Effects of tumor size on prognosis in differentiated thyroid carcinoma smaller than 2 cm

Benefits of subdividing small-differentiated thyroid carcinoma (sDTC) by tumor size are controversial. We conducted a meta-analysis to investigate whether tumor size is associated with prognosis of sDTC. PubMed and Web of Science databases were searched from their inception to September 2018. The identified studies according to the inclusion/exclusion criteria were analyzed using fixed/random-effects models. Data were calculated and results of the meta-analysis were expressed as odd ratio (OR). sDTC was classified as S1 (≤1 cm) and S2 (>1 cm and ≤2 cm). A systematic analysis was performed to compare the difference of recurrence, survival and clinicopathological factors between the two subgroups of sDTC (S1 vs. S2). A total of 21 studies published between 2004 and 2017 enrolling 219,291 patients were included. Findings showed that, S2 was associated with higher recurrence risk compared with S1 (OR=1.575, 95% CI=1.428-1.738; P<0.05). There was no statistical difference in survival between S1 and S2, but significant statistical heterogeneity (OR=1.160, 95% CI=0.810-1.662; P=0.448; I²=75.8%). Meta-regression analysis revealed publication year potentially caused the heterogeneity (P<0.05). Comparison of small papillary thyroid carcinoma alone agreed with the results of sDTC. T1b increased the risk of recurrence (OR=1.520; 95% CI=1.072-2.155; P<0.05) and death (OR=1.504; 95% CI 1.353-1.672; P<0.05) compared with T1a. S2 associated with extrathyroidal extension (OR=2.575; 95% CI=1.603-4.135; P<0.05), bilaterality (OR=2.278; 95% CI=1.905-2.723; P<0.05), vascular invasion (OR=4.494; 95% CI=2.812-7.183; P<0.05) and lymph node metastases (OR=1.12; 95% CI=1.10-1.14; P<0.05). Our analysis suggested it is necessary to subdivide sDTC into S1 and S2 owing to their different effects on prognosis, especially recurrence.