RET Fusion Test: Lung and Thyroid Cancer, Targeted Therapy, and Results

A RET fusion test looks for an acquired rearrangement that joins part of the RET gene to another gene inside cancer cells. The fusion can switch on the RET kinase continuously, creating a growth signal that may drive certain non-small cell lung cancers, differentiated thyroid cancers, and a smaller number of other solid tumors. Finding a true, expressed RET fusion can make a patient eligible for RET-targeted therapy and may influence the order of treatment.

RET fusion testing is usually performed on tumor tissue with a next-generation sequencing panel, preferably one that evaluates RNA as well as DNA. Blood-based circulating tumor DNA testing can help when tissue is unavailable, but a negative blood result may need tissue confirmation. RET fusions are different from inherited activating RET variants that cause multiple endocrine neoplasia type 2. The report should identify the fusion partner, the testing method, the specimen, assay quality, and whether the alteration is considered oncogenic and clinically actionable.

• A positive, in-frame RET fusion can identify a targetable cancer driver, especially in lung adenocarcinoma and papillary thyroid cancer.
• RNA-based NGS is often the most direct way to confirm that a RET rearrangement produces an expressed fusion transcript.
• A negative plasma result does not exclude a RET fusion because some tumors release little circulating DNA.
• RET fusion and RET mutation are not interchangeable; they occur in different cancer settings and may require different interpretation.
• Treatment choice depends on cancer type, stage, prior therapy, drug approval, other biomarkers, and the patient’s overall condition.

Table of Contents

• What a RET Fusion Is
• When RET Fusion Testing Is Used
• Testing Methods and Samples
• How to Interpret Results
• Targeted Therapy and Treatment Planning
• Resistance and Repeat Testing
• Limitations and Questions to Ask

What a RET Fusion Is

RET encodes a receptor tyrosine kinase, a protein that carries growth signals from the cell surface into the cell. In normal tissues, RET activity is tightly controlled. A fusion rearranges DNA so that the kinase portion of RET becomes attached to a partner gene whose structure encourages constant activation. The resulting fusion protein can signal without the normal external trigger.

Common partners include KIF5B in lung cancer and CCDC6 or NCOA4 in thyroid cancer, but many other partners exist. A clinically meaningful fusion usually keeps the RET kinase domain intact and places the gene segments in the correct reading frame. The partner name can sometimes help confirm that the finding is biologically plausible, but treatment eligibility usually depends more on whether the fusion is functional and validated than on one specific partner.

RET fusions are generally somatic, meaning they arose in the tumor and are not present throughout the body. They do not usually imply that children or siblings inherited a cancer syndrome. This is distinct from a germline RET pathogenic variant associated with MEN2 and medullary thyroid cancer. A germline RET genetic test answers that inherited-risk question.

A fusion also differs from a point mutation. RET mutations are especially important in medullary thyroid cancer, while RET fusions are characteristic drivers in subsets of non-small cell lung cancer and differentiated thyroid cancer. Reports that simply say “RET positive” are incomplete; clinicians need to know whether the result is a fusion, mutation, amplification, protein-expression finding, or uncertain rearrangement.

In lung adenocarcinoma, RET fusions account for a small percentage of tumors, commonly around 1% to 2%. They are often found in people with little or no smoking history, but smoking history should never be used to decide who deserves testing. In papillary thyroid cancer, prevalence varies with age, histologic subtype, population, and prior radiation exposure. RET fusions can be particularly enriched in pediatric and radiation-associated thyroid tumors.

When RET Fusion Testing Is Used

Broad molecular testing is standard for many patients with advanced nonsquamous non-small cell lung cancer because several actionable drivers can look identical under the microscope. RET should usually be evaluated alongside EGFR, ALK, ROS1, BRAF, MET, NTRK, KRAS, ERBB2, and other clinically relevant alterations. A comprehensive solid tumor NGS panel reduces tissue use and can detect multiple alteration types in one workflow.

RET fusion testing may be considered in:

• Newly diagnosed advanced or metastatic lung adenocarcinoma.
• Other nonsquamous NSCLC and selected squamous tumors when clinical features or limited biopsy material make histology uncertain.
• Recurrent or advanced differentiated thyroid cancer, particularly when radioactive iodine no longer controls disease and systemic treatment is being considered.
• Pediatric thyroid cancer, where gene fusions are relatively common and can influence treatment.
• Anaplastic thyroid cancer as part of urgent broad profiling, because rapidly identifying an actionable driver may alter therapy.
• An advanced solid tumor without a satisfactory treatment option when tumor-agnostic profiling is appropriate.
• A tumor with a suspicious RET rearrangement found by another method that needs confirmation.

Testing is most useful before selecting first systemic therapy in advanced driver-positive lung cancer. Starting immunotherapy before a full driver panel returns can complicate later treatment and may expose a patient to a therapy less likely to control a kinase-driven tumor. When the patient is clinically stable, the oncology team often waits for complete molecular results rather than acting on one rapid marker alone.

In early-stage disease, the role of broad profiling depends on the cancer type and evolving evidence. Testing may support clinical-trial eligibility, help characterize recurrence risk, or prepare for treatment if disease returns. It does not automatically mean targeted therapy is indicated after surgery.

Thyroid tumors may be tested after surgery, on a biopsy, or through a molecular classifier used for an indeterminate thyroid nodule. A fusion detected in a diagnostic nodule assay can support malignancy risk assessment, but that use is different from selecting systemic therapy for advanced cancer. The same result must be interpreted within the clinical question that prompted the test.

Testing Methods and Samples

RET fusions can be detected by RNA-based NGS, DNA-based NGS, fluorescence in situ hybridization, reverse-transcription PCR, or other validated methods. Each approach has strengths and weaknesses.

Method	Strengths	Important limitations
RNA-based NGS	Directly detects expressed fusion transcripts; can identify many partners; efficient for multiplex testing	RNA degrades more easily than DNA, especially in old or poorly fixed tissue
DNA-based NGS	Can assess mutations, copy-number changes, and some fusions in one assay; DNA is relatively stable	Large or repetitive introns can reduce fusion sensitivity; a structural call may not prove expression
FISH	Works on limited tissue and does not require knowledge of the partner	Can be difficult to interpret, does not identify the partner, and may detect nonfunctional rearrangements
RT-PCR	Fast and sensitive for known fusion transcripts	Misses uncommon or novel partners not included in the assay
Plasma ctDNA NGS	Requires a blood draw, samples multiple disease sites, and may return results quickly	False negatives occur when tumor DNA shedding is low; fusion detection varies by platform

RNA-based testing is often favored when fusion detection is a primary purpose. Some laboratories use DNA NGS first and reflex to RNA when the DNA result is negative, atypical, or suggests a rearrangement without defining an expressed transcript. An integrated DNA-and-RNA panel can provide a more complete answer from one specimen.

The specimen may be a formalin-fixed paraffin-embedded biopsy, surgical tissue, cytology cell block, fine-needle aspirate, or plasma. Pathology staff estimate tumor content and select an area with enough viable cancer cells. Decalcified bone specimens can perform poorly because some decalcification methods damage nucleic acids. A new biopsy may be preferable when the only sample is old, tiny, heavily necrotic, or exhausted by prior tests.

Preanalytic quality matters. Delayed fixation, excessive fixation, low tumor percentage, and degraded RNA can cause an “quantity not sufficient” or failed result. That is not the same as a true negative. The report should state whether quality-control metrics passed and whether the assay could assess all intended fusion targets.

Liquid biopsy is valuable when tissue acquisition is unsafe or would delay treatment. A positive, well-characterized RET fusion in plasma is usually actionable. A negative plasma test should be followed by tissue testing when feasible, especially in a patient whose cancer type warrants comprehensive profiling. The principles are similar to a liquid biopsy cancer test: positive findings can be highly informative, while negative findings depend heavily on DNA shedding.

Discordant RET results deserve review rather than an automatic assumption that one assay is wrong. A DNA panel may flag a rearrangement without proving that an in-frame fusion transcript is expressed. An RNA assay may fail because the tissue RNA is degraded even when a real fusion is present. FISH can show that the RET locus is rearranged but may not identify the partner or confirm whether the product is functional. When the result is unexpected, borderline, or central to treatment, the molecular pathologist may recommend an orthogonal method using another tissue section or a new specimen.

Specimen handling can be decisive. Small lung biopsies are often shared among diagnosis, PD-L1 staining, and several molecular tests. Reflex comprehensive testing can conserve tissue better than a long sequence of single-gene assays. Bone metastases exposed to strong acid decalcification may be unsuitable for RNA, and a sample with heavy necrosis can produce a false-negative result. The report should state tumor percentage, DNA and RNA quality, coverage, and which fusion partners or intronic regions the test can detect.

How to Interpret Results

A useful report does more than list “RET.” It should identify the partner genes, exons or breakpoints when available, reading frame, evidence of transcript expression, variant allele fraction or supporting reads, clinical tier, and any therapy or trial associations.

Positive for an oncogenic RET fusion

A positive result means the laboratory found a fusion with evidence that it can activate RET signaling. In the right cancer and clinical setting, this is a predictive biomarker for a selective RET inhibitor. The result also supports the idea that the fusion is a dominant driver, especially when other mutually exclusive drivers are absent.

A positive result does not guarantee response. Treatment can fail because of tumor heterogeneity, co-alterations, inadequate drug exposure, central nervous system disease, or pre-existing resistance mechanisms. The report also does not show whether the cancer is localized or metastatic; imaging and pathology determine stage.

Negative

A negative result means no reportable RET fusion was detected within the assay’s capabilities. It does not mean no targetable alteration exists. The clinician should review the rest of the panel and confirm whether the method was adequately designed for fusion detection.

Questions after a negative result include:

• Was RNA tested, or only DNA?
• Did the sample pass quality controls?
• Was tumor content adequate?
• Was the result from plasma with low or undetectable tumor fraction?
• Did the assay cover RET introns and uncommon partners?
• Is another tissue sample available?

A failed or insufficient result should be labeled as such rather than interpreted as negative.

Rearrangement detected, fusion uncertain

FISH or DNA sequencing may show a RET rearrangement without proving that a functional transcript exists. Reflex RNA testing can resolve whether the event is in-frame and expressed. A novel partner is not automatically nonactionable, but it requires careful review of the breakpoint, kinase-domain retention, and supporting evidence.

RET mutation or amplification instead of fusion

A RET point mutation in medullary thyroid cancer may be actionable and may also raise a germline-testing question. In lung adenocarcinoma, an isolated RET mutation may not carry the same meaning as a fusion. RET amplification alone is generally less established as a predictive biomarker. The oncology team should not substitute one alteration type for another when matching therapy.

Targeted Therapy and Treatment Planning

Selective RET inhibitors are designed to block RET kinase signaling while reducing off-target inhibition compared with older multikinase drugs. Selpercatinib has demonstrated substantial and durable activity in RET fusion-positive lung and thyroid cancers, including activity in brain metastases. Pralsetinib is another selective RET inhibitor, although approved uses and availability differ by country and can change over time.

For advanced RET fusion-positive NSCLC, randomized evidence has shown better progression-free outcomes with first-line selpercatinib than with platinum-based chemotherapy with or without pembrolizumab. That makes complete biomarker testing before treatment especially important. The final regimen still depends on local approvals, comorbidities, drug interactions, symptoms, and access.

For advanced RET fusion-positive differentiated thyroid cancer, a selective RET inhibitor may be considered when systemic treatment is needed, especially after radioactive iodine is no longer effective or appropriate. The timing should account for tumor pace, symptoms, lesion location, prior multikinase therapy, and whether local treatments can control limited disease.

Treatment monitoring usually includes imaging, laboratory tests, blood pressure, liver tests, and review of adverse effects. Potential toxicities vary by drug and can include elevated liver enzymes, hypertension, diarrhea, dry mouth, edema, fatigue, changes in heart rhythm, bleeding risk, wound-healing concerns, and thyroid-function changes. Dose interruptions or reductions can manage many adverse effects without abandoning an effective drug.

The presence of a RET fusion may also open clinical trials in earlier-stage disease, uncommon tumors, or resistance settings. Trial matching should verify that the assay and fusion meet the protocol’s eligibility rules.

Immunotherapy biomarkers such as PD-L1 and tumor mutational burden can still appear on the report, but they should not be interpreted in isolation from an oncogenic driver. RET fusion-positive lung cancers may show PD-L1 expression yet obtain less benefit from single-agent immunotherapy than expected from PD-L1 alone. Sequencing treatment thoughtfully can also reduce overlapping toxicity.

RET fusions should also be distinguished from activating RET point mutations. Germline or somatic RET mutations are central in medullary thyroid cancer and MEN2, whereas RET fusions are more typical of papillary and other non-medullary thyroid cancers and a subset of lung cancers. The treatment families can overlap, but the hereditary implications, tumor biology, and recommended confirmatory workup differ. A fusion found in tumor tissue generally does not diagnose MEN2. Conversely, a germline MEN2 result does not mean the patient has a RET fusion.

The timing of targeted therapy depends on the cancer and treatment setting. A positive result should be reviewed with disease stage, symptoms, brain metastases, prior systemic therapy, and current approvals. Selective RET inhibitors can produce meaningful responses, including intracranial activity, but they also require monitoring for blood pressure, liver tests, heart rhythm, drug interactions, and other treatment-specific risks. The oncology team should use the current prescribing information rather than a laboratory drug list alone.

Resistance and Repeat Testing

Even when a selective RET inhibitor works well, resistant cancer-cell populations can eventually emerge. Resistance can occur through changes in RET itself or through alternative signaling pathways that bypass RET.

On-target mechanisms include solvent-front mutations such as RET G810 substitutions and other kinase-domain changes that reduce drug binding. Off-target mechanisms can involve MET or KRAS amplification, activation of other pathways, or histologic transformation. The mechanism is not predictable from the original fusion alone.

Repeat molecular testing is most useful when the result could change the next treatment. Options include a new tissue biopsy, plasma ctDNA, or both. Plasma can capture alterations from several metastases, while tissue provides histology, tumor architecture, and a direct sample from a progressing lesion. A negative resistance test does not prove the absence of resistance; it may reflect low tumor fraction or a mechanism the assay does not measure.

Clinicians may continue a RET inhibitor while treating one or a few progressing sites locally when the rest of the disease remains controlled. Widespread progression more often prompts systemic reassessment. Clinical trials of next-generation RET inhibitors may be available for specific resistance mutations.

Repeat testing can also correct an earlier incomplete evaluation. A tumor originally tested with a small DNA panel may deserve RNA-based fusion analysis when disease becomes advanced. Conversely, a fusion established by a robust prior assay usually does not need to be rediscovered before every treatment unless a protocol specifically requires a recent sample.

Limitations and Questions to Ask

RET fusion testing can fail because of poor specimen quality, limited tumor cells, degraded RNA, incomplete intronic coverage, uncommon partners, low ctDNA shedding, or bioinformatics filters. Different laboratories may classify a rare fusion differently. The report should be reviewed in a molecular tumor board when the finding is novel, borderline, or discordant across methods.

A positive fusion also cannot answer every treatment question. It does not predict the exact duration of response, compare all available RET inhibitors for one individual, or replace assessment of stage, symptoms, organ function, and patient preferences.

Useful questions for the oncology team include:

1. Was the test performed on DNA, RNA, or both?
2. What tissue or blood sample was analyzed, and did it contain enough tumor?
3. What is the exact fusion partner and is the transcript in-frame and expressed?
4. Is the result considered an established oncogenic driver or an uncertain rearrangement?
5. Which RET inhibitors are approved for this cancer and line of treatment where I live?
6. Does the drug have activity against brain metastases?
7. What baseline blood pressure, liver, heart-rhythm, and medication checks are needed?
8. Should the tumor be retested at progression, and would tissue, plasma, or both be best?
9. Could this RET finding suggest an inherited syndrome, or is it clearly tumor-only?

The last question is especially relevant in medullary thyroid cancer, where a RET mutation may be germline. In contrast, a classic fusion in lung or papillary thyroid cancer is usually somatic. When the distinction is unclear, a genetics referral and confirmatory normal-tissue test can resolve it.

References

• First-Line Selpercatinib or Chemotherapy and Pembrolizumab in RET Fusion-Positive NSCLC 2023 (RCT)
• Efficacy and Safety of Selpercatinib in Patients With RET-Activated Thyroid Cancer: Long-Term Safety and Efficacy From LIBRETTO-001 2024 (Clinical Trial)
• Expert consensus on the diagnosis and treatment of RET fusion-positive non-small cell lung cancer in China 2023 (Consensus Statement)
• 2021 European Society for Medical Oncology (ESMO) Recommendations on Laboratory Diagnostics for RET Gene Fusions and Mutations 2021 (Guideline)
• Selpercatinib in RET Fusion–Positive Non–Small Cell Lung Cancer: Final Efficacy and Safety Results From LIBRETTO-001 2025 (Clinical Trial)
• Advancing RET-targeted therapy in thyroid cancer: current evidence and future directions 2025 (Review)

Disclaimer

This article provides general information and does not replace molecular-pathology review or treatment advice from an oncology team. RET testing methods, drug approvals, and guideline recommendations change over time and differ by country. Treatment decisions must account for the exact fusion, cancer type, stage, prior therapy, other biomarkers, medical conditions, and current prescribing information.