Is 18F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography useful to discriminate metachronous lung cancer from metastasis in patients with oncological history?

BACKGROUND: Solitary pulmonary nodules detected during follow-up in patients with previous cancer history have a high probability of malignancy being either a metachronous lung cancer or a metastasis. This distinction represents a crucial issue in the perspective of "personalized medicine", implying different treatments and prognosis. Aim - to evaluate the role of 18F-Fluorodeoxyglucose (18F-FDG) Positron Emission Tomography/Computed Tomography (PET/CT) in distinguishing whether solitary pulmonary nodules are metachronous cancers or metastases and the relationship between the nodule's characteristics and their nature. METHODS: From a single-institution database, we retrospectively selected all patients with a previous cancer history who performed 18F-FDG PET/CT to evaluate pulmonary nodules detected during follow-up, ranging from 5mm to 40mm, and histologically diagnosed as malignant. RESULTS: Between September 2009 and August 2017, 127 patients (80 males; mean age=70.2±8.5years) with 127 malignant nodules were included: 103/127 (81%) metachronous cancers, 24/127 (19%) metastases. In both groups, PET/CT provided good and equivalent detection rate of malignancy (81% vs 83%). No differences between metachronous cancers and metastases were found in: patient's age (70.3±8.1years vs 69.5±9.7years), gender (males=63.1% vs 62.5%), interval between previous cancer diagnosis and nodules' detection (median time=4years vs 4.5years), location (right-lung=55% vs 54%; upper-lobes=64% vs 67%; central-site=31% vs 25%), size (median size=17mm vs 19.5mm), 18F-FDG standardized uptake value (median SUVmax=5.2 vs 5.9). CONCLUSIONS: In oncological patients, despite its high detection rate, 18F-FDG PET/CT, as well as any other clinico-anatomical features, cannot distinguish whether a malignant solitary pulmonary nodule is a metachronous lung cancer or a metastasis, supporting the need of histological differential diagnosis.