incidental thyroid lesions found on CT or MRI

Radiologists find satisfaction in detecting abnormalities and making diagnoses. However, our judgment is required when we identify incidental findings, especially if the cost of workup is high and the benefit of workup may be low. The incidental thyroid nodule is a scenario in which we need to carefully consider the consequences of our report and recommendations (1).

A few facts about incidental thyroid nodules

• 1. Incidental thyroid nodules are common whereas thyroid cancer is uncommon. 16-18% of patients will have an incidental nodule seen on CT and MRI (2,3). Only 1.6% of patients with one or more thyroid nodules will actually have thyroid cancer (4).

  2. Costs of workup of incidental thyroid nodules add up. The 2012 USA Medicare reimbursement totals over $1000 for the following steps in the workup of a thyroid nodule: office consultation, diagnostic ultrasound, US guided FNA and cytopathology (5). Other costs to consider are patient anxiety, time lost and potential complications of diagnostic lobectomy.

• 1. Small thyroid cancers are typically indolent and most patients die with rather than of thyroid cancer 6,7. More than 96% of thyroid cancers are papillary and follicular cancers (well-differentiated) and have an excellent prognosis (8).

  2. The observed incidence of thyroid cancer is increasing exponentially and has doubled in the last decade (8). Mortality has not changed significantly despite this trend which raises concern that the apparent increase in incidence is due to overdiagnosis of subclinical thyroid cancers (9).

How should we be reporting thyroid nodules on CT and MRI?

Reporting practices are highly variable among radiologists which really highlights the need for clear practice guidelines (10). The American College of Radiology has formed an Incidental Thyroid Nodules Committee, and published a white paper (15) based on the Duke 3-tiered system reporting guidelines for incidental thyroid nodules on CT and MRI.

The Duke 3-tiered system guidelines have now been studied in several cohorts. The following is a summary of the findings.

• • The 3-tiered system reduces FNA rate for incidental thyroid nodules by 34-46% (3,11)

  • The 3-tiered system captures the same proportion of thyroid cancers compared to a 1cm size cutoff (3)

  • Incidental thyroid cancers missed by the 3-tiered system represent 1% of all thyroid cancers having surgery and are small papillary cancers that are lower in stage (12)

  • The 3-tiered system results in a net 54% reduction in ITN reported in the impression section of the report (13).

​The decision to workup ITN should also account for the patients’ comorbidities and life expectancy. In patients with significant comorbidities and advanced age, the diagnosis and treatment of thyroid cancer is unlikely to alter their quality of life or life expectancy. A high proportion of patients having imaging are older and/or have comorbidities. Grady, et al. evaluated the indication for imaging in patients with ITNs reported on CT and MRI and found 52% are imaged for a known malignancy and 17% are imaged for vascular disease (14).

1. Exclusions: (a) normal findings, including hypodense ovary, crenulated enhancing wall of corpus luteum, asymmetric ovary

(within 95% confidence interval for size) with normal shape; (b) unimportant findings, including calcifications without associated

noncalcified mass; (c) previous characterization with ultrasound or MRI; and (d) documented stability in size and appearance

for >2 years.

2. Cyst: should have all of the following features: (a) oval or round; (b) unilocular, with uniform fluid attenuation or signal

(layering hemorrhage acceptable if premenopausal); (c) regular or imperceptible wall; (d) no solid area, mural nodule; and

(e) <10 cm in maximum diameter.

3. Refers to an adnexal cyst that would otherwise meet the criteria for a benign-appearing cyst except for one or more of the

following specific observations: (a) angulated margins, (b) not round or oval in shape, (c) a portion of the cyst is poorly imaged

(eg, a portion of the cyst may be obscured by metal streak artifact on CT of the pelvis), and (d) the image has reduced signalto-

noise ratio, usually because of technical parameters or in some cases because the study was performed without intravenous

contrast.

4. Features of masses in this category include (a) solid component, (b) mural nodule, (c) septations, (d) higher than fluid attenuation,

and (e) layering hemorrhage if postmenopausal.

5. This indicates that ultrasound should be performed promptly for further evaluation, rather than in follow-up.

6. A benign-appearing cyst >5 cm with suspected internal hemorrhage in a patient aged >55 years, or within 5 years of menopause,

should be followed in 6 to 12 weeks because hemorrhagic cysts in early postmenopause are possible, although rare.

7. May decrease threshold from 3 cm to lower values down to 1 cm to increase sensitivity for neoplasm.

Source: White Paper: Managing Incidental Findings on Abdominal and Pelvic CT and MRI, Part 1: Adnexal Findings, JACR Sept. 2013