Incidental thyroid nodule

Incidental thyroid nodules (ITNs) are are a common finding in cross-sectional imaging, often performed without a head-and-neck indication ¹. About 5% of the population will have a palpable thyroid nodule, with autopsy rates approaching 50% (often multiple) ². As techniques improve, the detection of ITNsincidental thyroid nodules on imaging has increased moving closer to pathologists' pick up rate rather than the clinicians' ³. Despite this, mortality figures associated with thyroid cancer have not matched this trend ⁴.

ITN ratesRates on imaging

Incidental thyroid nodules are common and are identified at different rates depending on the modality:

• extrathyroidal ultrasound: 67% ^5,6
• CT (covering lower neck / upper mediastinum; e.g. CT neck, CT cervical spine, CT chest): 25% ^7,8
• MRI: 18% ⁸
• ¹⁸FDG-PET: <2% ⁹

There is also variable practice in the reporting of ITNsincidental thyroid nodules by radiologists ^8,10.

Malignancy rates

Despite the lower detection rate, around one-third of ¹⁸FDG-PET-positive nodules will be malignant ⁹ (due to the metabolic activity and increased uptake, often papillary thyroid cancers).

Rates of malignancy from other cross-sectional modalities (CT / MRI/MRI) only reach 12% in some studies ^8,11. Ultrasound is lower still at <2% on a population-based study, illustrating the high proportion of benign nodules ¹².

CT and MRI lack the spatial resolution to characterise suspicious features, with limited correlation with US findings (including the number of nodules). Size, however, can be used to stratify follow up.

Multinodular goitres can have similar malignancy rates as solitary thyroid nodules, but typically still require ultrasound analysis to identify the highest grade (not necessarily the largest) nodule ¹.

What to do with incidental thyroid nodules on CT / MRI / PET/MRI/PET

The American Thyroid Association published theirits guidelines on thyroid nodule management in 2015, recommending an ultrasound assessment of all nodules >10 mm detected on cross-sectional imaging ¹³.

The 2015 ACR White Paper stratified ITNincidental thyroid nodule assessment using the Duke 3-tiered system ¹⁴:

• category 1:
  • focally avid on PET, locally invasive nodules, suspicious lymph nodes (cystic, calcified, enhancing)
  • US workup for any size of nodule
• category 2:
  • solitary nodule in a young patient (<35 years)
  • US if ≥10 mm in adults, US for any size in paediatrics
• category 3:
  • solitary nodule in an older adult (≥35 years)
  • US if ≥15 mm

N.b.NB: multiple nodules can be prioritised by the same characteristics.

Younger patients are at higher risk of malignancy, hence the lower size criterion. PET avid lesions also merit FNAfine needle aspiration cytology (FNAC) given the higher proportion of malignancy.

Small non-suspicious nodules can be included in the report text, but should not appear in the conclusion and do not require any further evaluation. If known significant co-morbidities / limited/limited life expectancy this can also be considered before recommending assessment.

What to do with incidental thyroid nodules on extrathyroidal ultrasound

The same cut-off criteria are used for cross-sectional imaging (10 mm for <35 years, 15 mm for ≥35 years). Suspicious sonographic features which necessitate repeat assessment included in the ACR White Paper have been updated on the ACR TI-RADS 2017 publication ¹⁵.

Clinical impact

The Duke 3-tiered system has been found to reduce the number of ITNsincidental thyroid nodules requiring further evaluation by nearly half compared with standard practice, with only 1% of potential (low-grade) malignancies missed ^14,16. There can be a measurement discordance between modalities (with CT / MR/MRI +/- 4.7 mm compared to US), but without any missed malignancies applying the Duke 3-tiered system ¹⁷.