Which thyroid tumor cannot be confirmed with Fine Needle Aspiration Cytology (FNAC)?

Hurthle Cell Carcinoma Cannot Be Confirmed with FNAC

Hurthle cell carcinoma is the thyroid tumor that cannot be definitively confirmed with Fine Needle Aspiration Cytology (FNAC). While FNAC can identify Hurthle cells, it cannot distinguish between benign Hurthle cell adenomas and malignant Hurthle cell carcinomas because this distinction requires evidence of capsular or vascular invasion, which can only be determined through histological examination of the surgical specimen.

Diagnostic Limitations of FNAC for Different Thyroid Tumors

Hurthle Cell Carcinoma

• FNAC can only identify the presence of Hurthle cells but cannot determine malignancy 1
• The diagnosis of Hurthle cell carcinoma specifically requires evidence of capsular or vascular invasion, which cannot be assessed through cytology alone 1
• FNAC has high specificity but low sensitivity for Hurthle cell neoplasms due to sampling errors 2
• Molecular diagnostics are not recommended for Hurthle cell neoplasms as they may not perform well for these lesions 1

Other Thyroid Carcinomas and FNAC Capabilities

Papillary Thyroid Cancer

• Can be reliably identified on FNAC through characteristic nuclear features
• FNAC is a very sensitive test for papillary carcinoma 1
• Cytological examination can categorize specimens as "carcinoma (papillary, medullary, or anaplastic)" 1

Medullary Thyroid Cancer

• Can be confirmed with FNAC, though may occasionally require additional immunohistochemical studies (e.g., calcitonin) 1
• FNAC combined with calcitonin measurement has high sensitivity for MTC 1
• Cytological examination can categorize specimens as medullary carcinoma 1

Anaplastic Thyroid Cancer

• Can be identified on FNAC based on characteristic morphological features
• Cytological examination can categorize specimens as anaplastic carcinoma 1
• May require core or surgical biopsy in cases where FNAC results are suspicious or not definitive 1

Diagnostic Challenges with Hurthle Cell Lesions

• Hurthle cells can be seen in both non-neoplastic and neoplastic thyroid lesions, creating diagnostic dilemmas 3
• Studies show that cytological diagnosis of Hurthle cell neoplasm corresponds to a higher incidence of malignancy (32.1%) compared to non-Hurthle cell follicular neoplasms (15.5%) 4
• Key cytomorphologic features that suggest malignancy include:
  • Nuclear budding
  • Granular chromatin
  • Cellular dyshesion 5
• However, these features alone cannot definitively distinguish between adenoma and carcinoma

Management Approach for Thyroid Nodules with Hurthle Cells

1. Initial evaluation: Ultrasound assessment of the nodule
2. FNAC: Perform on nodules >1 cm or smaller nodules with suspicious features 1
3. When Hurthle cells are identified:
   • Consider the nodule as a potential neoplasm
   • Surgical excision (typically lobectomy) is required for definitive diagnosis 1
   • Clinical risk factors, sonographic patterns, and patient preference should guide management 1

Common Pitfalls to Avoid

• Misinterpreting Hurthle cell-like cells in papillary thyroid carcinoma as benign Hurthle cells 5
• Relying solely on FNAC for definitive diagnosis of Hurthle cell carcinoma
• Undersampling of nodules with extensive cystic degeneration can lead to false-negative results 2
• Failure to recognize that Hurthle cell neoplasms require histological examination for definitive classification

In clinical practice, when FNAC reveals Hurthle cells, the patient should be counseled that surgical excision is necessary for definitive diagnosis, as the distinction between benign and malignant Hurthle cell neoplasms cannot be made on cytology alone.