Background and Objective: Type 2 diabetes is known to increase the risk and progression of certain types of cancer. Metformin treatment of diabetic patients is reported to have beneficial effects on some cancers. We evaluated the clinical outcome of diabetic patients with differentiated thyroid cancer (DTC) according to metformin treatment. Methods: We reviewed 943 patients diagnosed with DTC after total thyroidectomy between 1995 and 2005 in a tertiary hospital. The study involved 60 diabetic patients and 210 control patients matched for age, sex, body mass index (BMI), and tumor size. Results: There were no differences in the clinicopathological features and disease-free survival (DFS) between diabetic patients and the control group over 8.9 years of follow-up. Of the diabetic patients with DTC, 35 patients (58%) were treated with metformin. There were no differences in age, sex, BMI, tumor size, antidiabetic medication, glycated hemoglobin, or C-peptide levels in metformin and nonmetformin groups. However, cervical lymph node (LN) metastasis was more prevalent in the metformin group than in the nonmetformin group (OR 3.52, p = 0.035). Among diabetic patients with cervical LN metastasis of DTC, the metformin subgroup (17.1 years) was associated with longer DFS than the nonmetformin subgroup (8.6 years) (HR 0.16, p = 0.021); metformin treatment was also associated with longer DFS in this subgroup in multivariate analysis after adjusting age, BMI, duration of diabetes, presence of tumor at resection margin, and serum thyroglobulin level at ablation (HR 0.03, p = 0.035). Conclusions: Metformin treatment is associated with low recurrence in diabetic patients with cervical LN metastasis of DTC.

Background: It infrequently occurs that cytologic diagnosis of papillary thyroid carcinoma (PTC) cannot be confirmed by histology after surgery. This phenomenon may be a false-positive cytology or a true disappearing tumor. Objectives: We evaluated patients who had consistent findings of PTC at fine needle aspiration cytology (FNAC) and no evidence of PTC in surgical specimens. Methods: Positive cytology findings and a negative histological diagnosis of PTC in the thyroid was defined as thyroid nodules with FNAC findings of PTC prior to surgery and no evidence of malignancy on histological examination of surgically removed thyroids. We retrospectively reviewed patients who underwent fine needle aspiration (FNA) and thyroidectomy in Asan Medical Center from 2004 to 2012. Results: Six patients were found who fit the definition of positive cytology findings and a negative histological diagnosis of PTC in the thyroid. The FNAC diagnosis of 6 patients was ‘malignancy' suggesting PTC according to the Bethesda system. All patients underwent thyroidectomy with central neck dissection. Three patients had reactive changes after FNA due to needle passage. Among these 3 patients, 2 had pathologically confirmed metastatic PTC in dissected lymph nodes. These 2 patients could be defined as true disappearing PTC in the thyroid after FNA. The remaining 3 patients had neither histologic alterations nor evidence of PTC in the thyroid and lymph nodes specimens. Conclusions: Both disappearing PTC and a false-positive result of FNAC should be considered in patients with positive cytology findings and a negative histological diagnosis of PTC in the thyroid.