A double-hit lymphoma FISH panel looks for chromosome rearrangements involving MYC, BCL2, and BCL6 in an aggressive B-cell lymphoma. These rearrangements matter because certain combinations define biologically distinct, high-risk lymphomas that cannot be identified reliably from cell appearance or protein staining alone. The terminology has changed in modern classifications: the strongest established “double-hit” entity is now lymphoma with concurrent MYC and BCL2 rearrangements, while MYC/BCL6-rearranged cases are handled separately in current WHO and International Consensus Classification frameworks. A FISH panel can still test all three genes because the pattern helps classify the tumor and distinguishes a true genetic double hit from “double-expressor” lymphoma, which refers to increased MYC and BCL2 proteins on immunohistochemistry without necessarily having either rearrangement. The report therefore needs to be read gene by gene and then interpreted as a combination, not simply as “positive” or “negative.”
- A true genetic double hit requires specific rearrangements detected by FISH or another validated genomic method; MYC/BCL2 protein co-expression alone is not a double hit.
- Concurrent MYC and BCL2 rearrangements are the defining combination in the current WHO double-hit category; an additional BCL6 rearrangement can create a triple-hit pattern.
- MYC plus BCL6 without BCL2 is no longer grouped identically with MYC/BCL2 disease in modern classifications.
- FISH results are usually reported for each gene as rearranged or not rearranged, often with the percentage of abnormal nuclei and a laboratory-specific cutoff.
- No special patient preparation is needed for the FISH assay itself; testing is usually performed on tissue from a lymphoma biopsy.
Table of Contents
- What the Double-Hit FISH Panel Tests
- How FISH Testing Is Performed
- How to Read MYC, BCL2, and BCL6 Results
- Double-Hit, Triple-Hit, and Double-Expressor Lymphoma
- Why Modern Classification Changed
- Testing Strategy, Limitations, and Pitfalls
- What the Result Means for Prognosis and Next Steps
What the Double-Hit FISH Panel Tests
The panel detects structural chromosome changes that place MYC, BCL2, or BCL6 under abnormal regulatory control. These genes have different normal functions, but when rearranged in a B-cell lymphoma they can cooperate to promote rapid growth and resistance to cell death.
MYC is a transcription factor that drives cell growth, metabolism, and proliferation. A MYC rearrangement is a defining feature of Burkitt lymphoma, but it also occurs in a subset of diffuse large B-cell lymphoma (DLBCL) and other high-grade B-cell lymphomas. The clinical meaning depends on the partner abnormalities and the overall tumor phenotype. A dedicated MYC rearrangement test focuses on this gene alone, while a double-hit panel asks whether MYC is rearranged together with BCL2 and/or BCL6.
BCL2 encodes a protein that blocks programmed cell death. The classic BCL2 rearrangement, often involving IGH and BCL2, is common in follicular lymphoma and can also be found in MYC/BCL2-rearranged aggressive lymphoma. A BCL2 rearrangement FISH result therefore has different implications depending on morphology and the presence or absence of MYC rearrangement.
BCL6 is a transcriptional repressor involved in germinal-center B-cell biology. BCL6 rearrangements occur in a subset of large B-cell lymphomas and can coexist with MYC. Current classification systems do not treat a MYC/BCL6 combination as biologically identical to MYC/BCL2 disease, which is an important update from older terminology.
A three-gene panel is useful because the combinations carry the meaning. If only MYC is rearranged, that is a single-hit result in informal language. If MYC and BCL2 are rearranged, the result supports the modern double-hit entity when the pathology is otherwise compatible. If MYC, BCL2, and BCL6 are all rearranged, the tumor has a triple-hit pattern and is classified within the MYC/BCL2-rearranged category in WHO-HAEM5.
How FISH Testing Is Performed
FISH stands for fluorescence in situ hybridization. The laboratory applies fluorescent DNA probes to tumor-cell nuclei. These probes bind defined regions of a chromosome and create signal patterns that can reveal whether a gene has been broken apart or abnormally joined to another locus.
For MYC, BCL2, and BCL6, laboratories commonly use break-apart probes. Each probe set places differently colored signals on opposite sides of the gene. In a normal nucleus, the signals remain together or overlap. If the gene is rearranged, the signals separate beyond the laboratory’s validated distance threshold.
Some laboratories also use dual-fusion probes to detect a specific partner, such as IGH::BCL2. Break-apart probes are broader because they can detect rearrangements with multiple possible partners, but they usually do not identify the partner gene by themselves.
The test is most often performed on formalin-fixed paraffin-embedded tissue from a lymph node or extranodal biopsy. It can also be performed on bone marrow or other suitable samples. No fasting or medication changes are needed specifically for FISH. The quality of the sample matters more than patient preparation.
A typical FISH report may include:
| Report item | Typical wording | Meaning |
|---|---|---|
| MYC | Rearrangement detected / not detected | Shows whether MYC has a structural rearrangement |
| BCL2 | Rearrangement detected / not detected | Shows whether BCL2 is rearranged |
| BCL6 | Rearrangement detected / not detected | Shows whether BCL6 is rearranged |
| Abnormal nuclei | Percentage above a laboratory cutoff | Quantifies how many scored nuclei show the abnormal signal pattern |
There is no universal percentage cutoff that patients can apply across laboratories. Each laboratory validates its own thresholds based on probe design, specimen type, normal controls, and scoring method. A result should therefore be interpreted against the reference cutoff printed on that report.
How to Read MYC, BCL2, and BCL6 Results
The safest way to read the report is to identify the status of each gene first, then interpret the combination. A single word such as “positive” can be misleading if it is not clear which probe was positive.
The common patterns are:
| MYC | BCL2 | BCL6 | General interpretation |
|---|---|---|---|
| Not rearranged | Not rearranged | Not rearranged | No rearrangement-based double-hit pattern detected by this panel |
| Rearranged | Not rearranged | Not rearranged | MYC rearrangement only; interpret with morphology and lymphoma type |
| Rearranged | Rearranged | Not rearranged | MYC/BCL2 double-hit pattern |
| Rearranged | Rearranged | Rearranged | Triple-hit pattern involving MYC, BCL2, and BCL6 |
| Rearranged | Not rearranged | Rearranged | MYC/BCL6-rearranged lymphoma; classified separately from MYC/BCL2 disease in modern systems |
A negative panel does not mean there is no lymphoma. It means the tested rearrangements were not detected. Most DLBCL does not have a MYC/BCL2 double hit. The diagnosis still rests on histology, immunophenotype, and other molecular findings.
A positive rearrangement also does not replace the biopsy diagnosis. For example, MYC rearrangement can occur in Burkitt lymphoma, DLBCL, and other high-grade B-cell lymphomas. The pathologist determines which disease entity fits the entire case.
The percentage of abnormal nuclei is not the same as the percentage of the body affected by lymphoma, and it is not a stage. It is simply the proportion of scored cells in that specimen that show the FISH signal pattern. A 70% abnormal-nuclei result does not mean “70% cancer in the body.”
Similarly, FISH does not measure the amount of MYC or BCL2 protein. Protein expression is evaluated by immunohistochemistry, which answers a related but different question.
Double-Hit, Triple-Hit, and Double-Expressor Lymphoma
The terms double hit and double expressor are often confused, but they describe different laboratory findings.
A genetic double hit refers to two defining gene rearrangements, most importantly MYC plus BCL2 in the current classification. A triple hit adds BCL6 rearrangement. These are chromosome-level abnormalities demonstrated by FISH or another genomic method.
A double-expressor lymphoma refers to increased MYC and BCL2 protein expression by immunohistochemistry. Laboratories commonly use cutoffs such as MYC expression in at least 40% of tumor cells and BCL2 expression in at least 50%, although exact thresholds and clinical use can vary. Double-expressor status is more common than a true double hit.
A tumor can be:
- double expressor but FISH negative for MYC/BCL2 rearrangements;
- genetically double hit and also show high MYC/BCL2 protein expression;
- MYC rearranged without BCL2 rearrangement;
- or negative for both patterns.
This distinction matters because protein overexpression can happen through mechanisms other than chromosome rearrangement. Immunohistochemistry is useful for tumor characterization and may prompt FISH testing, but it cannot establish a rearrangement-based double-hit diagnosis by itself.
The same principle applies to BCL6. BCL6 protein positivity is common in germinal-center-derived lymphomas and does not mean the BCL6 gene is rearranged. FISH is needed to establish rearrangement status.
A lymphoma flow cytometry panel can confirm a clonal B-cell population and define surface markers, but it does not substitute for FISH when the classification depends on MYC and BCL2 rearrangements.
Why Modern Classification Changed
Older classification grouped aggressive B-cell lymphomas with MYC plus BCL2 and/or BCL6 rearrangements under one broad high-grade category. WHO-HAEM5 and the International Consensus Classification refined that approach because MYC/BCL2 and MYC/BCL6 tumors are not molecularly uniform.
In WHO-HAEM5, the defining entity is diffuse large B-cell lymphoma/high-grade B-cell lymphoma with MYC and BCL2 rearrangements. Cases may also have BCL6 rearrangement. MYC/BCL6-rearranged tumors without BCL2 rearrangement are classified according to their morphology and other features, commonly as DLBCL, not otherwise specified, or high-grade B-cell lymphoma, not otherwise specified.
The International Consensus Classification likewise separates MYC/BCL2 from MYC/BCL6 biology, although the exact naming structure differs and includes a provisional MYC/BCL6 category. This means a pathology report may use slightly different formal wording depending on which classification system the institution follows.
The practical patient-level lesson is simple: not every MYC plus BCL6 result is equivalent to a MYC plus BCL2 result. If an older report says “double-hit lymphoma” based on MYC/BCL6, a modern hematopathology review may describe the same biology using a different diagnostic label.
The classification change also explains why a three-probe FISH panel remains useful. The panel is not merely trying to generate the label “double hit.” It maps the relevant rearrangements so the tumor can be placed in the correct modern category.
Testing Strategy, Limitations, and Pitfalls
One major challenge is deciding which large B-cell lymphomas should undergo FISH. MYC/BCL2-rearranged tumors often have a germinal-center B-cell phenotype, high proliferation, or aggressive morphology, but no combination of morphology and immunohistochemistry detects every case reliably. For that reason, many hematopathology practices favor broad FISH screening of newly diagnosed DLBCL or high-grade B-cell lymphoma, while others use an institution-specific testing algorithm.
FISH also has technical limitations. A break-apart probe can detect that a gene is rearranged without identifying the partner. Rare cryptic rearrangements may escape a particular probe design. Poor tissue quality, decalcification, low tumor content, overlapping nuclei, and signal truncation can make a case uninterpretable or borderline.
Copy-number gains create another source of confusion. Extra MYC, BCL2, or BCL6 signals are not automatically rearrangements. Amplification or copy gain may have biological relevance, but it is not the same finding as a break-apart pattern and does not by itself create a double-hit diagnosis.
A result close to the laboratory cutoff may require pathologist review, repeat scoring, another tissue block, or an alternative method. The report should state whether a specimen was adequate and whether the result was definitive, borderline, or technically limited.
Another pitfall is ordering FISH without enough morphologic context. In follicular lymphoma, for example, a BCL2 rearrangement is common and has a different meaning than BCL2 rearrangement in a MYC-rearranged high-grade tumor. In Burkitt lymphoma, MYC rearrangement is expected but BCL2 rearrangement generally points away from classic Burkitt biology.
The final integrated pathology report—not the isolated FISH worksheet—should be used for diagnosis.
A further detail is the rearrangement partner. MYC can be rearranged with an immunoglobulin gene or with a non-immunoglobulin partner. In some studies, partner identity has correlated with tumor biology or outcome, but partner testing is not required in every routine case. A standard MYC break-apart assay tells the laboratory that MYC is rearranged without naming the partner. If the distinction is clinically or diagnostically important, additional fusion probes, chromosome studies, or sequencing may be used.
The same principle applies to BCL2 and BCL6. A break-apart result describes structural disruption of the gene locus, not the exact mechanism by which the rearrangement changes gene activity. This is why two patients can both have a “BCL6 rearrangement” yet have different partner genes and different co-mutation profiles. The panel is powerful for classification, but it is not a complete genomic map of the lymphoma.
Preanalytic quality is especially important in small needle biopsies. The laboratory needs enough intact tumor nuclei to score a representative population. If a block contains extensive necrosis or only a tiny focus of lymphoma, a technically negative result may be less reassuring than a negative result from a well-preserved, tumor-rich specimen. Pathologists may mark a different area of the block, select another tissue block, or request a repeat biopsy if the molecular answer is essential and the first sample is inadequate.
When a result is unexpected, comparing FISH with immunohistochemistry can help identify the problem. Strong MYC protein expression with a negative MYC break-apart probe does not prove the probe failed; many tumors overexpress MYC without rearranging the gene. Conversely, a MYC rearrangement may be present even when protein expression is not striking. The assays measure different biological layers and should not be expected to match perfectly.
What the Result Means for Prognosis and Next Steps
MYC/BCL2-rearranged lymphoma is generally considered an aggressive, higher-risk form of large B-cell lymphoma. Historically, these tumors had poorer outcomes with standard R-CHOP than many DLBCL cases without the double hit. Prognosis still varies with age, stage, LDH, performance status, extranodal disease, treatment, and response.
A positive result often changes the formal lymphoma classification and prompts the hematology team to consider the disease in a higher-risk treatment framework. The exact regimen cannot be chosen from the FISH result alone. Clinical trials, patient fitness, disease burden, central nervous system risk, and evolving evidence all influence management.
The result may also affect how urgently the workup is completed. Patients with aggressive lymphoma typically undergo staging with PET/CT, blood testing, and evaluation for relevant organ function before treatment. Serum LDH is often included in prognostic tools, but LDH cannot determine double-hit status; the genetic finding requires tissue-based testing.
When the panel is negative but the lymphoma still appears highly aggressive, the clinical team does not dismiss the risk. Other high-grade entities and molecular subtypes can behave aggressively without MYC/BCL2 rearrangements. Conversely, the presence of a double hit does not predict an individual patient’s exact survival time.
If the report uses older terminology, ask which specific genes are rearranged. “Double-hit lymphoma” is less informative than “MYC and BCL2 rearranged, BCL6 not rearranged.” The gene-by-gene description remains clear even as classification names evolve.
For follow-up, FISH is usually a diagnostic classification tool, not a routine serial blood marker. Response is assessed by clinical examination, imaging, and repeat tissue or marrow evaluation when clinically needed. If relapse occurs and new tissue is biopsied, repeat molecular characterization may be useful because lymphoma biology can evolve.
The most helpful discussion with the hematologist or pathologist centers on three questions: Which rearrangements were detected? What is the final modern diagnostic category? How does that category, together with stage and clinical risk factors, affect the treatment plan?
References
- Large B-Cell Lymphomas in the 5th Edition of the WHO-Classification of Haematolymphoid Neoplasms—Updated Classification and New Concepts 2023 (Review)
- Diagnostic and prognostic molecular pathology of lymphoid malignancies 2024 (Review)
- Double hit lymphoma: contemporary understanding and practices 2025 (Review)
- Clinical and Pathological Features That Predict High-Grade B-Cell Lymphomas (HGBCLs) with MYC and BCL2 or BCL6 Translocations (Double-Hit Lymphoma): A Systematic Review and Meta-Analysis 2026 (Systematic Review)
- The International Consensus Classification of Mature Lymphoid Neoplasms: a report from the Clinical Advisory Committee 2022 (Position Statement)
Disclaimer
MYC, BCL2, and BCL6 FISH results must be interpreted with the biopsy morphology, immunophenotype, and current lymphoma classification. Terminology has changed, so an older “double-hit” label may not map exactly to a current WHO or ICC category. Treatment decisions should be made with a hematologist or oncologist using the complete pathology and staging information.
