Cardiac mR imaging and the s pecter of double- s trand

Radiology: Volume 277: Number 2—November 2015 n radiology.rsna. org 329 A wide range of imaging modalities are available for the assessment of cardiovascular disease, including echocardiography, computed tomography (CT), nuclear imaging (single photon emission CT [SPECT]/positron emission tomography), and magnetic resonance (MR) imaging (1). The advantages and disadvantages of the various tests include certain considerations, such as the performance characteristics and appropriateness of each to detect a specific condition, availability, cost, exposure to ionizing radiation, and use of contrast agent. MR imaging, a powerful diagnostic imaging modality, utilizes electromagnetic fields of three different frequency bands: static magnetic field, time-varying gradient magnetic fields, and pulsed radiofrequency fields (2). Unlike ionizing radiation, which is an established carcinogen, the magnetic and radiofrequency fields used in MR imaging have not been clearly linked to subsequent cancer risks (3,4). Concern about potential cancer risks after cardiac MR imaging has been raised, however, by a few small studies, including a study (with 20 patients) by Fiechter et al (5) that reported increased double-strand breaks (DSBs). The finding was surprising because of the lack of evidence or mechanism for cancer risks and the small sample size, but warranted further exploration. In this edition of the journal, Brand et al (6) report results from a replication study. In their study of 45 patients who underwent cardiac MR imaging, Brand et al did not find evidence of increased DSBs in the lymphocytes of patients immediately after the procedure. The two studies used broadly similar methods (5,6). Blood was drawn before and after MR imaging, and DSBs were quantified in isolated blood lymphocytes by using immunofluorescence microscopy after staining against the phosphorylated histone variant g-H2AX. The patients were of similar age (mean ages, 50 years vs 53 years), they were undergoing procedures for similar conditions, and all examinations were performed with a 1.5-T MR imager. Brand et al (6) found that the mean number of DSBs at baseline was 0.116 per cell before MR imaging and 0.117 per cell 5 minutes after MR imaging (P = .71). Fiechter et al (5) reported that the mean number of DSBs before MR imaging was 0.143 per cell and this increased to 0.270 per cell after MR imaging. There are some differences in study design that could have affected the results, including clear exclusion criteria by Brand et al of patients who had history of lymphoma or leukemia and those who had undergone recent radiographic examination, radiation therapy, or systemic chemotherapy. Differences in timing of blood draw could also be a factor, although increased DSBs have also been reported immediately after CT scans (7,8). Brand et al also noted a surprisingly wide range of DSBs in the study by Fiechter et al in the levels both before and after MR imaging. In Brand et al, the baseline values ranged from 0.092 to 0.169 DSB per cell, whereas in Fiechter et al they were 0–0.661 DSB per cell before and 0–1.065 DSB per cell after MR imaging. The very low readings are unusual for this age range because most people accumulate damage from a variety of sources, and levels of 1 DSB per cell or more would usually be seen after ionizing radiation doses of 50 mGy or more (9). Small sample size and low power to detect small effects is likely a limitation of both studies. Small underpowered studies with positive findings are most likely to be from chance, and the effect size is likely to be exaggerated (10). Small null studies may just be underpowered (10). The International Agency for Research on Cancer has classified both extremely low frequency (ELF) magnetic 1 From the Radiation Epidemiology Branch, Division of Cancer Epidemiology & Genetics, National Cancer Institute, 6120 Executive Blvd, Bethesda, MD 20892 (A.B.d.G., R.A.K., P.R.); and Department of Cardiology, National Institutes of Health Clinical Center, Bethesda, Md (D.M.). Received September 2, 2015; final version accepted September 2. Address correspondence to A.B. d.G. (e-mail: berringtona@mail.nih.gov).