Incidental T1 stage medullary thyroid carcinoma: The effect of tumour diameter on prognosis and therapeutic implications

OBJECTIVE: The definition of the tumour diameter of micro-medullary thyroid carcinoma (micro-MTC) is insufficient. It is controversial to perform a completion thyroidectomy immediately for incidental T1 stage MTC. DESIGN: We used the Surveillance, Epidemiology and End Results (SEER) registry to retrospectively analyze all patients with T1 stage MTC diagnosed between 2004 and 2015. The tumour diameter 1.0 and 0.5 cm were used as the cut-off points to group and analyze the differences of clinicopathological features. We analyzed the prognosis of patients with less than total thyroidectomy. METHODS: The disease-specific survival was the main outcome. Survival was estimated with Kaplan-Meier curves and Cox regression models estimated hazard ratios for tumour characteristics. RESULTS: A total of 908 patients diagnosed with T1 stage MTC in the SEER database were included. Our study found that tumour diameter 1.0 cm is a key point affecting the prognosis of T1 stage MTC patients, although patients with tumour diameter ≤ 0.5 cm had a lower rate of lymph node metastasis and no distant metastasis. Cox proportional hazard multivariate analysis showed that distant metastasis was the only risk factor for survival in patients with T1 stage MTC. Kaplan-Meier survival analysis showed that, regardless of tumour diameter, there was no significant difference between less than total thyroidectomy and total thyroidectomy in T1 stage patients. CONCLUSIONS: For incidental MTC with tumour diameter ≤ 1.0 cm and without distant metastasis, if there is no significant increase in serum calcitonin level after surgery and ret proto-oncogene (RET) gene mutation is negative, it may be not necessary to perform completion thyroidectomy immediately.