Mycosis Fungoides, Sézary Syndrome, and Dupixent Claims Explained

Why This Diagnosis Keeps Coming Up

If you or someone in your family was treated with Dupixent (dupilumab) for eczema or atopic dermatitis and later received a diagnosis of cutaneous T-cell lymphoma, you have probably heard two unfamiliar terms from your dermatologist or oncologist: mycosis fungoides and Sézary syndrome.

Those two names appear repeatedly in the lawsuits that have been filed against Sanofi and Regeneron. Reported filings include claims such as one described by AboutLawsuits.com alleging that a Dupixent user was later diagnosed with mycosis fungoides.

This post explains what those diagnoses are, why they are so easily confused with severe eczema, and how they relate to the claims plaintiffs have begun bringing. It is educational. Nothing here is a statement that Dupixent has been proven to cause cancer — that question is disputed and is exactly what the litigation is examining.

What Cutaneous T-Cell Lymphoma Is

Cutaneous T-cell lymphoma, usually shortened to CTCL, is a group of rare cancers of a type of white blood cell called a T lymphocyte. Unlike most lymphomas, which are first found in lymph nodes, CTCL first shows up in the skin.

Because it begins in the skin, CTCL frequently looks like a common skin condition. The National Cancer Institute maintains general information on lymphoma types, staging, and treatment for patients and caregivers.

CTCL is uncommon. It is also a spectrum rather than a single disease, which matters for anyone trying to understand their own pathology report.

Mycosis Fungoides: The Most Common Form

Mycosis fungoides is the most frequently diagnosed form of CTCL. Despite the name, it has nothing to do with a fungus — the term is a historical holdover.

It is often described in stages that can unfold slowly:

• Patch stage — flat, discolored, sometimes itchy areas that can resemble eczema or psoriasis
• Plaque stage — thicker, raised, well-defined lesions
• Tumor stage — raised nodules that may ulcerate
• Advanced disease — involvement of lymph nodes, blood, or internal organs

Many people spend years in the earliest stage. Some are treated for eczema that entire time.

Sézary Syndrome: When CTCL Involves the Blood

Sézary syndrome is a less common and more aggressive presentation. It typically involves widespread reddening of the skin (erythroderma), intense itching, and the presence of malignant T cells circulating in the blood.

People with Sézary syndrome often describe severe, whole-body itch and skin that feels like it is burning — symptoms that overlap heavily with severe atopic dermatitis. Blood work and flow cytometry are usually part of how it is distinguished.

Some reported Dupixent lawsuits describe diagnoses that required multiple rounds of chemotherapy, according to coverage by AboutLawsuits.com.

Why CTCL Is So Often Mistaken for Eczema

This is the single most important clinical point for families trying to make sense of a late diagnosis.

Early mycosis fungoides and moderate-to-severe atopic dermatitis can look nearly identical: itchy, red, scaly patches that wax and wane. Early biopsies of CTCL are frequently read as non-specific dermatitis, and it is not unusual for a patient to need several biopsies over months or years before a definitive diagnosis is reached.

This diagnostic difficulty is well documented in the dermatology literature; searchable summaries and abstracts are available through PubMed. It is also the medical backdrop against which the litigation’s second theory is being argued.

What Dupixent Is and How It Is Used

Dupixent is a biologic given by injection. It is prescribed for atopic dermatitis and several other conditions and works on part of the immune signaling pathway associated with type 2 inflammation.

For many patients it substantially reduces itching and clears skin. That benefit is not in dispute, and no one should stop a prescribed medication because of a blog post — that is a conversation for your treating physician.

Approved prescribing information and safety communications for prescription drugs are published by the U.S. Food and Drug Administration. If you want to know what a label currently says about lymphoma or malignancy, the FDA’s materials are the primary source rather than litigation coverage.

The Two Theories Plaintiffs Have Pleaded

The Dupixent lawsuits are not built on a single allegation. Two distinct theories appear in the reported filings, and understanding both matters because they describe very different patients.

First — that the drug may cause or accelerate a cutaneous T-cell lymphoma. Some complaints allege that treatment preceded a new CTCL diagnosis, or that an existing cancer progressed rapidly afterward. AboutLawsuits.com has reported filings framed this way.

Second — that the drug may mask the early signs of a lymphoma the patient already had. Because Dupixent can improve the appearance of skin lesions, some complaints allege that improvement made early CTCL harder to recognize and delayed the point at which a correct diagnosis was made.

The practical consequence of the second theory is worth stating plainly: a person diagnosed with CTCL before starting Dupixent is not automatically outside the litigation. Timing alone does not settle eligibility.

Both theories are allegations under examination. Neither has been established as a proven biological mechanism, and the manufacturers dispute them.

Where the Litigation Stands

Federal Dupixent CTCL cases have been centralized into a multidistrict litigation (MDL 3180) in the District of New Jersey. Centralization decisions are made by the Judicial Panel on Multidistrict Litigation, and the consolidated proceeding is administered through the U.S. District Court for the District of New Jersey.

It is important not to misread that step. An MDL is a case-management tool. It groups cases with common questions so that discovery and pretrial rulings happen once instead of dozens of times. It is not a ruling that the drug caused anyone’s cancer, and it is not a finding against Sanofi or Regeneron.

This litigation is early. Core scientific questions have not been tested in front of a jury, and there is very little established fact to report. Anyone offering you projections about size, timing, or value at this stage is guessing.

What Records Tend to Matter

If you are considering having a claim reviewed, these are the documents that usually do the work:

• Dermatology records covering the eczema or atopic dermatitis diagnosis and its treatment history
• Prescription and pharmacy records showing when Dupixent was started, how long it continued, and when it stopped
• All skin biopsy and pathology reports, including earlier ones read as non-specific — these are often the most important documents in a masking claim
• Oncology and hematology records, including staging, flow cytometry, and treatment

You do not need to assemble all of this yourself before making a call. Much of it is obtainable with a signed authorization.

You can read more about the claims we handle on our Dupixent lymphoma page.

Who May Want to Have a Case Reviewed

Generally, the people looking at this litigation are those who used Dupixent for eczema or atopic dermatitis and were later diagnosed with CTCL, mycosis fungoides, or Sézary syndrome — and also those who had a CTCL diagnosis first and allege their disease was obscured or worsened during treatment.

Every state sets its own deadlines for filing, and those deadlines can be short. Whether a particular claim fits is a question for a lawyer who has looked at the actual records, not a checklist.

Take the Next Step

A CTCL diagnosis is disorienting, and sorting out whether a medication played any role in it is not something you should have to research alone. A review costs nothing and carries no obligation.

If you or a family member was treated with Dupixent and later diagnosed with cutaneous T-cell lymphoma, mycosis fungoides, or Sézary syndrome, you can request a free case review and we will look at the timeline with you.

This post is general information, not legal or medical advice, and does not create an attorney-client relationship. Do not change or discontinue any prescribed treatment without speaking to your physician.