Background: International guidelines for the management of differentiated thyroid cancers are based on the 7th TNM classification: pT3 tumors are defined as differentiated thyroid cancers (DTCs) measuring more than 4 cm in their greatest dimension that are limited to the thyroid or any tumor with minimal extrathyroidal extension (ETE; sternothyroid muscle or perithyroid soft tissues). Differences in clinicohistological features and prognosis among patients with pT3 tumors remain controversial, and studies regarding pT3 subgroups are lacking.
Objective: To analyze the prognosis of four subgroups of pT3 DTCs (papillary, PTC; or follicular, FTC).
Design and setting: The data of patients who underwent surgery for pT3 DTC between 1978 and 2015 in a surgical department specialized in endocrine surgery were reviewed. Patients were classified into four groups as follows: the pT3a (≤ 10 mm with ETE), pT3b (10-40 mm with ETE), pT3c (> 40 mm without ETE), and pT3d groups (> 40 mm with ETE). Recurrence-free survival (RFS) was analyzed using the Kaplan-Meier method.
Results: One thousand eighty-eight patients with pT3 DTC were included, of whom 311 (29%) had pT3a; 548 (50%), pT3b; 165 (15%), pT3c; and 64 (6%), pT3d. For the 916 patients with lymph node (LN) dissection, metastatic LNs were more frequent in the pT3b and pT3d groups (61 and 61%, respectively) than in the other groups (44% pT3a and 10% pT3c; p < 0.001). During the median follow-up period of 9 years (range, 2-38 years), recurrence occurred in 169 patients with T3 tumors (16%), including 18 with pT3a (6%), 100 with pT3b (18%), 20 with pT3c (12%), and 31 with pT3d (48%). In a multivariate analysis, LN metastases (< 0.0001), extranodal extension (p = 0.03), FTC (vs. PTC) (p = 0.006), pT3b (p = 0.016), and pT3d (p = 0.047) were associated with an increased risk of recurrence. The 5-year RFS rates were 94.5, 82.2, 91.1, and 50.3% for the pT3a, pT3b, pT3c, and pT3d groups, respectively (p < 0.01).
Conclusion: Except for microcarcinoma, the risk of LN involvement is high and similar for the DTC patients with minimal ETE, regardless of the size of the tumor. The association of a tumor size of > 4 cm and ETE are associated with a poor prognosis and should justify the classification of these cases as a high-risk group. Other pT3 patients with no LN metastases could be individualized as a low-risk group.
Keywords: Clinicopathological features; Lymph node dissection; Neoplasm recurrence, local; Thyroid cancer, well differentiated, papillary, follicular; Tumor staging.