By analyzing the learning curve of ultrasound-guided fine-needle aspiration biopsy (US-FNAB) of thyroid nodules that was conducted by six doctors at the ultrasound department, we aimed to obtain the number of cases that beginners need to perform to achieve the average and stable level of US-FNAB, so as to effectively control the quality of US-FNAB of thyroid nodules.

A total of 6147 thyroid nodules that underwent US-FNAB (performed by 6 doctors; Dr1 and Dr6 were experienced doctors, and Dr2-Dr5 were inexperienced doctors) were retrospectively studied between March 2011 and November 2014 at West China Hospital, Sichuan University. To evaluate the operator's diagnostic inadequacy over time, the procedures were chronologically subdivided into several consecutive groups of 100 cases each. Yields of US-FNAB were divided into two levels according to the classification standard of the Bethesda system: adequacy and inadequacy. The learning curve was plotted based on the inadequate diagnostic rate of US-FNAB according to every group data of each doctor. The average rate of diagnostic inadequacy for the 6147 thyroid nodules was analyzed. Then, the inadequate diagnostic rates for the first 100 (F100) and second 100 cases (S100) for each doctor from Dr2 to Dr5 was analyzed, respectively. Pearson Chi-Square test was used to compare the inadequate diagnostic rate between the average rate of diagnostic inadequacy for the 6147 thyroid nodules and the F100 and S100 of four doctors (Dr2-Dr5), respectively.

The rates of non-diagnostic procedures for each group from Dr1 to Dr6 were: Dr1 (17 groups): 25%, 23%, 17%, 17%, 14%, 15%, 24%, 14%, 12%, 7%, 8%, 9%, 6%, 10%, 11%, 16%, and 8%; Dr2 (5 groups): 40%, 27%, 24%, 21%, and 13%; Dr3 (10 groups): 22%, 9%, 20%, 23%, 13%, 17%, 14%, 11%, 10%, and 15%; Dr4 (15 groups): 21%, 22%, 28%, 26%, 22%, 29%, 24%, 19%, 14%, 20%, 8%, 11%, 11%, 9%, and 4%; Dr5 (8 groups): 39%, 24%, 13%, 21%, 12%, 13%, 8%, and 9%; Dr6 (4 groups): 13%, 15%, 5%, and 9%. The average rate of diagnostic inadequacy for the 6147 thyroid nodules was 16.04%. The inadequate diagnostic rates of the F100 cases and the S100 cases for each doctor from Dr2 to Dr5 were: Dr2: 40% and 27%; Dr3: 22% and 9%; Dr4: 21% and 22%; Dr5: 39% and 24%. The inadequacy rate of the F100 cases in Dr2 and Dr5 was significantly higher than the average rate of diagnostic inadequacy for the 6147 thyroid nodules (Dr2: χ²=14.286, P<0.001; Dr5: χ²=13.266, P<0.001). There was no difference in inadequate diagnostic rates between the S100 cases in each doctor from Dr2 to Dr5 and the average rate of diagnostic inadequacy for the 6147 thyroid nodules (P>0.05).

The learning curve of US-FNAB of thyroid nodules for beginners has been drawn and observed. According to the training amount that beginners require to achieve the average and stable level of US-FNAB, we can determine the time accurately when beginners could perform US-FNAB of thyroid nodules independently. In this way, we can get an effective method to manage the quality of US-FNAB of thyroid nodules.