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