Nodule Characterization [7

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Although US examination of the thyroid is very sensitive it is not very specific for identifying the cause of solid nodules. The appearances of benign and malignant nodules themselves can be similar. Examination of the surrounding structures for additional features of malignancy can be helpful. Local invasion (Figure 26.2) or large lymph nodes within the neck clearly indicate malignancy.

The majority of malignant nodules (approximately 90%) have a lower echo return than the normal thyroid without distal enhancement.

Hyperechoic nodules (Figure 26.3) are usually benign (malignancy rate less than 1%) but this only accounts for approximately 20% of benign nodules. Around 50-60% of benign

nodules are hypoechoic, causing clear confusion with malignant nodules.

Cysts are usually benign and easily seen as very echo poor regions within the thyroid generally with thin walls and septa. Post cystic enhancement (a region of brighter echo return behind the cyst) confirms the fluid content (Figure 26.4). The liquid can be echo free (black) or contain floating echogenic "dots" indicating a benign lesion. Hemorrhage into a cyst, which may cause clinical enlargement of the thyroid nodule, shows as echogenic liquid of varying density and appearance when compressed or moved with respect to gravity. A solid element within the cyst wall should be looked for as this increases the probability of malignancy.

The appearance of the margins of a nodule can help in the differentiation of benign from malignant: benign generally have well-defined margins and malignant have poorly defined and irregular margins, sometimes with a thick hypoechoic halo. The difference, however, is often unclear in practice.

Tiny areas of calcification (microcalcification) are suggestive (30%) of malignancy (medullary and papillary carcinomas) but larger regions of calcification are less specific (Figure 26.5). The finding of calcification in a solitary nodule, however, indicates malignancy in 55% [8].

The blood flow pattern within the nodule as seen using Doppler (color and "power" -Figure 26.6) has been used to help in the differentiation between benign and malignant. An intranodular vascular pattern is more sugges-

Figure 26.4 Normal right lobe and isthmus.Cysts are shown on the patient's left side, demonstrating post cystic enhancement.

Figure 26.2 Malignant thyroid displacing the common carotid artery.

Figure 26.4 Normal right lobe and isthmus.Cysts are shown on the patient's left side, demonstrating post cystic enhancement.

Figure 26.5 Calcification shown as bright echoes with shadowing beyond.

Figure 26.6 Power Doppler (A) showing peripheral flow in a benign nodule (and the common carotid artery) and color Doppler (B) showing central intranodular flow in a malignant nodule.

tive of malignancy than a peripheral pattern or no visible vessels [9,10].

Studies are currently under way to investigate whether the new US contrast can improve the benign/malignant differentiation. Promising early results show that contrast arrives within the nodule (from an arm intravenous injection) quicker in malignant nodules than benign (8 seconds versus 20 seconds) [11].

Nodule size can be easily measured with the calipers on modern US machines. Size has been used as a criterion for risk of malignancy but this view has recently been questioned [7,12]. Similarly, single or multiple nodules, easily determined by US, was considered an important risk factor, as the risk of malignancy was thought to be lower in multinodular goiters. The difference is now thought not to be significant [13].

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