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This copy is for personal use only. To order printed copies, contact reprints@rsna.org Malignancies with Low Fluoro- deoxyglucose Uptake at PET/CT: Pitfalls and Prognostic Importance 1 Abbreviations: FDG = fluorine 18 fluoro- deoxyglucose, HCC = hepatocellular carci- noma, SUV = standardized uptake value RadioGraphics 2016; 36:293–294 Published online 10.1148/rg.2015150073 Content Codes: From the Department of Radiology and Bio- medical Imaging, University of California–San Francisco, 505 Parnassus Ave, San Francisco, CA 94143. Recipient of a Certificate of Merit award for an education exhibit at the 2014 RSNA Annual Meeting. Received March 19, 2015; revision requested August 28 and received October 3; accepted October 23. All authors have disclosed no relevant relationships. Ad- dress correspondence to S.C.B. (e-mail: spen- cer.behr@ucsf.edu). Funding: The work was the National Institutes of 5T32EB001631-10). supported by Health (grant R.R.F. supported by an RSNA Research Fel- low Grant and a Society of Nuclear Medicine and Molecular Imaging–Educational Research Foundation Mitzi & William Blahd, MD, Pilot Research Grant. RSNA, 2016 The full digital presentation is available online. Fluorine 18 fluorodeoxyglucose (FDG) positron emission tomography (PET) is commonly performed for staging and restaging of solid tu- mors. Although most solid tumors demonstrate high uptake of FDG, many others do not. Low FDG uptake may be due to various reasons, including tumors with low glucose metabolism or low cellularity, improper patient preparation, and small tumor size. The presence of low-level FDG uptake could be a source of scan misinterpretation in these low-cellularity or low-glucose-metabolizing tumors, including low-grade lung adenocarcinomas, renal cell cancers, and mucinous neoplasms. The ability to detect lesions at PET/computed tomography (CT) stems from many factors, including size of the lesion, ability of the tumor to concentrate FDG, proper patient preparation, back- ground FDG uptake in surrounding tissues, and type of scanner used. Several examples of low-grade lung adenocarcinoma, renal cell cancer, and mucinous neoplasms are presented that have low FDG uptake. For example, Figure 1 depicts a renal cell cancer without associated FDG avidity above background activity. In many neoplasms, including hepatocellular carcinoma (HCC), lymphoma, and prostate cancer, there is strong evidence that increas- ing FDG avidity correlates with poor prognosis and poor response to treatment. In these cases, high FDG uptake likely correlates with de- differentiation or transformation to a more aggressive form of cancer. For example, in HCC, high FDG uptake predicts poor response to radiation therapy, transarterial chemoembolization, and liver trans- plantation and is also associated with higher stage and the presence of metastatic disease. Similarly, lesions with high FDG uptake in a patient with a known low-grade lymphoma are suspicious for high- grade transformation (also called Richter transformation). Therefore, in lymphoma, prostate cancer, and HCC, it is important for radiolo- gists to report the degree of FDG uptake. TEACHING POINTS Some malignancies will demonstrate low-level or absent FDG uptake, including renal cell cancer, low-grade lung adenocarcinomas, and mucinous neoplasms. In some malignancies, including HCC, prostate cancer, and low-grade lymphomas, the presence of high-level FDG uptake correlates with poor prognosis. In general, higher FDG uptake in these malignancies correlates with a poorly differenti- ated neoplasm that will have a relatively poor treatment response. In some malignancies, uptake of a second radiotracer is typically inversely correlated with uptake of FDG. This property is termed the flip-flop effect and is commonly seen in thyroid cancer and neuroendocrine tumors. RESIDENT AND FELLOW EDUCATION FEATURE Robert R. Flavell, MD, PhD David M. Naeger, MD Carina Mari Aparici, MD Randall A. Hawkins, MD, PhD Miguel H. Pampaloni, MD, PhD Spencer C. Behr, MD NUCLEAR MEDICINE |

Malignancies with Low Fluorodeoxyglucose Uptake at PET/CT: Pitfalls and Prognostic Importance:Resident and Fellow Education Feature