Incidental thyroid nodule

Incidental thyroid nodules, sometimes called thyroid incidentalomas, are a common findingdiscrete lesions in the thyroid gland found on cross-sectional imaging, often performed without a head-and-neck indication for indications other than thyroid evaluation. They are common but occasionally represent thyroid cancer¹. This article discusses the epidemiology of incidental thyroid nodules and recommendations for further evaluation.

Epidemiology

About 5% of the population will havehas a palpable thyroid nodule, withwhile autopsy rates approachingapproach 50% (often multiple nodules)². As imaging techniques improve, the detection of incidental thyroid nodules on imaging has increased, moving closer to pathologists' pick uppickup rate rather than the bedside clinicians'³. Despite this, mortality figures associated with thyroid cancer have not matched this trend ⁴.

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

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

There is also variable practice in the reporting

Risk of incidental thyroid nodules by radiologists ^8,10.

Malignancy rates

malignancy

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) only reach 12% in some studies ^8,11. Ultrasound is lower still at <2% onmalignancy in 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.

WhatManagement

There is variable practice in radiologists' reporting of incidental thyroid nodules ^8,10. Professional organizations have developed recommendations in recent years to do with incidentalaid radiologists and other clinicians in their management.

Incidental thyroid nodules on CT/MRI/PET

CT, MRI, PET, and other nuclear medicine scans lack the spatial resolution to characterise suspicious features and do not perfectly agree with the size and number of nodules found on US ¹⁶. Size on CT or MRI, however, can be used to stratify need for evaluation.

American Thyroid Association recommendations

The American Thyroid Association published its guidelines on thyroid nodule management in 2015, recommending an ultrasound assessmentthyroid US for all patients in whom another imaging study suggested the presence of all nodules >10 mm detected on cross-sectional imaginga thyroid nodule ¹³. The guidelines also state separately that in general, only nodules >1 cm merit evaluation.

The 2015 ACR White Paper stratified incidentalWith the finding of a thyroid nodule assessment using>1 cm, a serum thyroid-stimulating hormone (TSH, thyrotropin) level should also be obtained. If the Duke 3-tiered system¹⁴:

• category 1:
  • focally avid on PETlevel is low, locally invasive nodules, suspicious lymph nodes (cystic, calcified, enhancing)
  • US workup for any sizea radionuclide thyroid scan should be obtained.

    The findings

    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

NB: multiple nodules can be prioritised bythese tests then help guide the same characteristics.

Younger patients are at higher risk of malignancy, hence the lower size criterion. PET avid lesions also meritneed for fine needle aspiration cytology (FNAC) given, the higher proportionmost accurate method of malignancyevaluating thyroid nodules.

Small non-suspicious nodules can be includedThe American College of Radiology Incidental Thyroid Findings Committee published a white paper in the report text, but should not appear in2015 with recommendations based on the conclusion and do not requireso-called 3-tiered system initially developed at Duke University ¹⁴. A nodule meeting any of the following criteria is recommended for further evaluation by thyroid US:

• focal thyroid uptake on FDG-PET or other nuclear medicine scans (e.g. If known significant co-morbidities/limitedoctreotide, sestamibi)
• thyroid nodule with local tissue invasion
• thyroid nodule with suspicious lymph nodes (enlarged, cystic, calcified, or hyperenhancing)
• thyroid nodule ≥1 cm (in axial plane) in patients <35 years old
• thyroid nodule ≥1.5 cm (in axial plane) in patients ≥35 years old

When multiple thyroid nodules are present, the criteria apply to the largest nodule.

Patients who have limited life expectancy this can also be considered before recommending assessment.

Whator serious comorbidities (that increase the risk of treatment or pose more risk to do withthe patient than possible thyroid cancer) should not undergo further evaluation of incidental thyroid nodules on extrathyroidal ultrasound

The same cut-off criteria are usedlacking suspicion for cross-sectional imaging (10 mm for <35 years, 15 mm for ≥35 years). Suspicious sonographic features which necessitate repeat assessment included inlocal invasion or nodal metastases unless the ACR White Paper have been updated on the ACR TI-RADS 2017 publication ¹⁵patient or referring physician specifically requests so.

The Duke 3-tiered system has been found to reduce the number of incidental thyroid nodules requiring further evaluationbiopsy by one-third to nearly halfone-half compared with standard practicepractices, with only 1%a false negative rate of potential (low-grade) malignancies missed 13% ^14,1617-19. There can

Incidental thyroid nodules on extrathyroidal ultrasound

Per the American College of Radiology White Paper recommendations, the same size cutoffs used for nonsonographic imaging also apply for extrathyroidal ultrasound:

• ≥1.0 cm for age <35 years
• ≥1.5 cm for age ≥35 years.

Dedicated thyroid ultrasound would also be a measurement discordance between modalities (with CT/MRI +/- 4.7 mm comparedindicated if suspicious features happen to US), but without any missed malignancies applyingbe captured on the Duke 3-tiered systemnonthyroidal ultrasound, such as

• microcalcifications
• marked hypoechogenicity
• lobulated/irregular margins
• taller-than-wide shape

The list of suspicious sonographic features been updated in the ACR TI-RADS 2017 publication ¹⁷¹⁵.