Key Points
- Though thyroid cancer is associated with high overall survival rates, exceeding 90% for most subtypes, the risk of recurrence has been reported to be as high as 35%.
- Most of these recurrences are detected within the first five years after diagnosis and thus may actually represent persistent rather than truly recurrent disease.
- It is known that the majority of reoperations for thyroid cancer are preventable and that inadequate preoperative imaging frequently is the root cause of incomplete initial surgery.
- Ultrasound remains the most important imaging modality in the evaluation of thyroid cancer and should be used routinely to assess both the primary tumor and all associated cervical lymph node basins preoperatively.
- Ultrasound evaluation (‘‘mapping’’) of bilateral lymph node compartments 1–6 should be performed routinely in the preoperative evaluation of patients with definitive cytologic evidence of carcinoma (positive FNA).
- Screening for distant metastasis is generally not performed prior to initial surgery for differentiated thyroid cancers.
Diagnosis
Functional Imaging
- Functional imaging with positron emission tomography (PET), PET-CT or PET-18F-fluoro-2-deoxyglucose is currently limited to the detection of recurrent disease.
Ultrasound-guided Fine-needle Aspiration of Cervical Lymph Nodes
- Ultrasound evaluation (‘‘mapping’’) of bilateral lymph node compartments 1–6 should be performed routinely in the preoperative evaluation of patients with definitive cytologic evidence of carcinoma (positive FNA).
- Screening for distant metastasis is generally not performed prior to initial surgery for differentiated thyroid cancers.
- It is mandatory that the needle tip is visualized within the target lymph node during the FNA to assure specimen accuracy.
- For a description of technique please see full text article.
(http://online.liebertpub.com/doi/full/10.1089/thy.2014.0096)
Table 1. Preoperative Ultrasound Scanning Technique
Equipment:
High-frequency linear array probe.Positioning:
Hyperextension of neck.Primary lesion:
Assess size, multiplicity, margin, invasion of deep structures.Central compartment lymph nodes (level 6):
Scan from submental area to sternal notch. Scan three distinct areas: pretracheal, right paratracheal, and left paratracheal. Turn head away from side of interest to image tracheoesophageal groove. Angle transducer inferiorly to examine mediastinum.Lateral compartment lymph nodes (levels 2, 3, and 4):
Scan from mandible to clavicle. Angle transducer inferiorly at clavicle to image infraclavicular nodes at base of level 4.Posterior compartment lymph nodes (level 5):
Sweep laterally along clavicle to posterior border of sternocleidomastoid muscle, then trace posterior border superiorly to mastoid process.
Table 2. Ultrasound Features Predictive of Malignant Lymph Node Involvement
| Criterion | Sensitivity | Specificity |
|---|---|---|
| Size >1 cm | 68% | 75% |
| Shape (ratio of long axis to short axis <2.0) | 46% | 64% |
| Punctate calcifications | 46% | 100% |
| Peripheral hypervascularity | 86% | 82% |
| Adapted from Leboulleux et al. Neuroimaging Clin N Am 18:479–489, vii–viii. | ||
Figure 1. Ultrasound Features of Benign Lymph Nodes
Figure 2. Ultrasound Features of Malignant Lymph Nodes
Computed Tomography and Magnetic Resonance Imaging
- Ultrasound has certain limitations related to the underlying technology, particularly in the imaging of deep structures and those acoustically shielded by air or bone.
- For this reason, cross-sectional imaging with CT or MRI may play a supplemental role in preoperative imaging for thyroid cancer in a minority of cases. (See Table 3)
- CT imaging of the neck is optimized by iodinated intravenous contrast.
Notes:- This advantage must be balanced against the impact the iodine load will have in causing what is usually a minor delay in subsequent postoperative radioactive iodine ablation. Thus, preoperative communication between the surgeon and endocrinologist is important.
- After the administration of iodinated contrast, a waiting period of at least one month is recommended to allow urinary iodine levels to return to baseline levels before moving forward with radioactive iodine ablation. At present, there is no evidence to suggest that delays of this scale adversely affect thyroid cancer outcomes.
- MRI with gadolinium contrast is an alternative axial scanning modality that avoids the use of iodine but may be less informative to surgeons as compared to CT in the central compartment due to motion artifact arising from swallowing and respiration.
Table 3. Findings That May Prompt Axial Imaging
- Hoarseness with vocal cord paresis/paralysis
- Progressive dysphagia or odynophagia
- Mass fixation to surrounding structures
- Respiratory symptoms, hemoptysis, stridor, or positional dyspnea
- Large size of tumor or mediastinal extension, incompletely imaged on ultrasound
- Rapid progression/enlargement
- Sonographic suspicion for significant extrathyroidal invasion (cT4)
- Bulky, posteriorly located, or inferiorly located lymph nodes incompletely imaged by ultrasound
- Ultrasound expertise not available
