Preoperative diagnosis of thyroid cancer by FNAB has a sensitivity of 83% (65-98%), specificity of 92% (72-100%), positive predictive rate of 75% (50-96%), false negative rate of 5% (1-11%), and false positive rate of 5% (0-7%). fnab cannot distinguish follicular carcinoma of the thyroid from follicular cell adenoma. Preoperative FNAB can help reduce unnecessary thyroid nodule surgery and help determine the appropriate surgical plan.
FNAB can be considered for any thyroid nodule >1 cm in diameter. However, FNAB is not routinely performed in the following cases.
1. “hot nodules” with autonomic uptake as confirmed by thyroid nuclide imaging.
2. nodules that are purely cystic on ultrasound.
3. nodules that are highly suspicious of malignancy based on ultrasound images.
FNAB is not recommended for thyroid nodules <1 cm in diameter, but ultrasound-guided FNAB may be considered in the following cases.
1. ultrasound suggestive of malignant signs in the nodule.
2, with abnormal ultrasound images of the cervical lymph nodes.
3, history of radiation exposure to the neck or radiation contamination exposure during childhood.
4, history or family history of thyroid cancer or thyroid cancer syndrome.
5. Positive 18F-FDGPET imaging.
6, with abnormally elevated serum Ct levels.
Compared with FNAB under palpation, ultrasound-guided FNAB has a higher success rate of sampling and diagnostic accuracy. To improve the accuracy of FNAB, the following methods can be used: repeated puncture sampling at multiple sites of the same nodule; sampling at sites where ultrasound suggests suspicious signs; sampling at solid sites of cystic nodules with concurrent cyst fluid cytology. In addition, experienced operators and cytopathological diagnostic physicians are also important to ensure the success rate and diagnostic accuracy of FNAB.
Based on international standards and domestic reports, this guideline recommends the following classification in determining FNAB results.
Determination of FNAB results
FNAB results
Likelihood of nodule being malignant
Possible lesion type
Undiagnostic or unsatisfactory sampling
1-4%
Too little cellular component or inflammatory component only
benign
0-3%
Glial nodules, Hashimoto’s thyroiditis, subacute thyroiditis or cystic lesions
Uncertain
5-30%
Active cell proliferation or follicular lesions
Suspected malignancy
60-75%
Suspected papillary, medullary, metastatic or lymphoma
Malignant
97-99%
Papillary carcinoma, medullary carcinoma, metastatic carcinoma or lymphoma