3-D analysis of vascular and capsular invasion in thyroid follicular carcinoma

To clarify the mechanism of vascular invasion (VI) and capsular invasion (CI) in thyroid follicular carcinoma (FC), 3-D reconstruction of tumor tissues, vessels and capsules was performed from serial sections in five FC and three follicular adenomas (FA). Outflow veins in the lesions were well developed under the capsule, narrowed just beneath the capsule, and dilated within the capsule. Both VI and CI were observed at the poststenotic intracapsular dilated part of outflow veins. The tumor tissue extended along the intracapsular horizontal veins which flowed in parallel with the capsule in VI, or along the vertical veins which directly flowed outside the capsule in CI. There was tumor tissue in the subendothelial layer of the outflow veins within the capsule. Intravascular tumor nests and nodular lesions within the capsule continued from the main tumors. It can be concluded from these results that most VI and CI may not be true invasions but pre-invasive lesions.