Objective: Considering the diameter of the largest tumor while determining T stage in multifocal papillary thyroid microcarcinomas (PTMCs) might cause underestimation of tumoral stage. We aimed to investigate the effect of total tumor diameter (TTD) on tumor node metastasis (TNM) classification in multifocal T1a PTMCs.
Methods: T1 tumors were grouped as T1a or T1b according to 7th TNM edition. For patients with multifocal T1a, TTD (the sum of the maximal diameter of each focus) was calculated, and these patients were further subgrouped as TTD ≤1 cm or TTD 1 to 2 cm.
Results: There were 724 patients with T1 tumors. Multifocality was observed in 150 (28.5%) of 527 patients with T1a and 84 (42.6%) of 197 patients with T1b tumors (P<.001). Lymph node metastasis (LNM), thyroid capsule invasion, and lymphovascular invasion were significantly higher in T1b compared to T1a (P<.001, P<.001, and P = .015, respectively). There were 8 (1.5%) patients with persistence but not any with recurrence in the T1a group. Persistence and recurrence were observed in 3 (1.5%) and 5 (2.5%) patients in the T1b group, respectively. Among 150 T1a patients with multifocal tumors, TTD was ≤1 cm in 89 (59.3%) and 1 to 2 cm in 61 (40.7%) patients. Number of tumor foci, LNM, and thyroid capsule invasion were significantly higher in multifocal T1a patients with TTD 1 to 2 cm compared to with TTD ≤1 cm (P<.001, P = .032, P = .014, respectively).
Conclusion: TTD might be used as a parameter to determine patients at higher risk for persistence, and T1a multifocal PTMCs with TTD 1 to 2 cm can be reclassified as T1b.
Abbreviations: ETE = extrathyroidal extension LNM = lymph node metastasis PTC = papillary thyroid carcinoma PTMC = papillary thyroid microcarcinoma RAI = radioactive iodine TNM = tumor, node, metastasis TTD = total tumor diameter.