Background: FDG-avid thyroid incidentalomas (TI) are seen in approximately 2.5% of patients imaged for staging or response assessment of malignancy and represent thyroid cancer in approximately 35% of cases. Consequently, the 2015 ATA guidelines strongly recommend investigation of all FDG-avid nodules ≥1cm with US and fine needle aspiration cytology (FNA). This study aims to assess the overall and thyroid cancer specific survival in a large cohort of patients with FDG-avid TI with long-term follow-up to assess the validity of this approach. Methods: Retrospective review of 45,680 PET/CT scans performed at a comprehensive cancer center from January 2007 to January 2015 identified 2,588 FDG PET/CT reports referring to the thyroid. After exclusion of non-avid thyroid nodules, diffuse FDG-uptake, known thyroid cancer, abnormalities adjacent to thyroid and repeat studies, 500 patients (1.1%) with TI were identified of whom 362 had confirmed death and/or > 12 months’ clinical follow-up. Variables including age, gender, primary malignancy, overall survival, thyroid cancer-specific survival, FNA and histopathology were collected until January 2016. Multivariate logistic regression and survival analysis were performed. Results: 362 analyzed patients (65% female) had median age 65-years (range 19-96) and follow-up of 24-months (range 1-103). Lymphoid, lung and colorectal malignancy were the most common staging indications. Median overall survival was 20-months (IQR 9.5-39). The majority of 180 observed deaths were due to the primary malignancy under investigation (92.2%) or non-cancer related causes (7.2%); one patient (0.6%) died from incidentally-detected medullary thyroid cancer. FDG-avidity in index malignancy, advanced stage of that malignancy and clinician decision to not investigate FDG-avid TI were all predictors of mortality with hazard ratios of 8.5 (95%CI 4.6-15.8), 3.0 (95%CI 2.3-3.9) and 3.3 (95%CI 2.0-5.0) respectively (P<0.001). Of 131 patients suitable for cytological/histopathological evaluation, 47 (36%) had incidental thyroid cancer (24 papillary, 11 malignant FNA, 5 oncocytic/Hürthle cell carcinoma, 2 medullary, 1 follicular and 4 metastases from underlying malignancy). Conclusion: Overall survival with FDG-avid TI was poor due to the prognosis associated with underlying malignancy, which must be considered prior to investigation of FDG-avid TI and certainly before aggressive treatment. Active surveillance should be considered in this group of patients.
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