Ultrasound in Thyroid Malignancy
Cancer treatment has progressed by leaps and bounds. The
advent of newer technologies have made the fight against cancer an easier one.
Ultrasound is one such weapon which is of great help to diagnose and treat thyroid
malignancy, yet another form of cancer. Dr Nagaraj KR Consultant Radiologist,
HCG Bangalore, elaborates on the myriad aspects of ultrasound and its varied
The thyroid gland is a small butterfly-shaped endocrine gland situated in the
neck. The thyroid gland secretes hormones to regulate body's heart rate, blood
pressure, temperature, and metabolism.
As per the National Cancer Registry, the prevelance of thyroid cancer is 2.4
per 100,000. Although a diagnosis of thyroid or any type of cancer is frightening,
the vast majority of thyroid cancers is highly treatable and in most cases curable
with surgery and other treatment. Females are more likely to have thyroid cancer
at a ratio of three to one. Most patients are between 25 to 65 years of age
when first diagnosed.
Risk factors for thyroid carcinoma include age of less than 20 years or more
than 60 years, a history of neck irradiation, and a family history of thyroid
cancer. In India, thyroid cancer has a widespread distribution with papillary
cancer, occurring in coastal areas of Tamil Nadu, Andhra Pradesh and Kerala
which are iodine rich. The iodine content of soil modifies the development of
these cancers. The medullary subtype cancer is familial and inherited as an
autosomal dominant trait, it occurs in the syndromes of familial medullary cancer
and Multiple Endocrine Neoplasia (MEN).
Ultrasound in Thyroid Malignancy
- Detection of thyroid nodules.
- Differentiation of benign from malignant nodules.
- FNAC guidance
Instrumentation and technique: Linear array high frequency
transducers (7.5 15 MHz) probes, provide both deep ultrasound penetration
(upto 5 cm) and high definition images with a resolution of 0.7 1.0 mm.
Pathological Types of Thyroid Malignancy
Large heterogeneous thyroid mass
The main pathological types of thyroid carcinoma are papillary,
follicular, medullary, and anaplastic. Papillary and follicular thyroid carcinomas
both have an excellent prognosis, with a 20- year survival of 90 per cent95
per cent and 75 per cent respectively. Medullary thyroid carcinoma is more aggressive,
with a 10-year survival of 42 per cent90 per cent. Anaplastic thyroid
carcinoma has an extremely poor prognosis, with a five-year survival of five
Thyroid lymphoma, usually of the non-Hodgkin type, is uncommon.
It may occur as part of generalised lymphoma or as a primary tumour, usually
in the setting of Hashimoto thyroiditis.
Metastases to the thyroid are rare and usually originate
from primary lung, breast, and renal cell carcinomas. Metastatic disease should
be suspected when a solid thyroid nodule is found in a patient with a known
Ultrasound (US) Features Suggestive of Malignancy
Calcifications: Thyroid calcifications may occur in
both benign and malignant disease. Thyroid calcifications can be classified
as micro calcification, coarse calcification, or peripheral calcification.
- Thyroid microcalcifications are psammoma bodies, which
are 10100 microns round laminar crystalline calcific deposits. They
are one of the most specific features of thyroid malignancy, with a specificity
of 85.8 per cent95 per cent. Micro calcifications are found in 29 per
cent59 per cent of all primary thyroid carcinomas, most commonly in
papillary thyroid carcinoma. Their occurrence is also seen in follicular and
anaplastic thyroid carcinomas as well as in benign conditions such as follicular
adenoma and Hashimoto thyroiditis. During US, micro calcifications appear
as punctate hyperechoic foci without acoustic shadowing.
- Large, irregularly shaped dystrophic calcifications also
may occur and are secondary to tissue necrosis. They may appear as spicules,
fragmented plates, or granular deposits within fibrous septa in the thyroid
gland. They are commonly present in multinodular goiters; however, when found
in solitary nodules, they may be associated with a malignancy rate of nearly
75 per cent.
Coarse calcifications are the most common type of calcification in medullary
At US, dense coarse calcifications cause posterior acoustic shadowing. Inspissated
colloid calcifications in benign thyroid lesions may mimic micro calcifications
in thyroid malignancies, but the former can be distinguished from malignant
calcifications by the observation of ring-down or reverberation artifact.
- Peripheral calcification is one of the patterns most commonly
seen in a multinodular thyroid but also may be seen in malignancy.
Margins, Contour, and Shape: The halo or hypoechoic
rim around a thyroid nodule is produced by a pseudocapsule of fibrous connective
tissue, a compressed thyroid parenchyma, and chronic inflammatory infiltrates.
A completely uniform halo around a nodule is highly suggestive of benignity,
with a specificity of 95 per cent . However, a halo is absent at US in more
than half of all benign thyroid nodules. Moreover, 1024 per cent of papillary
thyroid carcinomas have either a complete or an incomplete halo.
A thyroid nodule is considered ill-defined when more than 50 per cent of its
border is not clearly demarcated. An ill-defined and irregular margin in a thyroid
tumor suggests malignant infiltration of adjacent thyroid parenchyma with no
pseudocapsule formation. However, unless frank invasion beyond the capsule is
demonstrated, the US appearance of the nodule margins alone is an unreliable
basis for determining malignancy or benignity.
Vascularity: Vascular flow within a thyroid nodule
can be detected with colour or power doppler US. The most common pattern of
vascularity in thyroid malignancy is marked intrinsic hypervascularity. This
occurs in 69 per cent74 per cent of all thyroid malignancies. However,
it is not a specific sign of thyroid malignancy.
Frates et al showed that more than 50 per cent of hypervascular solid thyroid
lesions were benign. Peri-nodular flow is defined as the presence of vascularity
around at least 25 per cent of the circumference of a nodule. This flow pattern
is more characteristic of benign thyroid lesions but also has been found in
22 per cent of thyroid malignancies. In contrast, complete avascularity is a
more useful sign: completely avascular nodule is very unlikely to be malignant.
Hypoechoic Solid Nodule
Thyroid nodule FNAC with needle in situ
Malignant nodules, both carcinoma and lymphoma, typically
appear solid and hypoechoic when compared with normal thyroid parenchyma. When
a thyroid nodule is markedly hypoechoic, with a darker appearance than that
of the infrahyoid or strap muscles of the neck, the specificity for detection
of malignancy is increased to 94 per cent. Marked hypoechogenicity is very suggestive
Direct tumour invasion of adjacent soft tissue and metastases
to lymph nodes are highly specific signs of thyroid malignancy. Suggestive clinical
symptoms include dyspnea, hoarseness, and dysphagia. Aggressive local invasion
is common with anaplastic thyroid carcinoma, lymphoma, and sarcoma.
At US, direct tumour invasion of adjacent soft tissues may appear as a subtle
extension of the tumour beyond the contours of the thyroid gland or as frank
invasion of adjacent structures.
Lymph Node Metastases
Malignant hypervascular thyroid nodule
Metastases to regional cervical lymph nodes have been reported
to occur in 19.4 per cent of all thyroid malignancies. They are most common
in papillary thyroid carcinoma.
US features that should arouse suspicion about lymph node metastases include
a rounded bulging shape, increased size, replaced fatty hilum, irregular margins,
heterogeneous echotexture, calcifications, cystic areas, and vascularity throughout
the lymph node instead of normal central hilar vessels at Doppler imaging.
Nonspecific US Features
- Size of nodule: The size of a nodule is not helpful
for predicting or excluding malignancy. There is a common but mistaken practice
of selecting the largest nodule in a multinodular thyroid for fine needle
- Number of nodules: Although most patients with
nodular hyperplasia have multiple thyroid nodules and some patients with thyroid
carcinoma have solitary nodules, the presence of multiple nodules should never
be dismissed as a sign of benignity. The risk of malignancy in a thyroid with
multiple nodules is comparable to that with a solitary nodule.
- Interval growth of a nodule: In general, interval
growth of a thyroid nodule is a poor indicator of malignancy. Benign thyroid
nodules may change in size and appearance over time, with the potential to
either enlarge or decrease in size. Rapid interval growth, which most commonly
occurs in anaplastic thyroid carcinoma but also may occur in lymphoma, sarcoma,
and, occasionally, high-grade carcinoma.
Common interpretative pitfalls that may lead to failure to recognise a malignancy
include mistaking cystic or calcified nodal metastases for nodules in a multi-nodular
thyroid, mistaking diffusely infiltrative thyroid carcinomas and multifocal
carcinomas for benign disease, and failing to recognise micro-calcifications
in papillary thyroid cancer.
Fine Needle Aspiration Cytology (FNAC) Guidance
is recommended for the following situations in an incidentally detected nodule:
Micro-calcifications in a nodule with a diameter of 1 cm or greater; coarse
calcification or a solid nodule with a size of 1.5 cm or greater; and a mixed
cystic and solid nodule with a size of 2 cm or greater.
These size limitations for each category are based on consideration
of the excessive number of biopsies of small nodules and the likelihood that
the treatment of microcarcinomas (1cm) does not improve life expectancy.
To conclude, US is valuable for identifying many malignant or potentially malignant
thyroid nodules. Although there is some overlap between the US appearance of
benign nodules and that of malignant nodules, certain US features are helpful
in differentiating between the two. These features include micro calcifications,
local invasion, lymph node metastases, and markedly reduced echogenicity.