa-

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tients in our study, with the average length of follow-up imaging being 20.0 months. The earliest identified fracture occurred 94 days after ablation, with no fractures at immediate postprocedure computed tomographic fluoroscopy. The relatively long time frame for fractures to occur makes the theory that fractures are related to mechanical force unlikely. Instead, we speculate that they are a result of osteonecrosis or “pleural tethering.” To address the point that the incidence of rib fractures following ablation seemed high, studies evaluating other focal thermal therapies have shown an even greater incidence of fracture (1,2). The length of follow-up provided by our study may explain why the other cited articles did not capture rib fractures. Kashima et al (3) report radiographs being obtained up to 7 days after RF ablation. In addition, all but two patients in our study were asymptomatic, with most fractures incidentally found. Thus, it is possible that fractures go undetected because patients lack complications or pain. Dr Yamakado and colleagues propose that they may have avoided fractures by treating away from the pleura or osseous structure. However, patients with postablation fractures were significantly more likely to have lesions close to the chest wall, meaning that appropriately sized ablation zones would undoubtedly involve these structures. To answer the question presented at the end of the letter, we did not find that the number of electrodes was a statistically significant indicator of fracture. Although women may be apt to experience a fracture due to thinner ribs, we theorized that the increase might result from the greater incidence of osteoporosis in women. Disclosures of Conflicts of Interest: E.A. No relevant conflicts of interest to disclose. D.D. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: received grants from Neuwave Medical; received personal fees from BSd Medical, Perfint Tech, and Covidien. Other relationships: none to disclose.

References 1. Pettersson N, Nyman J, Johansson KA. Radiation-induced rib fractures after hypofractionated stereotactic body radiation therapy of non–small cell lung cancer: a dose- and volume- response analysis. Radiother Oncol 2009;91(3):360–368. 2. Voroney JPJ, Hope A, Dahele MR, et al. Chest wall pain and rib fracture after stereotactic radiotherapy for peripheral non–small cell lung cancer. J Thorac Oncol 2009; 4(8):1035–1037. 3. Kashima M, Yamakado K, Takaki H, et al. Complications after 1000 lung radiofrequency ablation sessions in 420 patients: a single center’s experiences. AJR Am J Roentgenol 2011;197(4):W576–580.

Coronary CT Angiography Cannot be Recommended in Patients with Atrial Fibrillation From Georg M. Schuetz, MD,* Peter Schlattmann, MD, PhD,†‡ and Marc Dewey, MD, PhD* Department of Radiology,* and Institute of Medical Statistics,† CharitéUniversitätsmedizin Berlin, Charitéplatz 1, 10117 Berlin, Germany e-mail: [email protected] Department of Medical Statistics, Informatics and Documentation, University Hospital of Friedrich Schiller University Jena, Jena, Germany‡ Editor: It was with great interest that we read the systematic review study in the May 2013 issue of Radiology by Drs Vorre and Abdulla (1) about the diagnostic accuracy of coronary computed tomographic (CT) angiography in patients with atrial fibrillation. Only seven eligible primary studies including 247 patients with atrial fibrillation and comparing CT with conventional coronary angiography were available. This limits the representativeness and generalizability of the review. Moreover, we believe that the authors’ conclusion that CT has high diagnostic accuracy in patients with atrial fibrillation may represent an overinterpretation of their

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data and that too much “spin” (2,3) is given to that conclusion and the “Implication for Patient Care,” in which it is stated that CT is useful for excluding coronary artery disease in such patients. The small number of studies and patients prospectively included in comparison studies is a limitation that should not lead to such optimistic conclusions. The clinically commonly encountered nondiagnostic coronary angiography test results with CT in patients suspected of having coronary artery disease and atrial fibrillation limit the application further and have resulted in appropriate use criteria rendering this indication clearly as inappropriate (4). Nondiagnostic test results are commonly encountered but infrequently and only variably reported (5). For this meta-analysis, Drs Vorre and Abdulla used the reported diagnostic accuracy data, although different approaches of including nondiagnostic test results were applied in the primary studies. For example, patients with nondiagnostic segments were excluded in the largest primary study by Yang et al (6) but also declared as positive in other studies (eg, the study by Bettencourt et al [7]). We have recently shown in a meta-analysis that including nondiagnostic test results if available in the calculation of diagnostic accuracy using a 3 × 2 table instead of a 2 × 2 table results in significantly reduced test performance that likely better resembles the clinical utility of the diagnostic test and indication under investigation (5). Disclosures of Conflicts of Interest: G.M.S. Financial activities related to the present article: receives money from the German Federal Ministry of Education and Research. Financial activities not related to the present article: none to disclose. Other relationships: none to disclose. P.S. Financial activities related to the present article: received a grant from the German Federal Ministry of Education and Research. Financial activities not related to the present article: received a grant from the German Science Foundation. Other relationships: none to disclose. M.D. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: is a paid consultant for Guerbet; institution has grants/grants pending from the European Regional Development Fund, the German Heart Foundation/Ger-

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Rib fractures after radiofrequency and microwave ablation of lung tumors.

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