Breach of the thyroid capsule and lymph node capsule in node-positive papillary and medullary thyroid cancer: Different biology

AimsThe higher incidence of extranodal growth (breach of a lymph node capsule) in the presence of extrathyroidal extension (breach of the thyroid capsule) in papillary thyroid cancer prompted conclusions that the biology of thyroid cancer is conferred to the lymph nodes, causing invasion of perinodal tissues. This study aimed at quantifying the independent contributions of clinical-pathological factors to extranodal growth in thyroid cancer.MethodsMultivariate analyses of 1250 patients operated on for node-positive papillary (PTC; 702 patients) or node-positive medullary thyroid cancer (MTC; 548 patients), 138 and 130 of whom harbored extranodal growth.ResultsAfter correction for multiple testing, extranodal growth correlated with number of lymph node metastases (means of 17.0 vs. 10.1 nodes for PTC, 20.6 vs. 13.4 nodes for MTC; each P < 0.001) and male gender (49 vs. 35% for PTC, P = 0.005; 62 vs. 46% for MTC; P = 0.002); and in MTC also with extrathyroidal extension (46 vs. 30%; P = 0.002). On multivariate analysis, independent determinants of extranodal growth were number of lymph node metastases (odds ratios of 2.1, 3.7 and 3.7 for PTC (P ≤ 0.01) and 2.7, 3.3, and 4.0 for MTC (P ≤ 0.004) looking at 6–10, 11–20 and >20 involved nodes against a 1–5 node baseline) and male gender (odds ratio 1.6 for PTC, 1.7 for MTC; each P = 0.02), but not extrathyroidal extension.ConclusionsIn PTC and MTC, extranodal growth develops independently from extrathyroidal extension. This finding argues against mere transference of primary tumor characteristics to lymph nodes, pointing more to accrual of invasive properties by nodal tumor deposits.