Validation of Ultrasound and Computed Tomography-Based Risk Stratification System and Biopsy Criteria for Cervical Lymph Nodes in Preoperative Patients With Thyroid Cancer

Abstract

Objective: This study aimed to validate the risk stratification system (RSS) and biopsy criteria for cervical lymph nodes (LNs) proposed by the Korean Society of Thyroid Radiology (KSThR).

Materials and methods: This retrospective study included a consecutive series of preoperative patients with thyroid cancer who underwent LN biopsy, ultrasound (US), and computed tomography (CT) between December 2006 and June 2015. LNs were categorized as probably benign, indeterminate, or suspicious according to the current US- and CT-based RSS and the size thresholds for cervical LN biopsy as suggested by the KSThR. The diagnostic performance and unnecessary biopsy rates were calculated.

Results: A total of 277 LNs (53.1% metastatic) in 228 patients (mean age ± standard deviation, 47.4 years ± 14) were analyzed. In US, the malignancy risks were significantly different among the three categories (all P < 0.001); however, CT-detected probably benign and indeterminate LNs showed similarly low malignancy risks (P = 0.468). The combined US + CT criteria stratified the malignancy risks among the three categories (all P < 0.001) and reduced the proportion of indeterminate LNs (from 20.6% to 14.4%) and the malignancy risk in the indeterminate LNs (from 31.6% to 12.5%) compared with US alone. In all image-based classifications, nodal size did not affect the malignancy risks (short diameter [SD] ≤ 5 mm LNs vs. SD > 5 mm LNs, P ≥ 0.177). The criteria covering only suspicious LNs showed higher specificity and lower unnecessary biopsy rates than the current criteria, while maintaining sensitivity in all imaging modalities.

Conclusion: Integrative evaluation of US and CT helps in reducing the proportion of indeterminate LNs and the malignancy risk among them. Nodal size did not affect the malignancy risk of LNs, and the addition of indeterminate LNs to biopsy candidates did not have an advantage in detecting LN metastases in all imaging modalities.

Keywords: Computed tomography; Lymph nodes; Neoplasm metastasis; Thyroid neoplasm.

Fig. 1. Flow chart of the study population. A total of 55276 patients with screened from thyroid cancer patients who underwent US, CT and biopsy for cervical LNs. After excluding patients with past history of other malignancy, previous thyroid cancer surgery, and no CT images, 277 LNs from 228 patients were included in this study. US = ultrasound, FNA = fine-needle aspiration, CNB = core-needle biopsy, CT = computed tomography, LNs = lymph nodes

Fig. 2. A 79-year-old female with right papillary thyroid carcinoma. An enlarged lymph node (10 mm in short diameter) at the right level II with right papillary thyroid carcinoma showing a LN with a preserved echogenic hilum, which was classified as probably benign on US (A). This LN shows loss of the hilum (arrow) on axial contrast-enhanced CT (B) and is classified as indeterminate. Core needle biopsy confirmed a benign hyperplastic lymph node. CT = computed tomography, US = ultrasound, LN = lymph node

Fig. 3. A 65-year-old female with left papillary thyroid carcinoma. A small lymph node (3 mm in short diameter) at the left level III showing suspicious features of hyperechogenicity on US (arrow) (A). Strong enhancement is noted on axial contrast-enhanced CT (arrow in B) and is classified as suspicious LN. Fine-needle aspiration confirmed a metastatic papillary thyroid carcinoma. US = ultrasound, CT = computed tomography, LN = lymph node

Fig. 4. Comparison of diagnostic performance and unnecessary biopsy rates according to US based classifications and size threshold simulations in Korean Society of Thyroid Radiology (KSThR) guideline for cervical LNs. Graphs show sensitivity (A), specificity (B), accuracy (C), and unnecessary biopsy rate (D) of LN classifications according to the size cutoffs in KSThR guideline. Criteria (1): Any suspicious LNs. Criteria (2): Suspicious LNs > 3 mm in SD. Criteria (3): Suspicious LNs > 5 mm in SD. Criteria (4): Any suspicious and indeterminate LNs. Criteria (5): Suspicious LNs > 3 mm and indeterminate LNs > 5 mm in SD. Criteria (6): Suspicious LNs > 5 mm and indeterminate LNs > 5 mm. US = ultrasound, LN = lymph node, SD = short diameter

Fig. 5. Comparison of diagnostic performance and unnecessary biopsy rates according to CT based classifications and size threshold simulations in Korean Society of Thyroid Radiology (KSThR) guideline for cervical LNs. Graphs show sensitivity (A), specificity (B), accuracy (C), and unnecessary biopsy rate (D) of LN classifications according to the size cutoffs in KSThR guideline. Criteria (1): Any suspicious LNs. Criteria (2): Suspicious LNs > 3 mm in SD. Criteria (3): Suspicious LNs > 5 mm in SD. Criteria (4): Any suspicious and indeterminate LNs. Criteria (5): Suspicious LNs > 3 mm and indeterminate LNs > 5 mm in SD. Criteria (6): Suspicious LNs > 5 mm and indeterminate LNs > 5 mm. CT = computed tomography, LN = lymph node, SD = short diameter

Fig. 6. Comparison of diagnostic performance and unnecessary biopsy rates according to US + CT based classifications and size threshold simulations in Korean Society of Thyroid Radiology (KSThR) guideline for cervical LNs. Graphs show sensitivity (A), specificity (B), accuracy (C), and unnecessary biopsy rate (D) of LN classifications according to the size cutoffs in KSThR guideline. Criteria (1): Any suspicious LNs. Criteria (2): Suspicious LNs > 3 mm in SD. Criteria (3): Suspicious LNs > 5 mm in SD. Criteria (4): Any suspicious and indeterminate LNs. Criteria (5): Suspicious LNs > 3 mm and indeterminate LNs > 5 mm in SD. Criteria (6): Suspicious LNs > 5 mm and indeterminate LNs > 5 mm. US = ultrasound, CT = computed tomography, LN = lymph node, SD = short diameter