Medullary thyroid microcarcinomas (microMTCs) are medullary thyroid carcinomas (MTCs) that measure ≤1 cm in size for which there is a paucity of data on incidence, characteristics, and clinical significance.
Patients who had a diagnosis of microMTC were abstracted from the Surveillance, Epidemiology, and End Results database (1988-2007). The data were analyzed using chi-square tests, t tests, and log-rank tests; multivariate logistic regression was used to identify factors that were associated independently with lymph node metastases. Tests for diagnostic accuracy, including likelihood ratio tests and post-test probability tests, were computed to evaluate the size-specific likelihood of developing lymph node metastases among patients with microMTC.
In total, 310 patients had microMTC; its incidence increased during the study period (P(trend) = .033), and microMTC as a proportion of all MTCs increased by 39%. The mean tumor size was 5.7 mm. Thirty-one percent of tumors were multifocal, and 7.8% had extrathyroid extension. Lymph node metastases occurred in nearly 37% of patients who had any lymph nodes removed (65 of 176 patients). Nearly 20% of patients had regional disease, and 5% had distant metastases. The overall 10-year survival rates for patients with localized, regional, and distant disease stages were 96%, 87%, and 50%, respectively (P < .001). After adjustment, extrathyroid extension (odds ratio [OR], 41.9; P < .001) and tumor size (OR, 1.2; P = .008) retained an independent association with lymph node metastases. MTCs that measured ≤5 mm were associated with a probability of lymph node metastases of approximately 23%, and the probability increased for patients who had tumors >5 mm.
The current results indicated that microMTCs have significant rates of poor prognostic features known to impact the survival of patients with MTC. These microcarcinomas are an important clinical entity that requires comprehensive evaluation and surgical management.