What specimen is needed to assess a RET mutation?

Specimen for RET Mutation Testing

The specimen needed depends on the tumor type: for NSCLC, non-medullary thyroid cancer, and other solid tumors, use formalin-fixed paraffin-embedded (FFPE) tumor tissue as first-line; for medullary thyroid carcinoma (MTC), use blood or sputum for germline analysis first. 1

Specimen Selection by Clinical Scenario

For NSCLC, Non-MTC Thyroid Cancer, and Other Solid Tumors

Primary specimen: FFPE tumor tissue

• This is the gold standard for detecting RET fusions in these malignancies 1
• Tumor tissue testing is required to definitively exclude RET fusion 1
• Can be obtained from primary tumor or metastatic sites

Alternative when tissue unavailable:

• Liquid biopsy (cell-free nucleic acid) when FFPE specimens are not available or exhausted 1
• Use cell-free nucleic acid NGS panel from blood
• This is explicitly recommended as the backup approach when tissue is insufficient

For Medullary Thyroid Carcinoma (MTC)

Initial specimen: Blood or sputum

• Start with germline analysis using blood or sputum 1
• This identifies hereditary RET mutations (MEN2A/2B syndromes)
• If known familial mutation exists, simple Sanger sequencing on blood leukocyte DNA is sufficient 1

Secondary specimen if germline negative:

• FFPE tissue from metastatic lesions if patient develops metastatic disease 1
• This identifies somatic RET mutations that may be present in sporadic MTC
• Approximately 7.3% of apparently sporadic MTC cases harbor germline mutations 2

Important Clinical Caveats

Fine-needle aspiration (FNA) specimens can be used:

• FNA material is viable for RET mutation detection 3
• Particularly useful for preoperative diagnosis in MTC to distinguish sporadic from familial disease
• Can identify somatic mutations in thyroid nodules and lymph nodes before surgery 3

Do NOT use:

• RET immunohistochemistry (IHC) is not recommended for any indication 1
• IHC lacks the specificity and sensitivity needed for clinical decision-making

Practical Algorithm

1. Identify tumor type first

   • MTC → Start with blood/sputum for germline
   • NSCLC/non-MTC thyroid/other solid tumors → Start with FFPE tissue

2. If tissue unavailable → Proceed to liquid biopsy (blood-based cell-free DNA)

3. If germline testing negative in MTC and disease progresses → Obtain FFPE tissue from metastatic site for somatic mutation analysis

This specimen selection strategy ensures appropriate identification of patients eligible for selective RET inhibitor therapy while avoiding unnecessary testing and optimizing the chance of detecting clinically actionable RET alterations.