Thyroid Cancer



Thyroid Cancer


Todd M. Blodgett, MD

Alex Ryan, MD

Marios Papachristou, MD









Coronal PET (A), axial CT (B) and fused PET/CT (C) show a subtle recurrent thyroid carcinoma image in a patient with rising thyroglobulin levels and a negative iodine study.






Axial CT (top) and fused PET/CT (bottom) show focal thyroid cancer recurrence in the thyroidectomy bed image.


TERMINOLOGY


Abbreviations and Synonyms



  • Well-differentiated thyroid cancer (WDTC)


  • Medullary thyroid carcinoma (MTC)


  • Anaplastic thyroid carcinoma


Definitions



  • WDTC: Carcinoma of the thyroid arising from papillary &/or follicular cell origin


  • MTC: Uncommon malignant neuroendocrine neoplasm arising from thyroid parafollicular “C cells”


  • Anaplastic: Aggressive form of mostly undifferentiated cells


IMAGING FINDINGS


General Features



  • Best diagnostic clue



    • WDTC



      • Non-physiologic, focal, asymmetric uptake of FDG


      • However, many WDTC may not be FDG avid when iodine avid


    • MTC



      • Solid, low attenuating, discrete thyroid masses with punctate calcification and nodal mets


    • Anaplastic



      • Diffuse, intense FDG activity correlating with an infiltrative thyroid mass


  • Location



    • WDTC



      • Primary and recurrent disease arise mostly in the parenchyma and bed of the thyroid gland


      • Metastatic disease travels to cervical/mediastinal lymph nodes and then to bone, lungs, and mediastinum


      • Papillary: Lymphatic invasion and spread to multifocal nodal regions


      • Follicular: Hematogenous spread to lung and bone


    • MTC



      • Intraglandular


      • Often multifocal and bilateral (2/3 sporadic, almost 100% familial)


      • Lymph nodes: Level VI and superior mediastinal; also retropharyngeal and levels III & IV


    • Anaplastic



      • May involve the entire thyroid gland



      • Early metastatic disease


  • Size



    • WDTC



      • Often diffuse microscopic disease


      • Lymph node and pulmonary metastases may be below limits of detection


      • Metastases to bone, in contrast, may grow very large


    • MTC



      • Up to 2.5 cm


    • Anaplastic



      • Bulky


  • Morphology



    • WDTC



      • Typical lymph node findings of roundedness and calcification may be absent


      • Differentiate from typical thymus morphology (variable by age)


      • Skeletal metastases typically lytic


    • MTC



      • Solid, nonencapsulated mass


      • Calcification in larger tumors


      • May be infiltrative in familial forms


Imaging Recommendations



  • Best imaging tool



    • Ultrasound for initial evaluation of all thyroid masses with fine needle aspiration


    • WDTC



      • For iodine-avid disease: Diagnosis, staging, and follow-up best performed with I-123 or I-131 whole body scan


      • For non-iodine-avid tumor, FDG PET/CT is superior


    • MTC



      • Consider FDG PET/CT for staging and restaging


      • Current insurance coverage restrictions for MTC


    • Anaplastic



      • Most are intensely FDG avid, but there are current insurance coverage limitations


  • Protocol advice



    • General



      • Iodine scan: Withdrawal or thyrogen-stimulated


      • Mediastinal lymph nodes near heart may be blurred due to motion, leading to false negatives


    • FDG PET



      • Thyroid-stimulating hormone (TSH) elevation/administration improves performance


      • Considerations for IV contrast for PET/CT; need to know if patient will be treated with radioactive iodine


      • Increased thyrocyte metabolism, glucose transport, hexokinase I levels, and overall glycolysis contribute to specific FDG uptake


      • Hormone withdrawal and administration of recombinant TSH (rhTSH = thyrogen)


      • Thyrogen dosage schedule not established, but Medicare pays for two injections


      • Recommended dosing: 0.9 g IM on day 1 and day 2, and FDG PET on day 3, 4, or 5


    • Correlative tests



      • Thyroglobulin measurement also best with elevated TSH


      • Serum thyroglobulin (Tg)


      • Correlate with radioiodine scan


      • Insensitive in presence of anti-Tg antibodies


      • Elevated levels post-therapy indicate residual thyroid tissue (> 2.0 ng/mL)


CT Findings



  • WDTC



    • Normal thyroid findings include



      • Cystic changes (hypodense)


      • Calcifications (hyperdense)


      • Well-defined borders


    • Primary tumor



      • Typically highly variable morphology


      • May mimic normal gland


      • Low attenuation nodule within gland


      • May have dystrophic calcifications


    • Signs of more aggressive tumor



      • Large size


      • Diffuse infiltration


      • Ill-defined, heterogeneous morphology



      • Extension to surrounding tissues


    • Lymph node appearance also highly variable



      • Large to small (may appear as benign reactive nodes)


      • Solid to heterogeneous/hemorrhagic to cystic


      • Variable calcification


      • Isolated retropharyngeal nodal metastasis may occur


  • MTC



    • Solid, low density, well-circumscribed mass in thyroid


    • Multifocality more common in familial types


    • Calcification in tumor and involved lymph nodes may be fine and punctate


    • Bone metastases typically lytic


  • Anaplastic



    • Large infiltrative mass


Nuclear Medicine Findings



  • WDTC



    • No current indication for pre-operative PET or PET/CT staging of WDTC


    • Consider in patients with anaplastic thyroid carcinoma for staging, although not covered by Medicare


    • Currently covered by Medicare for patients with



      • Documented history of follicular origin WDTC


      • Status post-thyroidectomy


      • Radioactive I-131 therapy


      • Current elevation in serum thyroglobulin


      • Negative I-131 whole-body scan


    • Consider performing FDG PET or PET/CT in all patients with these parameters


    • WDTC normally demonstrates mild to moderate FDG uptake (mean SUV ˜ 2.5 at 60 min)



      • When iodine avid, may not have any FDG uptake


    • Elevated TSH may result in double the SUV of WDTC vs. suppressed state



      • Best with stimulated thyroglobulin > 10 mU/L


    • Invaluable for identifying recurrence and metastases in soft tissue, lymph nodes, liver, lungs, and bone



      • Many of these lesions not visible or detected prospectively by CT


    • FDG PET can follow a negative I-131 or I-123 whole-body scan in patients with elevated thyroglobulin (Tg)



      • 15-20% of patients with WDTC and high serum thyroglobulin have negative diagnostic I-131 whole-body scans


      • I-131 or I-123 whole-body scan should be performed prior to injection of FDG if both scans are performed on same day


      • Small deposits may produce false negatives on I-131 scan


      • Metastases tend to become more aggressive and FDG avid as they dedifferentiate and lose ability to concentrate I-131


      • 15% of these patients have persistent, recurrent, or metastatic disease


      • Generally 75% or better sensitivities for local recurrences and distant metastases


      • PET/CT imaging has diagnostic value regardless of thyroglobulin level


    • Use of TSH to increase uptake by thyroid tissue is controversial, but has been shown to be effective in some studies


    • Non-iodine-avid recurrence: FDG PET may help identify areas amenable to surgical removal


  • MTC



    • MTC has low avidity for iodine, making radioiodine imaging and therapy ineffective



      • FDG PET effective for detection of disease


    • FDG PET improves detection of suspected recurrent disease undetectable by CT/MR



      • Elevated tumor markers, but no gross disease on cross-sectional imaging


      • Sensitivity 70-100%, specificity 79-90%


      • Poorer sensitivity for liver and lung foci, especially when < 1 cm


      • Controversy exists as to whether PET can reliably assess recurrent, persistent MTC


      • May be significant overlap of serum calcitonin levels between positive and negative FDG PET scans


      • Elevated calcitonin not specific; can be elevated in conditions such as CRI


      • I-123-PET/CT combined with FDG PET/CT allows localization of both foci of highly specific I-123 uptake and iodine-negative tumors


Other Modality Findings



  • I-123 or I-131 whole-body scan



    • Tumors may become less well differentiated and lose iodine avidity


    • Whole-body scan may appear normal despite extensive metastatic disease


    • I-123 scans miss metastases in bone, lungs, and lymph nodes


  • I-131 scintigraphy and serial thyroglobulin measurements



    • Used after near/total thyroidectomy and ablation


    • Standard method to detect differentiated thyroid cancer recurrence


    • Thyroglobulin threshold of 10 ng/mL commonly used as cutoff for suspicion of recurrence



      • Anti-thyroglobulin antibodies may lead to falsely low levels of measured serum thyroglobulin


  • Surveillance imaging following I-131 typically performed with high resolution US



    • FNA can be performed at time of exam


    • FDG PET for suspicion of recurrence in sites inaccessible by US


DIFFERENTIAL DIAGNOSIS


Benign Thyroid Conditions



  • 50% of FDG-avid nodules are benign (usually follicular adenoma [FA])



    • FA: Solitary mass without adenopathy or evidence of invasion


  • Incidentally identified FDG-avid nodules should be biopsied, as 50% are malignant


  • Multinodular goiter: Diffusely enlarged gland with multiple nodules and coarse calcifications



Thyroid Non-Hodgkin Lymphoma (NHL)



  • Infiltrating mass associated with diffuse enlargement of gland


  • Calcification in mass or LN rare


Parathyroid Adenoma



  • May present with similar features to thyroid carcinoma on FDG PET


  • Usually extrathyroidal


Other Cancers of Head and Neck



  • Anaplastic thyroid cancer


  • Thyroid lymphoma


  • Squamous cell carcinoma


  • Neuroendocrine tumors


  • Metastatic disease


Normal/Benign Extrathyroidal Structures



  • Asymmetrical muscle uptake



    • Minimize activity and agitation (benzodiazepine useful)


    • Reschedule examination in hyperglycemic patients (> 200 mg/dL)


    • Provide comfortable support of head/neck


  • Vocal cords and cricoarytenoids



    • Minimize talking, activity, and agitation during FDG uptake period


    • Unilateral vocal cord paralysis (surgery, invasion) can cause asymmetric uptake


  • Tonsillar and adenoid tissue



    • FDG uptake observed due to inflammatory activity


    • Obtain careful history of recent illness and allergies


  • Reactive lymph nodes



    • Correlate with presence of enhanced tonsillar FDG uptake, recent illness


  • Salivary glands



    • Treatment with I-131 may lead to asymmetric salivary gland uptake


    • Accessory sites of salivary tissue may be difficult to distinguish from lymph nodes


  • Cervical spine arthritis



    • Due to degeneration or rheumatic disease


    • Focal uptake in facet joints may mimic metastatic disease


  • Tracheostomy sites


PATHOLOGY


General Features

Sep 22, 2016 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on Thyroid Cancer
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