Correlation between Thy grading system and histology diagnosis
Among the 471 patients (71.5%) who underwent preoperative FNAC, the postoperative histology was reported as benign in 352 (74.7%) and malignant in 119 cases (25.3%), Table 2 . Thy1 grade was reported in 165 cases, with 32 (19.4%) had final histological diagnosis of malignancy. Around 10% (N=16) of Thy1 lesions were cystic (Thy1c) with malignancy encountered in around 23% of these lesions. The majority of patients in Thy2 group had a benign final histological diagnosis. However, 28 patients (14.7%) with Thy2 cytology had malignant nodules on post-operative histology (false negative). The false negative rate increased to 50% for the Thy2c sub-category (Table 2 ). Thy3a and Thy3f FNAC categories were reported in 48 and 41 patients respectively, with histology confirming malignancy in 14 (29.2%) and 21 (51.2%) patients respectively (Table 2 ). Thy4 and Thy5 categories where only recorded in 18 (3.82%) and 7 (1.5%) patients respectively, with malignancy diagnosis confirmed in 16 (89%) of Thy4 and 7 (100%) of Thy5 patients. PTC was the commonest histological diagnosis in all six DCs. Interestingly, the only two patients with MTC and 2 out of 5 with TL had non-diagnostic (Thy1) FNAC, and were operated on mainly based on clinical and radiological suspicion.