Evidence of reliability of FNAs for parotid lesions is as follows:
There was a retrospective study performed in 2016 that looked at over 100 cases of parotid tumors that had been diagnosed over the past 10 years. Over 90 of them were initially evaluated by FNA; this study compared their initial FNA diagnosis to the final diagnosis made after the tumor was surgically removed. Pleomorphic adenoma (PA) was the most common neoplasm identified (found in 63% of cases), followed by other benign neoplasms and, less commonly, malignancies. The overall accuracy of FNAC for parotid masses was estimated to be 95%.
“False-negative diagnoses were found in mucoepidermoid carcinoma (n=1; 1.4%), acinic cell carcinoma (n=1; 1.4%), and epithelial-myoepithelial carcinoma (n=1; 1.4%) whereas there were false-positive diagnoses in cases of pleomorphic adenoma (n=2; 2.7%) and in normal parotid tissue (n=1; 1.4%).” (Gudmundsson et al.)*
In this case, “negative” is a shortened version of “negative for malignancy,” meaning benign; “positive” is short for “positive for malignancy.” A false negative means that the case was (on FNA) initially diagnosed as benign, but on final pathology was malignant (so in the above paragraph, for all the cases where the final pathology was mucoepidermoid carcinoma, 1 case was initially diagnosed falsely as “negative for malignancy” or benign, but when resected turned out to be a mucoepidermoid cancer, which is a malignant tumor). A false positive means that the case was initially diagnosed as malignant (with FNA), but on final pathology was found to be benign. The tumors found to be pleomorphic adenomas did have 2 cases falsely identified as malignant on FNA which on resection were found to be benign; no cases in this study that were diagnosed by FNA as benign were found to be malignant.
Based on the above study, the evidence shows that FNAs are excellent at diagnosing parotid tumors, including PAs, and that their findings of “benign” versus “malignant” are overall very reliable.
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