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 uses

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 cent–95 per cent and 75 per cent respectively. Medullary thyroid carcinoma is more aggressive, with a 10-year survival of 42 per cent–90 per cent. Anaplastic thyroid carcinoma has an extremely poor prognosis, with a five-year survival of five per cent.

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 non-thyroid malignancy.

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 10–100 microns round laminar crystalline calcific deposits. They are one of the most specific features of thyroid malignancy, with a specificity of 85.8 per cent–95 per cent. Micro calcifications are found in 29 per cent–59 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 thyroid carcinomas.

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, 10–24 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 cent–74 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 of malignancy.

Local Invasion

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 aspiration (FNA).
• 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

FNAC 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.