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FDG-PET Detected Thyroid Incidentaloma

FDG-PET Detected Thyroid Incidentaloma. Dr. LP Si Yan Chai Hospital. Background. With the increasing use of imaging modalities, more and more clinically inconspicuous thyroid lesions are discovered These are termed as thyroid incidentalomas

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FDG-PET Detected Thyroid Incidentaloma

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  1. FDG-PET Detected Thyroid Incidentaloma Dr. LP Si Yan Chai Hospital

  2. Background • With the increasing use of imaging modalities, more and more clinically inconspicuous thyroid lesions are discovered • These are termed as thyroid incidentalomas • Also include asymptomatic thyroid lesions identified during non-thyroid neck surgery, eg. parathyroidecotmy, carotid endarterectomy

  3. Prevalence • USG: 67% • CT / MRI: 16% • Carotid duplex: 9.4% • FDG-PET: 2-3% • Increasing use of FDG-PET • Thyroid carcinomas detected by FDG-PET is more aggressive than those detected by other imaging modalities

  4. FDG-PET • Fluorodeoxyglucose positron emission tomography • Mostly commonly used for • Disease staging for patients with known malignancy • Malignancy screening in healthy individuals

  5. Increase FDG uptake in metabolically active tissue,e.g. malignancy, infection, inflammation, etc • Normal thyroid gland not shown up

  6. FDG-PET detected thyroid incidentalomas • Unsuspected, asymptomatic thyroid lesions discovered on FDG-PET • Prevalence 1.2-4.8% • Increasing incidence but no increase in disease specific mortality • Risk of malignancy • 14-74% • higher than incidental thyroid nodules found by USG, CT, or MRI

  7. Is the thyroid incidentaloma on FDG-PET benign or malignant?

  8. Pattern of uptake • Focal • Increase in radioactivity concentration compared to that of the thyroid parenchyma • Clearly delineated from the surrounding thyroid gland • Diffuse • Even distribution of FDG in the entire thyroid gland • Radioactivity concentration is higher than that of the surrounding soft tissue

  9. Focal uptake

  10. Diffuse uptake

  11. Focal uptake of thyroid lesions on PET also known as • PETomas • PET-associated incidental neoplasms (PAINs) • PET detected thyroid incidentalomas (PETI) • Associated with an increased risk of malignancy than diffuse uptake

  12. 20 out of 27 patients who underwent operation proved to have thyroid malignancy (74%) Nishimori H et al. Incidental thyroid “PETomas”: clinical significance and novel description of the self-resolving variant of focal FDG-PET thyroid uptake. Can J Surg Vol. 54, No.2 , April 2011

  13. SUV • Standardized uptake value • Elevated in metabolically active tissue • Controversies about the use of SUVas an adjunct to differentiate between benign or malignant • No cut off value

  14. Katz SC et al. PET-Associated Incidental Neoplasms of the Thyroid. J Am Coll Surg (2008) 259-264

  15. SUV correlates with the size of the thyroid incidentaloma in focal uptake • r = 0.64, p < 0.05 Nilsson IL et al. Thyroid Incidentaloma Detected by Flurordeoxyglucose Positron Emission Tomography/Computed Tomography: Practical Management Algorithm. World J Surg (2011) 35:2691-2697

  16. Risk factors of malignancy • Clinically apparent thyroid nodules are associated with 5% malignant rate • Are et al. showed that risk of malignancy in the presence and absence of palpable thyroid nodule is 24% and 6.3% respectively (p = 0.01)

  17. Focal FDG uptake and unilateral uptake are associated with higher malignancy rate Are C et al. FDG-PET Detected Thyroid Incidentalomas: Need for Further Investigation? Annals of Surgical Oncology (2006) 14(1): 239-247

  18. FDG-PET detected incidental thyroid carcinomas • Most are papillary thyroid carcinoma (PTC) • Poor prognostic variants are more common • Tall-cell variant papillary thyroid carcinoma (TCVPTC) • Poorly differentiated thyroid carcinoma • As high as 50% • Extra-thyroidal extension is also more common • Higher treatment failure rate • Higher local, regional and distant recurrences • Higher disease-specific mortality

  19. TCVPTC • More aggressive with a worse prognosis when compared to classical PTC (cPTC) • High rate of extra-thyroidal extension • Up to 90% in one series • Commonly smaller than cPTC • FNAC is warranted regardless of the size if the lesion is suspicious

  20. Management • Most of the patients already have a primary malignancy → the whole plan of management should depend on the overall prognosis and life expectancy • USG thyroid is necessary for all patient • No evident nodule → follow up • Nodule ≥ 1cm or < 1cm with suspicious features → FNAC

  21. Suspicious sonographic features • Marked hypoechogenicity • Taller than wide • Irregular shape • Ill-defined margin • Punctate calcification • Increase in vascularity

  22. FNAC (Bethesda System) • Nondiagnostic • Benign • Atypia of undetermined significance (AUS) • Suspicious for follicular neoplasm / follicular neoplasm • Suspicious for malignancy • Malignant

  23. Conclusion • FDG-PET detected thyroid incidentalomas harbour a significant risk of malignancy • Thyroid carcinomas detected on FDG-PET are more aggressive and demonstrate poorer prognosis • They should be investigated by USG +/- FNAC • Definitive management plan depends on the overall prognosis and life expectancy of the patients

  24. References • American Thyroid Association Management Guidelines for Patients with Thyroid Nodules and Differentiated Thyroid Cancer (2009) • Jin J et al. Thyroid incidentaloma. Best Practice & Research Clinical Endocrinology & Metabolism 26 (2012) 83-96 • Nilsson IL et al. Thyroid Incidentaloma Detected by Flurordeoxyglucose Positron Emission Tomography/Computed Tomography: Practical Management Algorithm. World J Surg (2011) 35:2691-2697 • Ho TY et al. Prevalence and significance of thyroid uptake detected by 18F-FDG PET. Endocrine (2011) 40:297-302 • Bertagna F et al. F18-PDF_PET/CT thyroid incidentalomas and their benign of malignant nature: a critical and debated issue. Ann Nucl Med (2011) 25:151-152 • Nishimori H et al. Incidental thyroid “PETomas”: clinical significance and novel description of the self-resolving variant of focal FDG-PET thyroid uptake. Can J Surg Vol. 54, No.2 , April 2011 • Law TT et al. Incidental Thyroid Carcinoma by FDG-PET/CT: A Study of Clinicopathological Characteristics. Ann Surg Oncol (2011) 18:472-478

  25. Jin J et al. Incidental thyroid nodule: patterns of diagnosis and rate of malignancy. The American Journal of Surgery (2009) 197, 320-324 • Kang BJ et al. Incidental thyroid uptake on F-18 FDG PET/CT: correlation with ultrasonography and pathology. Ann Nucl Med (2009) 23:729-737 • Gough J et al. Thyroid Incidentaloma: An Evidence-based Assessment of Management Strategy. World J Surg (2008) 32:1264-1268 • Katz SC et al. PET-Associated Incidental Neoplasms of the Thyroid. J Am Coll Surg (2008) 259-264 • Are C et al. Histological Aggressiveness of Fluorodeoxyglucose Positron-Emission Tomogram (FDG-PET)-Detected Incidental Thyroid Carcinomas. Annals of Surgical Oncology (2007) 14(11):3210-3215 • Are C et al. FDG-PET Detected Thyroid Incidentalomas: Need for Further Investigation? Annals of Surgical Oncology (2006) 14(1): 239-247

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