Initial Size of Metastatic Lesions Is Best Prognostic Factor in Patients with Metastatic Differentiated Thyroid Carcinoma Confined to the Lung

Background: For patients with lung metastases of differentiated thyroid carcinoma (DTC), there is no consensus on the maximal size of metastatic lesions to use when determining the intensity of follow-up and additional therapeutic options. This study evaluated the clinical outcomes and survival of patients with metastatic DTC confined to the lung, using the maximal diameter of lung lesions in the initial computed tomography. Methods: This retrospective cohort study included 112 DTC patients with metastases confined to the lung. The clinical responses were evaluated according to changes in the serum levels of stimulated thyroglobulin or antithyroglobulin antibody, disease status was evaluated according to radiological findings, progression-free survival (PFS), and cancer-specific survival (CSS). Results: Macronodular lung metastases (>= 1 cm) were observed in 27 (24%) patients, and these patients had significantly poor biochemical responses and disease status (p < 0.001, and p < 0.001, respectively), irrespective of radioactive iodine (RAI) avidity. After adjusting for age, sex, primary tumor size, extrathyroidal invasion, cervical lymph node metastasis, time of lung metastasis, and RAI avidity, the macronodular group also had shorter PFS and CSS (p = 0.009 and p = 0.03, respectively) than the micronodular group. From the multivariate analyses, RAI avidity was not an independent prognostic factor predicting PFS and CSS. In the subgroup analyses, RAI avidity was a significant prognostic factor associated with better PFS and CSS (p = 0.013 and p = 0.021, respectively) in the micronodular group only. Conclusions: The initial largest diameter of metastatic lesions is the most important prognostic factor for predicting poor clinical outcomes and survival in patients with metastatic DTC confined to the lung.