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Technetium-99m (99mTc) pertechnetate is the radionuclide most frequently
employed in thyroid imaging. Pertechnetate ions are trapped by the thyroid
in the same way as iodine but are not organified, in contrast to the less
readily available and more expensive isotope iodine-123 (123I) which is
both trapped and organified. Although the majority of thyroid malignancies
do not concentrate radioisotopes (and thus appear 'cold'), less than 20%
of 'cold' nodules are due to cancerous lesions, the remaining 80% being
due to colloid nodules, haemorrhage, cysts or inflammatory lesions such
as Hashimoto's thyroiditis (22). 'Hot' nodules on iodine scintigrams are
associated with a low incidence of malignancy (23), most reported 'hot'
carcinomas probably representing the existence of a malignant lesion next
to a benign 'hot' lesion (24). The finding of apparently normal isotope
uptake (a 'warm' lesion) may be ambiguous since a hypofunctioning malignant
nodule may be obscured by overlying normal tissue. Radionuclide imaging
may also reveal multifocal abnormalities in patients thought clinically
to have solitary nodules, mixed areas of hyper- and hypofunction being
characteristic of multinodular goitres and less commonly Hashimoto's thyroiditis
(25). While the incidence of cancer is less in multinodular goitres than
in solitary nodules, the presence of 'cold' areas on such scans leads to
concern regarding the underlying pathology since the presence of a multinodular
goitre does not prevent the development of thyroid malignancy as a second
abnormality.
In view of these limitations of radionuclide scanning it is not surprising that this approach does not specifically distinguish benign from malignant lesions. This is illustrated by Ashcraft and Van Herle's review of 22 series (26), in which more than 5000 patients underwent surgery irrespective of the functional status of the nodule defined by isotope scanning. Of these, 84% were 'cold', 10.5% 'warm' and 5.5% 'hot' lesions. Malignancy was present in 16% of the 'cold', 9% of the 'warm' and 4% of the 'hot' nodules. Thus malignant nodules are more likely to be 'cold' than 'hot', but most 'cold' lesions are benign and the presence of a 'warm' or 'hot' nodule does not exclude malignancy. There are therefore no specific features which indicate the benign or malignant nature of a thyroid nodule and this poor specificity has led many centres to abandon radionuclide scintigraphy in the routine investigation of those presenting with nodular thyroid disease. |
