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.


Abbreviations and Synonyms

  • Well-differentiated thyroid cancer (WDTC)

  • Medullary thyroid carcinoma (MTC)

  • Anaplastic thyroid carcinoma


  • 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


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


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


General Features

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