Intratumoural lymph vessel density is related to presence of lymph node metastases and separates encapsulated from infiltrative papillary thyroid carcinoma

Papillary thyroid carcinoma (PTC) gives frequently rise to nodal metastases via lymphatic vessels while follicular thyroid carcinoma (FTC) metastasises mainly via blood vessels to lung and bones. The follicular variant of PTC (FVPTC) encompasses the infiltrative subtype (I-FVPTC), which shares most of the features of classic PTC (CPTC), and the encapsulated subtype (E-FVPTC), which appears to be related to minimally invasive FTC. In an attempt to contribute to the understanding of the aforementioned differences, we evaluated intratumoural and peritumoural lymph vessels density (LVD), using the immunomarker D2-40 in a series of E-FVPTC, I-FVPTC, and CPTC with known BRAF and RAS status. None of the E-FVPTC cases presented extra-thyroid extension, lymph vessel invasion or nodal metastases, at variance with I-FVPTC and CPTC cases. The BRAF V600E mutation was detected in 8.3% of E-FVPTC, 25.0% of I-FVPTC and in 40.7% of CPTC, while N-RAS Q61R mutation was detected only in 10.3% of FVPTC cases. Only one case of E-FVPTC (8.3%) had intratumoural D2-40-stained vessels in contrast to their presence in 76.5% of the cases of I-FVPTC. Intratumoural LVD determined by D2-40 expression correlated with the occurrence of extra-thyroid extension, lymph vessel invasion and lymph node metastases in PTC cases. At variance with intratumoural LVD, peritumoural LVD was not associated with any clinic-pathological or molecular feature, being similar in E-FVPTC, I-FVPTC and CPTC. Our study highlights the role of intratumoural lymph vessels in PTC nodal metastisation and reinforces the importance of distinguishing E-FVPTC from I-FVPTC regarding invasiveness, metastatic pattern and molecular profile.