The short answer
Mycosis fungoides is a lymphoma of T cells that appears first in the skin, as patches, plaques and tumors. Skin biopsy, immunophenotyping, T-cell receptor testing and blood counts confirm it. Staging weighs skin surface, lymph nodes and blood. A lesion that turns hot, swollen and tender needs a call right away, and one that ulcerates or weeps needs a same-day call.
Here the skin is both the disease site and the body's barrier, so broken skin can become infected fast.
A temperature of 100.4 F (38 C) with shivering, fast breathing, confusion, a fast heart rate or clammy or blotchy skin is an emergency.
Skin that turns red, peeling and painful over most of the body, with fever or shivering, needs emergency care.
Call right away, at any hour, if a lesion becomes hot, swollen and tender. Call the same day if a patch, plaque or tumor ulcerates, weeps or crusts.
On this page
Choose how you want to read this
The whole article, with every detail and caution.
This page is for people learning about a diagnosis of mycosis fungoides, a type of cutaneous T-cell lymphoma. It explains how the diagnosis is made and staged, when to get help quickly, and questions to ask your care team.
Why exact diagnosis matters
Knowing the exact subtype and what the tissue tests found can help you follow visits with your cancer team. It also shapes the plan. The stage and the pattern of your disease guide which treatments your team offers.
How this diagnosis is made
Mycosis fungoides is a cutaneous T-cell lymphoma. That means it is a cancer of T cells, a type of white blood cell, and it shows up first in the skin. Sézary syndrome is a related form in which the abnormal T cells are also found in the blood.
Tests of the skin and blood are used to make the diagnosis. They usually include:
- Skin biopsy. A small piece of skin is removed and examined under a microscope. Early-stage disease can be hard to confirm, so more than one biopsy is common.
- Immunophenotyping and flow cytometry. These lab tests look at the markers on the surface of the cells to identify the exact cell type.
- T-cell receptor gene rearrangement testing. This checks whether the T cells all came from one abnormal cell.
- Blood tests. A complete blood count with differential, and a Sézary cell count, look for abnormal cells in the blood.
How staging works
After the diagnosis, more tests are done to see whether the cancer has spread beyond the skin. Staging runs from stage I to stage IV. It takes into account how much of your skin surface is involved, whether lymph nodes or other organs are affected, and how many abnormal cells are in your blood.
When to get help sooner
In this cancer, the skin is both where the disease is and the body's barrier against germs. So broken skin can become infected fast.
- Call 911 or go to an emergency department if your temperature reaches 100.4°F (38°C) or higher with shivering, fast breathing or shortness of breath, confusion, a fast heart rate, clammy or sweaty skin, or pale, blue, or blotchy skin. Do not drive yourself. CDC lists these as possible signs of sepsis, a life-threatening emergency (CDC). The 100.4°F figure is the CDC threshold for fever during cancer treatment, which the CDC treats as an emergency (CDC).
- Call 911 or go to an emergency department if your skin turns red, peeling, and painful over most of your body, with fever or shivering. Do not drive yourself.
- Call your care team right away, at any hour, if a lesion becomes hot, swollen, and tender, redness is spreading outward from a lesion, or you see red streaking. MedlinePlus says red streaks that spread from an area of skin infection need a call to your provider right away (MedlinePlus).
- Call your care team the same day if a patch, plaque, or tumor ulcerates, weeps, or crusts.
- Call your care team the same day if you develop drenching night sweats, unexplained weight loss, or new lumps in the neck, armpit, or groin.
A same-day look at a lesion that has changed is a reasonable thing to ask for.
Questions for your oncology team
- Which specific diagnostic tests confirmed this subtype?
- How do these test results shape the treatment you recommend?
- Are there clinical trials of targeted therapies that fit my test results?
Frequently asked questions
Why do subtypes matter for cancer treatment?
The exact type and the stage of your disease guide which treatments your team offers.
Sources
- National Cancer Institute — Mycosis Fungoides (Including Sézary Syndrome) Treatment (PDQ) Patient Version
- National Cancer Institute — Mycosis Fungoides and Other Cutaneous T-Cell Lymphomas Treatment (PDQ) Health Professional Version
- CDC — Fever and Cancer Treatment
- CDC — About Sepsis
- MedlinePlus: Cellulitis
Common questions
When do skin changes become an emergency?
Call 911 or go to an emergency department if your temperature reaches 100.4°F (38°C) or higher with shivering, fast breathing, confusion, a fast heart rate, clammy skin, or pale, blue or blotchy skin. Do the same if your skin turns red, peeling and painful over most of your body, with fever or shivering. Do not drive yourself.
Which changes need a quick call?
Call your care team right away, at any hour, if a lesion becomes hot, swollen and tender, if redness is spreading outward from a lesion, or you see red streaking. Call the same day if a patch, plaque or tumor ulcerates, weeps or crusts, or if you develop drenching night sweats, unexplained weight loss, or new lumps in the neck, armpit or groin.
Why does the exact subtype matter?
The exact type and the stage of your disease guide which treatments your team offers. Ask your team what your test results mean for your plan.
What do the diagnostic tests actually look at?
A skin biopsy looks at the cells under a microscope. Lab tests look at markers on the cells and check whether the T cells all came from one abnormal cell. Blood tests look for abnormal cells in the blood.
Why is skin infection such a concern with this diagnosis?
Because the skin is both the disease site and the body's barrier, so broken skin can become infected fast. A same-day look at a lesion that has changed is a reasonable thing to ask for.
Questions to ask your doctor
Being prepared helps you get the most out of your appointments. Save or print these questions.
Tap a question to save it to your list (kept on this device).
Talk with a trained specialist or navigator
Cancer Explained provides educational guidance, but does not replace trained specialists, social workers, or your medical team.
Talk to a trained cancer information specialist
Free, confidential assistance from NCI Cancer Information Service via phone, chat, or email.
Contact your oncology team
Locate after-hours contact numbers, portal messages, or urgent triage phone lines.
Find a patient navigator
Get one-on-one help with appointments, logistics, translation, and care coordination.
Find a genetic counselor
Discuss inherited mutation risk, family history, and genetic testing options.
Find an oncology social worker
Access emotional counseling, family support groups, and mental health resources.
Find a financial navigator
Locate copay assistance foundations, grant programs, and lodging/travel support.
Find a clinical-trial specialist
Search matching studies and speak with NCI trial information specialists.
Get urgent help
Fever of 100.4°F (38°C) or higher during chemo, severe pain, or trouble breathing: call your cancer care team right away, day or night. Call 911 for an emergency.
Know someone who needs this?
Plenty of people are looking for something like this and do not know where to start. If this would help a friend or someone you love, send it on — we have written an opening line so you do not have to stare at an empty message. You can change every word of it.
Your message is written and sent in your own email or messaging app — we never see who you send it to, and nothing is added to any list.
Plain-language explanation of the published sources cited on this page. AI-assisted, source-checked, not clinician-reviewed.
Written by: Cancer ExplainedSources last checked: 2026-08-13 what this meansLast updated: 2026-09-29Next planned review: 2027-08-03
Review details and limits
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status — Source checked. This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
General education. Low-risk educational or organizational content. Medical facts are cited to authoritative sources.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
Words to know
Tap any term to see what it means.
How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
Read more about our editorial process, our use of AI, and our corrections policy.
Spotted a problem? Report an error — a factual mistake, broken or outdated source, confusing wording, or anything that seems unsafe. Please do not include names, medical record numbers, dates of birth, addresses, or other identifying medical information in your report.
After using this page, do you understand what to do next?
Anonymous — we only record the answer, never who gave it.