Clinical and demographic characteristics of the patients and lesions
As shown in Fig 1, a total of 108 and 105 thyroid nodules from 78 patients received FNAB-CT and FNAB-C test, respectively. All 57 micro thyroid nodules had FNAB-CT performed and 56 had FNAB-C performed. At the end of the study, 51 micro thyroid nodules from 39 patients and totally 87 thyroid nodules from 60 patients met endpoints: 56 patients underwent thyroidectomy, histology revealed 41 micro-MTC in 32 patients and totally 69 MTC in 52 patients. Eight thyroid nodules (six micro and two macro nodules) from four patients were identified as non-thyroid medullary carcinoma (non-MTC) lesions either had pathologically confirmed pancreatic neuroendocrine tumors (Pnet) with ectopic calcitonin secretion or test error interference by herbal medication.
Clinical and demographic characteristics of 60 patients(28 females and 32males) are shown in table 1 and table S1. The mean age was 47.90±14.18yrs(range: 23~69yrs), the mean basal level of serum calcitonin (sCT) was 438.06±582.25pg/ml (range: 10.22~2000pg/ml，values higher than 2000pg/ml were calculated as 2000pg/ml). Thirty-nine (65％) patients had at least one micro thyroid nodule with a mean age of 46.50 ± 15.73yrs(range: 23~69yrs)，the mean basal sCT was 167.48 ± 352.96 pg/ml(range: 10.22~2000pg/ml). Twenty-two patients (40％) had sCT values ranged from 10 to 100pg/ml，the other 36 patients (60％) had sCT＞100pg/ml (sCT values: 48.71 ± 27.93pg/ml vs 559.62 ± 652.15pg/ml; p＜0.001). Among 53 patients tested, germline RET mutation was found in 20 patients (16 with micro thyroid nodules) at codons 634(n=15), 618(n=2), 611(n=1) and 533(n=2).
Sonographic characteristics of the 87 thyroid nodules are shown in table 2. The maximum diameter of 87 thyroid nodules ranged from 2.9 to 39mm and 51 (58.62%) were micro nodules(＜10mm), with a mean maximum diameter of 7.23±2.11mm. The majority was in the middle(n=42,48.28％;micro nodules: n=27,52.94％)and middle-upper (n=16,18.39％; micro nodules: n=12, 23.53％) area of the lobes. Most of the thyroid nodules were solid(n=84, 96.55％; micro nodules: n=50, 98.04％) and hypochogenicity (n=84, 96.55％; micro nodules: n=51,100％). Other malignant sonographic features included the presence of microcalcifications(n=38, 43.68％; micro nodules: n=21, 41.18％), irregular margins(n=32, 36.78％; micro nodules: n=18, 35.29％) and tall shape(n=2, 2.30％; both were micro nodules, 3.92％). Following the TI-RADS malignancy risk stratification system, most of the thyroid nodules distributed in TIR4a (low suspicion for malignancy) classification(n=50, 57.47％; micro nodules: n=33, 64.71％) and only 13 of 87 thyroid nodules (micro nodules:7/51) were categorized as TIR5 (highly suggestive of malignancy)or TIR4c (moderate concern but not classic for malignancy).
Comparison of FNAB-CT with FNAB-C in diagnosing MTC
FNAB-CT were performed on all 87 thyroid nodules. With the cutoff value as 36pg/ml, the sensitivity and specificity was 98.55％ and 100％. Nineteen thyroid nodules showed negative FNAB-CT levels and 18 were confirmed as non-MTC lesions. False negative results occurred in a single case of MTC nodule with a size of 9.5*6.2*6.7mm; Sixty-eight thyroid nodules showed positive FNAB-CT level, all were histopathologically confirmed as MTC. The mean value of FNAB-CT in 69 MTC was significantly higher than in 9 non-MTC lesions ( 1993.40±326.34vs 3.45±2.57pg/ml, p＜0.0001).
FNAB-C were performed on 84 thyroid nodules. According to the Bethesda system for reporting thyroid cytopathology(TBSRTC), thyroid nodules were classified into 6 diagnostic categories (DCs): Bethesda I, non-diagnostic/unsatisfactory (ND/UNS) (n=6); Bethesda II, benign (B) (n=7); Bethesda III，atypia of undetermined significance or follicular lesion of undetermined significance (AUS/FLUS) (n=0); Bethesda IV, follicular neoplasm/suspicious for follicular neoplasm (FN/SFN) (n=26); Bethesda V, suspicious for malignancy (SM) (n=33) and Bethesda VI, malignant (M) (n=12). Sixty-nine nodules including 5 Bethesda I, 4 Bethesda II, 21 Bethesda IV, 27 Bethesda V and 8 Bethesda VI nodules were histologically diagnosed as MTC. Cytologic results as MTC or MTC suspicious were defined as positive. Consequently, 39 lesions were cytological positive and 38 was histologically confirmed MTC. Among the 45 cytological negative lesions, 31 were diagnosed as MTC in postoperative pathological analysis. Therefore, the sensitivity and specificity of FNAB-C when diagnosing MTC was 55.07% and 93.33%, respectively.
As in table 3, FNAB-CT had a greater performance over FNAB-C when diagnosing MTC regarding as sensitivity (98.55％ vs 55.07％), NPV (94.74％ vs 31.11％) and overall accuracy (98.85％ vs 61.90％).
Comparison of FNAB-CT with FNA-C in diagnosing micro thyroid MTC nodule
Subsequently, we explored the performance of FNAB-CT in detecting MTC in micro thyroid nodules. As in table 4, all 51 micro nodules had FNAB-CT performed and 50 had FNAB-C performed. FNAB-CT correctly identified 40(40/41) micro-MTC but FNAB-C misdiagnosed 21 (21/41) micro-MTC as non-MTC lesions, with diagnostic power as sensitivity (97.56％ vs 48.78％), NPV (90.91％ vs 30％) and overall accuracy (98.04％ vs 58％).
The limited diagnostic accuracy of FNAB-C should ascribe to the high occurrence of false negative diagnosis and micro-MTC had a high false negative rate of 41.18％. As in table 5, among total 31 FNAB-C negative MTC, 21 was smaller than 1cm in size, including four sample error and 17 interpretation error: 12 as follicular neoplasm, three as multinodular goiter, one as PTC suspicious and one as Hashimoto thyroiditis. The other 10 false negative results occurred in macroscopic nodules(＞1cm) and included one sample error and nine interpretation error as follicular neoplasm. The majority (30/31) of these FNAB-C negative MTC were revealed by FNAB-CT before surgery.
To be noted, four patients with thyroid nodules (two had only one micro thyroid nodule) and elevated sCT (92.44, 149.71, 467.7, ＞2000pg/ml) levels were excluded MTC diagnosis based upon FNAB-CT results. Subsequently, two of them found calcitonin-secreting pancreatic neuroendocrine tumors and the other two had normal sCT level after quitting herbal therapy. In addition, two patients with borderline elevated sCT (10.21, 16.43pg/ml) and micro thyroid nodules, received total thyroidectomy and central lymph node dissection due to markedly high levels of FNAB-CT(＞2000pg/ml), histopathological findings confirmed MTC in both patients and one with lymph node metastases exist.