What are the diagnostic imaging methods and treatment options for thyroid cancer?

Diagnostic Imaging for Thyroid Cancer

Neck ultrasound is the primary and most effective imaging modality for initial diagnosis and ongoing surveillance of thyroid cancer, with fine needle aspiration cytology (FNAC) guided by ultrasound serving as the definitive diagnostic tool for nodules >1 cm or smaller nodules with suspicious features. 1

Initial Diagnostic Imaging Approach

Ultrasound as First-Line Imaging

• Neck ultrasound should be performed for any suspected thyroid nodule, as it can detect clinically impalpable lesions and characterize them with high resolution due to the superficial location of the thyroid gland 1, 2

• Suspicious ultrasound features that warrant FNAC include:

  • Hypoechogenicity (darker appearance than surrounding tissue) 1
  • Microcalcifications (tiny calcium deposits within the nodule) 1
  • Irregular or poorly defined margins 1
  • Absence of peripheral halo 1
  • Increased internal vascularity on color Doppler 1
  • Taller-than-wide shape in transverse plane 1

• FNAC should be performed on nodules >1 cm, or on smaller nodules (<1 cm) if there is:

  • History of head/neck irradiation 1
  • Family history of thyroid cancer 1
  • Presence of cervical lymphadenopathy 1
  • Any of the suspicious ultrasound features listed above 1

Lymph Node Assessment

• Careful ultrasound exploration of cervical lymph node chains must be performed before any surgical intervention to assess for metastatic disease 1

• Suspicious lymph node features include:

  • Loss of elongated shape (becoming rounded) 1
  • Absence of normal hilar echoes 1, 2
  • Microcalcifications within the node 1
  • Cystic changes 1
  • Peripheral rather than central vascularity 1
  • Solid thyroid-tissue-like appearance 1

Advanced Imaging for Staging and Metastatic Disease

Cross-Sectional Imaging Indications

• CT scanning of the neck and chest is indicated when:

  • Locoregional or distant metastases are suspected or confirmed 1
  • Serum thyroglobulin (Tg) levels are rising without identifiable disease on ultrasound 1
  • There is concern for invasion of the trachea, larynx, or great vessels 1, 3
  • Patients have intermediate-to-high risk of persistent/recurrent disease 1

• Contrast-enhanced CT is used for neck and mediastinal lymph nodes but not for lung imaging 1

• Critical caveat: All radioactive iodine (RAI) treatment must be deferred for at least 6 weeks after administration of iodinated contrast medium 1

MRI Applications

• Contrast-enhanced MRI is appropriate for evaluating:
  • Neck structures (though CT is often superior due to less respiratory motion artifact) 1
  • Liver metastases 1
  • Bone metastases 1
  • Brain metastases 1

Nuclear Medicine Imaging

Whole Body Scan (WBS) with Radioactive Iodine

• Diagnostic WBS is NOT indicated during routine follow-up due to low sensitivity (27-55%), though specificity is high (91-100%) 1

• WBS should only be performed after therapeutic RAI administration for post-treatment assessment 1

FDG-PET/CT Imaging

• FDG-PET/CT is the first-line isotopic imaging for RAI-refractory thyroid cancer, with sensitivity around 94% and specificity 80-84% 1

• Specific indications for FDG-PET/CT include:

  • Negative cross-sectional imaging but serum Tg >10 ng/dL 1
  • Aggressive histologic subtypes (poorly differentiated thyroid cancer, widely invasive follicular carcinoma) 1
  • Rising Tg or thyroglobulin antibody (TgAb) levels without identifiable disease 1
  • Known RAI-refractory disease for extent assessment 1

• Important prognostic information: FDG uptake is associated with worse prognosis and refractoriness to RAI treatment, though it does not reliably predict tumor growth rate 1

Treatment Overview

Surgical Management

• Total or near-total thyroidectomy is the standard initial treatment for differentiated thyroid cancer (DTC) when:

  • Diagnosis is made preoperatively and nodule is ≥1 cm 1, 4
  • Any size nodule with metastatic, multifocal, or familial disease 1, 4

• Less extensive surgery (lobectomy) may be acceptable for:

  • Unifocal DTC discovered incidentally after surgery for benign disease 1, 4
  • Tumor must be small, intrathyroidal, and favorable histology (classical papillary or follicular variant) 1

Radioactive Iodine Therapy

• RAI is NOT recommended for small (≤1 cm) intrathyroidal DTC without locoregional metastases (low-risk cases) 4

• For other low-risk DTC, if RAI is given, use low activities (30 mCi, 1.1 GBq) following recombinant human TSH 4

• Intermediate-risk patients may receive 30-100 mCi (1.1-3.7 GBq) 4

• High-risk patients require 100 mCi (3.7 GBq) 4

Anaplastic Thyroid Cancer (ATC)

• ATC is almost uniformly fatal with median survival of 5 months, and complete resection is rarely possible 1

• Optimal outcomes require complete or near-complete resection followed immediately by high-dose external beam radiotherapy with or without chemotherapy 1, 4

• Incomplete palliative resection does not affect prognosis and is not recommended 1, 4

Follow-Up Imaging Strategy

Post-Treatment Surveillance Schedule

• All DTC patients should have neck ultrasound and serum Tg/TgAb assays at 6-18 months after primary treatment (surgery ± RAI) 1

• Low-risk patients with excellent response:

  • Tg/TgAb every 12-24 months 1
  • Repeat neck ultrasound as needed based on Tg/TgAb trends 1
  • TSH maintained at 0.5-2 mIU/mL 1

• Intermediate-risk patients with excellent response:

  • Same schedule as low-risk patients 1
  • TSH maintained at 0.5-2 mIU/mL 1

• Patients with biochemical incomplete or indeterminate response:

  • Tg/TgAb and neck ultrasound every 6-12 months 1
  • Rising Tg or TgAb levels warrant additional imaging (FDG-PET or cross-sectional imaging) 1
  • TSH suppression to 0.1-0.5 mIU/mL 1

• High-risk patients:

  • Tg/TgAb every 6-12 months 1
  • Cross-sectional or functional imaging if detectable Tg persists 1
  • More intensive imaging workup may be needed even with undetectable Tg (due to possible tumor dedifferentiation) 1
  • TSH suppression to <0.1 mIU/mL 1

Critical Prognostic Indicator

• Short serum Tg doubling time (<1 year) is associated with poor outcome and should prompt immediate comprehensive imaging staging 1