European Journal of Nuclear Medicine and Molecular Imaging Vol. 31, No. 2, February 2004 Abstract. In regions with a high prevalence of granulo- matous diseases, benign inflammatory fluorine-18 flu- orodeoxyglucose (FDG) uptake in the mediastinum is frequently observed even in healthy subjects. We exam- ined parameters of mediastinal FDG uptake to determine whether they can differentiate malignancy from benign lesions. Seventy patients with non-thoracic tumours who had mediastinal uptake on FDG positron emission to- mography (PET) were included (33 males, 37 females; age 57.5±16.9 years; 168 lymph nodes). Determination of metastasis was confirmed by biopsy or computed to- mography (CT) follow-up over 12 months (metastasis, 29; benign lesions, 41). No significant difference be- tween the metastasis group and the benign group was found in terms of residual disease in the primary site (48% vs 46%), lung invasion (29% vs 20%), number of sites of uptake (2.3 vs 2.4), smoking history (30.3% vs 46.3%) or bilateral uptake (52% vs 54%). Maximal stan- dardised uptake values (SUVs) in the mediastinal metas- tasis group were higher (4.9±1.8) than those in the be- nign group (2.5±0.9) (P<0.05). Using 3.4 as a cut-off value for maximal SUV, a sensitivity of 86% and a spec- ificity of 85% were achieved (AUC=0.917). Maximal SUV showed better predictive value than lymph node size measured on chest CT (P<0.05). In 8 of 51 normal subjects who underwent FDG PET as a routine check- up, mediastinal FDG uptake was observed. Maximal SUV in normal subjects was 2.5±0.8, which was similar to that in the benign group. In conclusion, maximal SUV was identified as a significant parameter for determining whether mediastinal FDG uptake represents malignant metastasis. When maximal SUV exceeded 3.4, the me- tastasis rate was high regardless of lymph node size. Keywords: FDG – Positron emission tomography – Neoplasm – Mediastinum – Lymph nodes Eur J Nucl Med Mol Imaging (2004) 31:202–207 DOI 10.1007/s00259-003-1368-x Introduction Fluorine-18 fluorodeoxyglucose positron emission to- mography (FDG PET) has been widely used in cancer patients for diagnosis, staging and therapeutic monitor- ing [1]. Mediastinal lymph nodes are the regional nodes of thoracic tumours, such as lung or oesophageal cancer, and the presence of mediastinal FDG uptake represents lymph node invasion. The accuracy of FDG PET for the nodal staging of lung cancer [2, 3, 4] or oesophageal cancer [5, 6] has been established. Although non-thoracic tumours have a lower inci- dence of mediastinal metastasis than thoracic tumours, the existence of mediastinal FDG uptake in patients with non-thoracic tumours could be interpreted as indicative of distant metastasis. However, benign inflammatory FDG uptake in mediastinal lymph nodes is frequently observed even in normal subjects, especially in regions with a high prevalence of granulomatous disease [7]. Therefore, mediastinal FDG uptake in patients with non- thoracic tumours is difficult to interpret, and this be- comes a major problem when analysing PET images. The conventional imaging method for evaluating medi- astinal lymph nodes is chest computed tomography (CT), but size-dependent methods like CT have several limita- tions, and the sensitivity and specificity of chest CT for differentiating mediastinal lymph nodes are not satisfac- tory. Therefore, a guideline is needed for differentiating metastatic FDG from benign FDG uptake in patients with non-thoracic tumours. In this study, we investigated the characteristics of mediastinal FDG uptake observed in patients with non- thoracic tumours and attempted to identify predictors of mediastinal FDG uptake that could differentiate malig- June-Key Chung ( ✉ ) Department of Nuclear Medicine, Cancer Research Institute, Seoul National University College of Medicine, 28 Yeongeon-dong, 110-744 Jongno-gu, Seoul, Korea e-mail: jkchung@plaza.snu.ac.kr Tel.: +82-2-7603376, Fax: +82-2-7457690 Original article Differentiation of mediastinal FDG uptake observed in patients with non-thoracic tumours Won Jun Kang, June-Key Chung, Young So, Jae Min Jeong, Dong Soo Lee, Myung Chul Lee Department of Nuclear Medicine, Cancer Research Institute, Seoul National University College of Medicine, Jongno-gu, Seoul, Korea Received: 20 March 2003 / Accepted: 20 September 2003 / Published online: 19 November 2003 © Springer-Verlag 2003