Management algorithm and clinical evaluation of incidental thyroid lesions detected in 18F-FDG PET / CT

Introduction: Today, 18F-FDG is widely used for PET / CT assessment of the cancer stage, reassessment and evaluation of treatment response. As a result, the incidence of incidental thyroid lesions is increasing, and this situation becomes clinically important Purpose: We aimed to evaluate the approach algorithm, clinical significance, and malignancy risk of focal or diffuse 18F-FDG involvement detected incidentally in the thyroid gland on 18F-FDG PET / CT images displayed in our oncology clinic. Methods: Seventy-one patients who underwent 18-F-FDG PET / CT for the evaluation of non-thyroid malignancies between January 2015 and October 2017 who had focal, diffuse or focal-diffuse FDG uptake in the thyroid gland with previously unknown primary thyroid pathology, were retrospectively evaluated and included in the study. Thyroid function tests were conduced in all patients. 69 patients underwent USG. Thyroid scintigraphy was ordered in patients with thyrotoxicosis. Definitive diagnosis was achieved by fine needle aspiration biopsy and / or total thyroidectomy. Findings: 34 (47.9%) of 71 patients were male and 37 (52.1%) were female. Mean age of the patients was 60.2 ± 11.3. Focal FDG uptake was detected in 60 (84.5%) of these patients, 6 (8.5%) patients had diffuse, and 5 (7.0%) patients had focal-diffuse uptake. Malignancy was detected in 26.3% of patients with focal involvement and in 2 patients with focal-diffuse involvement. Malignancy was not detected in patients with diffuse involvement pattern. 69 patients were evaluated with USG. There was a statistically significant correlation between high risk sonographic characteristics and malignancy (p = 0,036). On the other hand, the relationship between nodule size in USG and benign or malignant cytology results was not significant. In conclusion, the incidence of malignancy of incidentally detected thyroid lesions in 60 patients with fine needle aspiration biopsy and / or total thyroidectomy was 26.7%. Of 12 patients with malignant involvement, 8 patients had thyroid papillary carcinoma (66.7%), 4 patients had metastatic involvement (33.3%), in which 3 patients had lung cancer metastasis and 1 patient had laryngeal cancer metastasis. SUVmax1 was significantly higher in patients with malignancy (p = 0.03), but the relationship between SUVmax2 value and the likelihood of malignancy was not statistically significant. (p = 0.49 and p>0.05 respectively). For the cut-off value of SUVmax value of 7.0, sensitivity and specificity for this value were calculated as 75% and 54.8% respectively. Incidentally, a patient with focal FDG uptake had benign fine needle aspiration biopsy, however, total thyroidectomy revealed papillary thyroid carcinoma. Conclusion: Although thyroid incidentalomas by 18FDG-PET / CT are relatively rare, malignancy is highly likely to be detected. Although focal involvement and higher SUVmax values were predictors of malignant lesions, it is not possible to definitively diffentiate malignant and benign lesions. Further studies are needed to determine thyroid incidentaloma management algorithms and a valid SUVmax cut-off. Since a definite threshold value can not be determined for 18FDG-PET / CT, it is necessary to correlate thyroid incidentalomas with USG and if necessary, scintigraphy, and confirm possible malignant cases with fine needle aspiration biopsy. The effect of thyroid disease should be considered in the treatment plan and life expectancy of the primary disease should be considered in these patients.