Thyroid nodules are a common finding in the general population, and their detection is
increasing with the widespread use of ultrasound (US). The prevalence of thyroid
nodularity varies from 19% to 67%, and increases with age, affecting about 50% of the
population older than 40 years of age. The clinical significance of thyroid nodules relates to
the need to exclude thyroid cancer, which is found in 5–15% of cases, depending on sex,
age, and exposure to other risk factors. The incidence of thyroid cancer has increased about
fivefold in the last 50 years, mostly due to small papillary thyroid cancers, the most
indolent form of thyroid cancer.
Sonographic patterns such as hypoechogenicity, blurred or spiculated margins, spot
microcalcification, and intranodular vascularity are characteristics of malignant nodules,
but they yield a wide range of sensitivities (55–95%) and specificities (52–81%) for
diagnosis as malignant or benign. However, there is no information about the probability of
the US features associated with malignancy and which combination would be more
clinically useful. Diagnostic sensitivity ranges from 26.5% to 87.1% for hypoechogenicity,
54.3% to 74.3% for intranodular vascularity, and 26.1% to 59.1% for microcalcifications,
whereas specificity ranges from 43.4% to 94.3%, 78.6% to 80.8%, and 85.8% to 95%,
respectively
Fine-needle aspiration cytology (FNAC) is a standard method for triaging thyroid nodules
to surgery or clinical follow-up, and, with FNAC, the number of unnecessary surgeries has
decreased. The main limitation of FNAC, however, is nondiagnostic or unsatisfactory
results. According to the Bethesda System for Reporting Thyroid Cytopathology, a sample
is considered nondiagnostic or unsatisfactory when the specimen shows obscuring blood,
overlying thick smears, air drying of alcohol-fixed smears, or an inadequate number of
follicular cells. According to the Bethesda system, nondiagnostic results should ideally be
limited to less than 10% of all thyroid FNACs, but the rates of nondiagnostic results are
reported to be as high as 21%. Some authors have demonstrated that the best option for
reducing nondiagnostic results is on-site cytologic assessment. However, even with on-site
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assessment by cytopathologists, the prevalence of nondiagnostic results has still been
reported to be 10.7%.
Cytologic examination of thyroid nodules by FNA is diagnostic method for differentiating
benign from malignant nodules with reported sensitivity to be 60–98% and specificity,
54–90%. The risk of malignancy in a nodule reported as malignant or suspicious for
malignancy at FNAC is 97%–99% and 60%–75%, respectively. The accuracy of US-guided
FNAC (68%) is higher than that of palpation-guided FNAC (48%)
The main diagnostic method used is invasive FNA and
– 30% of FNA samples from thyroid nodules are not conclusive,
• 10–15% of FNA samples yielding non-diagnostic results
• 10–20% yielding indeterminate results.
– 5% false-negative rate
The thyroid imaging reporting and data system (TIRADS) developed by Kwak et al can help
stratify thyroid nodules according to malignancy risk by using the number of suspicious
ultrasonography (US) features such as solidity, hypoechogenicity or marked
hypoechogenicity, microlobulated or irregular margins, microcalcifications, and taller-
than-wide shape. TIRADS can help accurately predict malignancy and can be easily applied
in clinical practice owing to its simplicity. However, this reporting system has not been
applied to thyroid nodules with nondiagnostic results at cytologic examination, even when
it can be used in the continuous risk stratification of nodules in this category after FNAC. |