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High resolution ultrasonography can detect lesions in the thyroid as small
as 1 mm in diameter and the technique is therefore better than either physical
examination or radionuclide imaging in detecting nodules (27). A high prevalence
of nodular thyroid disease of up to 50% is revealed by such high resolution
scanning as mentioned above. In addition, in as many as 40% of those patients
thought clinically to have a solitary nodule, multiple nodules in the thyroid
are revealed by ultrasonography (28). Since data on the incidence of malignancy
in nodular thyroid disease are related to clinically palpable goitres,
it is unclear whether such multiple nodules detected ultrasonographically
are associated with a similar or lower risk of malignancy than those nodules
only obvious on clinical examination. Ultrasonography is of value in distinguishing
solid from cystic lesions of the thyroid, ultrasound scanning being reported
to classify nodules as solid, mixed cystic and solid or cystic with greater
than 90% accuracy (29). It has been reported that malignant nodules are
hypoechoic compared with the remainder of the gland, perhaps because of
their disordered architecture, while benign lesions are hyperechoic with
a sonolucent rim ("halo sign") (30). It has become clear, however, that
there are no sonographic criteria which are able to define benign or malignant
disease with good specificity. In Ashcraft and Van Herle's review (31),
more than 1000 patients with thyroid nodules underwent conventional ultrasonography
before proceeding to surgery. Eighty % were classified pre-operatively
as solid or mixed solid cystic lesions and 20% were cystic. At surgery,
17% of all the lesions were malignant. Although solid or mixed cystic lesions
harboured most of the malignancies, 6% of tumours occurred in those lesions
originally thought to be cystic, probably reflecting the observation that
some carcinomas undergo cystic degeneration.
The poor specificity of ultrasonography in terms of indicating the underlying pathology, together with the difficulties which arise if nodules which are impalpable are revealed by ultrasound scanning, means that such scanning is often not considered helpful in patients presenting with nodular thyroid disease and its routine use has been questioned. |
