There is wide variation in the management of thyroid nodules identified on ultrasound imaging.
To quantify the risk of thyroid cancer associated with thyroid nodules based on their ultrasound characteristics.
Retrospective case-control study of patients who underwent thyroid ultrasound between January 1st, 2000 and March 30th, 2005. Thyroid cancers were identified through linkage with the California Cancer Registry.
8 806 patients underwent 11 618 thyroid ultrasound examinations during the study period including 105 subsequently diagnosed with thyroid cancer. Thyroid nodules were common in patients diagnosed with cancer (97%) and patients not diagnosed with thyroid cancer (56%). Three ultrasound nodule characteristics–micro-calcifications (odds ratio [OR] 8.1 [95% CI 3.8, 17.3]), size greater than 2 cm (OR 3.6 [95% CI 1.7, 7.6]) and an entirely solid composition (OR 4.0 [95% CI 1.7, 9.2] - were the only findings associated with the risk of thyroid cancer. If a single characteristic is used as an indication for biopsy, most patients with thyroid cancer would be detected (sensitivity .88 [95% CI .80, .94]) with a high false positive rate (.44 [95% CI .43, .45]) and a low likelihood ratio positive (2.0 [95% CI 1.8, 2.2]), and 56 biopsies will be performed per cancer diagnosed. If two characteristics were required for biopsy, the sensitivity and false positive rates would be lower (sensitivity 0.52 [95% CI 0.42, 0.62]; false positive rate 0.07 [95% CI 0.07, 0.08]), the likelihood ratio positive would be higher (7.1 [95% CI 6.2, 8.2]), and only 16 biopsies will be performed per cancer diagnosed. In comparison to performing biopsy of all thyroid nodules greater than 5 mm, adoption of this more stringent rule requiring two abnormal nodule characteristics to prompt biopsy would reduce unnecessary biopsies by 90%, while maintaining a low risk of cancer, 5 per 1000 patients, for whom biopsy is deferred.
Thyroid ultrasound could be used to identify patients who have a low risk of cancer for whom biopsy could be deferred. Based on these results, these findings should be validated in a large prospective cohort.