Imaging at Diagnosis
High resolution ultrasound of the thyroid and neck with a high frequency (12-18 MHz) linear transducer is recommended as the primary imaging modality for tumor diagnosis. (GRADE: A; SOR 1.07, very strong recommendation) This enables evaluation of the morphologic features of the thyroid nodule as well as location within the thyroid, evaluation for extrathyroidal extension, and involvement of important adjacent anatomic structures, which may impact management.
At ultrasound, there are both pattern-based and point-based risk stratification guidelines that have been evaluated in both adults and children to differentiate benign and malignant thyroid nodules.9,12,16,20-25 Generally, features such as solid composition, taller than wide orientation, irregular margins, microcalcifications or punctate echogenic foci, and extrathyroidal extension are considered suspicious for malignancy. It is important to recognize intrathyroidal ectopic thymic tissue at ultrasound, which appears as a hypoechoic nodule with linear and punctate echogenic foci, is unique to children, and should not be mistaken for PTC.26,27 Although color Doppler may be useful for distinguishing solid components from debris in nodules and may be useful to predict bleeding risk during fine-needle aspiration (FNA), Doppler pattern appears to be less helpful in determining malignancy than grayscale appearance.28,29 An important distinction between adult and pediatric guidelines is the size threshold to guide FNA decisions. Although adult guidelines specify nodule size cut-offs to proceed to FNA, in children, size thresholds are not recommended in the decision-making process, but rather the ultrasound appearance is prioritized and a lower threshold to proceed with further diagnostic work-up recommended, particularly in children with risk factors. 5,9,12,16,23,30 (GRADE: B; SOR 1.64, strong recommendation) This is because in adults, the goal of imaging is not to diagnose every thyroid malignancy, but to balance the benefit of identifying clinically significant cancers against the cost of subjecting patients with benign nodules or indolent cancers to unnecessary treatment.20,31
Ultrasound lymph node mapping of the neck with a meticulous evaluation of lymph node levels in the central neck (level 6), lateral neck (levels 1-5), and mediastinum (level 7) is required because PTC metastasizes to regional lymph nodes in most children. 9 (GRADE: B; SOR 1.57, strong recommendation) Ultrasound is highly sensitive and specific for predicting cervical lymph node metastasis preoperatively.32 Preoperative suspicion of locoregional metastatic disease is important to plan an appropriate, compartment-oriented lymph node dissection at the time of initial surgery.6 Preoperative ultrasound has been shown to improve surgical outcome, decrease rate of recurrence or need for more surgeries, and to guide further medical therapy.33-35
Ultrasound guidance is recommended for FNA of the thyroid nodule, targeted to the solid or most suspicious component of the nodule to provide the highest diagnostic yield specimen. (GRADE: A; SOR 1.28, very strong recommendation) Ultrasound guidance is also recommended to guide FNA of suspicious lymph nodes, if needed preoperatively to plan lymph node dissection approach. 9 (GRADE: B; SOR 1.93, strong recommendation) Ultrasound-guided FNA is both sensitive and specific to diagnose pediatric thyroid cancer. Without ultrasound guidance, rates of non-diagnostic and false negative thyroid nodule cytologic results are higher.9,16
CT or MRI of the neck is not routinely recommended, but is reserved for select cases where bulky lymphadenopathy or large tumor burden can hinder ultrasound visualization of the deep compartments of the neck (levels 6 and 7, retropharyngeal, and supraclavicular regions) or if local invasion is suspected.18,36 Neck CT requires iodinated IV contrast material injection to adequately visualize anatomy, and therefore is not recommended. Neck CT without IV contrast is not recommended. Therefore, neck MRI is preferred over CT in the evaluation of the extent of bulky cervical metastatic disease prior to surgery. (GRADE C; SOR 2.0, moderate recommendation)
Although current ATA guidelines recommend either chest radiographs or CT in intermediate and high-risk patients to evaluate for pulmonary metastatic disease, CT is the most sensitive imaging modality for this purpose.37 Therefore, CT of the chest without IV contrast should be performed in initial staging to detect pulmonary metastases in patients in the ATA Intermediate and High-Risk categories. (GRADE: C; SOR 1.92, strong recommendation) Chest CT is not routinely recommended in patients categorized as Low-Risk.(GRADE: C; SOR 1.86, strong recommendation) While intravenous contrast material can improve detection of mediastinal and hilar lymphadenopathy in the chest, pulmonary metastases can be detected without IV contrast. Axial imaging with 3 mm or smaller slice thickness complemented by coronal and sagittal reconstructions is recommended, with maximal intensity projections (MIPs). Use of MIPs has been shown to improve the detection of small pulmonary nodules.38