Many people with thyroid nodules need surgery. You may ask your doctor before surgery, “Can my nodule be diagnosed as cancer now?” The doctor will say, “No, the diagnosis can only be made after surgery. Why does thyroid cancer have to be diagnosed by surgical pathology? What does a pre-operative fine needle aspiration biopsy of a thyroid cancer mean?
Limitations of fine needle aspiration biopsy
Fine needle aspiration cytology (FNAC) is currently recognized as one of the most effective methods for preoperative diagnosis of thyroid disease, with a 95% reliability of positive diagnosis reported abroad. However, it is influenced by the proficiency of the puncture and pathologist, and there is still a possibility of “false positives” and “false negatives” in practice.
Studies have shown that the rate of actual thyroid cancer that is “missed” by FNAC is 3.6% to 10.2%. When thyroid nodules are smaller than 5 mm, FNAC tends to miss surrounding normal tissue; in addition, false negatives can occur with multiple lesions, combined Hashimoto’s thyroiditis, or calcified lesions.
In addition, FNAC is inaccurate for follicular tumors because the cell morphology alone cannot distinguish malignant “carcinoma” from benign “adenoma.
Postoperative paraffin pathology is the “gold standard”
for diagnosis.
The FNAC takes very few specimens and looks at cell morphology alone, whereas surgical pathology looks not only at the cells but also at the tissue structure composed of a large number of cells, which is more accurate. The surgical pathology includes intraoperative frozen pathology and postoperative paraffin pathology, the latter of which is more accurate and is the gold standard for diagnosis.
Intraoperative frozen pathology, commonly referred to as “rapid pathology,” is considered to be as accurate as, or more accurate than, FNAC, which is performed immediately after thyroidectomy and is generally thicker and has fewer layers, and is only used to quickly determine the benignity or malignancy of the tumor, and can be diagnosed in about 30 minutes. Once thyroid cancer is diagnosed, the surgeon needs to immediately supplement with a cervical lymph node dissection procedure.
Postoperative paraffin pathology involves making wax blocks of the entire surgically removed specimen (including the tumor, surrounding thyroid tissue, lymph nodes, etc.) and slicing them layer by layer at a thickness of 3 to 8 microns, making dozens or even hundreds of sections, and then combining them with various techniques for diagnosis. Therefore, postoperative paraffin pathology is not only the most accurate in determining tumor benignity and malignancy, but can also clarify the specific type of tumor, exclude microscopic lesions, and comprehensively assess the size of the tumor, local invasion, surrounding thyroid, lymph node metastasis, etc. It contains much more information than fine needle aspiration and frozen pathology.
So, even if fine needle aspiration is done preoperatively, surgery still requires both frozen pathology and paraffin pathology, and one has to wait for paraffin pathology as the final diagnosis. Once the paraffin pathology report is available, the nature of the tumor is considered “definitive”.
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Co-written by Dr. Yiming Cao, Fudan University Cancer Hospital Dr. Xiaoke Zheng