ObjectiveTo assess the ultrasonographic features and the influence factors that affect the false-negative（FN）

false-positive（FP）diagnosis of extrathyroidal extensions（ETE）in papillary thyroid microcarcinoma（PTMC）.MethodsThe preoperative ultrasonic data of 472 nodules in 326 patients with PTMC confirmed by pathology were collected and analyzed.Taken pathological results as golden standard

the sensitivity

specificity

positive predictive value（PPV）

negative predictive value（NPV）and accuracy of ultrasound in evaluation of ETE were calculated.The factors associated with FN and FP for ETE were analyzed.Results There were 141 PTMC lesions with extrathyroidal extensions

73 lesions were true-positive and 68 were FN.The sensitivity

specificity

PPV

NPV and accuracy of ultrasoud in predicting ETE were 51.7%

86.7%

62.4%

80.8%and 76.3%

respectively.The univariate analysis showed that diameter of tumor ≤5 mm

aspect ratio ≥1

vicinity of the trachea（non-adjacent）and bilateral lobes lesions were associated with FN（all P<0.05）.Multiple Logistic regression analysis showed that vicinity of the trachea（non-adjacent）

bilateral lobes lesions were independent risk factors for FN（OR=4.916

7.237

both P<0.05）.The rest 331 PTMC lesions without extrathyroidal extensions

287 were true-negative for ultrasound diagnosis and 44 were FP

univariate analysis showed that diameter of tumor ≤5 mm was associated with FP.ConclusionSonographic features of the lesions

such as diameter

shape and location are important factors to affect the diagnosis of ETE in PTMC.