Extranodal NK/T Cell Lymphoma: Understanding Your Pathology Report

Section Editor: Jason Wasserman MD PhD FRCPC
July 16, 2026

Extranodal NK/T cell lymphoma is a rare, fast-growing cancer of the immune system. It starts in either natural killer (NK) cells or T cells, two closely related types of white blood cells that normally identify and destroy infected or abnormal cells. The word “extranodal” means that the lymphoma typically begins outside the lymph nodes, most commonly in the nasal cavity and its surrounding structures, though it can arise at many other sites in the body. Nearly all cases are driven by Epstein-Barr virus (EBV), a very common virus that normally causes no lasting harm but in rare circumstances can contribute to cancer development in NK or T cells. This article will help you understand the findings in your pathology report, what each term means, and why it matters for your care.

A note on the name: this disease was previously called “extranodal NK/T cell lymphoma, nasal type.” The World Health Organization removed “nasal type” from the name in its most recent classification because roughly one in five cases begins elsewhere. A separate international classification system still uses the older name, so you may see either version on your pathology report. Both refer to the same disease.

What causes extranodal NK/T cell lymphoma?

Extranodal NK/T cell lymphoma is driven by Epstein-Barr virus (EBV), which is present inside the lymphoma cells in virtually all cases. EBV is an extremely common virus; most people are infected during childhood or early adulthood, where it typically causes mononucleosis (“mono”) and then persists silently in B cells for life. In the overwhelming majority of infected people, EBV causes no long-term problems. In rare cases, however, EBV infects NK or T cells rather than B cells, and within those cells it activates genes that drive them to survive and multiply abnormally, contributing to lymphoma development. Why this happens in some individuals and not others is not fully understood, though genetic factors and immune system differences are thought to play a role.

This lymphoma is substantially more common in certain geographic populations. It accounts for a much higher proportion of lymphomas in East Asia (China, Japan, South Korea) and in parts of Latin America than in North America or Europe. This geographic pattern reflects differences in the EBV strains circulating in these populations and in genetic susceptibility. In Western countries, including Canada and the United States, the disease is rare but is diagnosed in people of all backgrounds. No specific environmental or lifestyle risk factor, such as smoking, diet, or chemical exposure, has been consistently identified as a cause. A weakened immune system (from HIV infection, organ transplantation, or inherited immune deficiencies) can increase the risk of EBV-related lymphomas in general.

What are the symptoms?

The symptoms of extranodal NK/T cell lymphoma depend on where in the body the lymphoma has developed. When it begins in the nasal cavity or the structures immediately surrounding it (the nasopharynx, palate, or upper airway), early symptoms often resemble a persistent sinus infection: nasal congestion, nosebleeds, postnasal drip, and a feeling of blockage that does not improve with standard treatment. As the lymphoma grows, it can destroy the surrounding bone and soft tissues, leading to facial swelling, destruction of the palate (the roof of the mouth), loss of facial sensation, or visible ulceration in the nose or mouth. This local destructive behavior was historically called “lethal midline granuloma,” a term that is now understood to describe this lymphoma rather than a separate condition.

When the lymphoma arises at other sites, called extranasal disease, symptoms depend on the location. Skin involvement may appear as lumps, nodules, or ulcers. Gastrointestinal involvement can cause abdominal pain, nausea, diarrhea, or bleeding. Involvement of the testis, orbit (the eye socket), or soft tissues of the limbs produces mass effects at those sites.

General symptoms, including fever, drenching night sweats, and significant weight loss, collectively called B symptoms, are common in more advanced disease. In a small proportion of patients, a serious complication called hemophagocytic lymphohistiocytosis (HLH) can develop, in which the immune system becomes dangerously overactivated. This causes high fevers, low blood counts, and organ dysfunction, and it requires urgent recognition and treatment.

How is the diagnosis made?

The diagnosis of extranodal NK/T cell lymphoma requires microscopic examination of tissue. A biopsy is taken from the affected area, most commonly the nasal cavity, but also skin, gastrointestinal tract, or other involved sites, depending on where the lymphoma is located. An adequate biopsy is critical because this lymphoma characteristically causes extensive necrosis (death of tissue) and ulceration. Superficial or small biopsies often contain only dead tissue without viable lymphoma cells, making the diagnosis impossible. When an initial biopsy is non-diagnostic, a repeat biopsy of deeper or peripheral viable tissue is usually required, and the need for a second biopsy does not mean anything has gone wrong.

Under the microscope, the lymphoma cells vary considerably in size. Most cases contain a mixture of small, medium, and large cells with irregularly shaped nuclei and pale or clear cytoplasm. Unlike some other lymphomas, in which all malignant cells look alike, extranodal NK/T cell lymphoma shows a spectrum of cell sizes that can initially be confusing. The cells divide frequently, and the pathologist will typically note abundant mitotic figures (cells caught in the act of dividing). One of the most distinctive and diagnostically important features is angioinvasion, the tendency of the lymphoma cells to invade the walls of blood vessels and grow inside them. This behavior cuts off blood supply to the surrounding tissue, which directly explains the extensive necrosis seen both clinically (as ulceration and tissue destruction) and under the microscope. Pathologists specifically look for lymphoma cells within blood vessel walls as a key diagnostic clue. The background tissue contains a mixed inflammatory infiltrate of normal-looking small lymphocytes, plasma cells, histiocytes, and eosinophils, which can make the tissue look more like an infection or inflammatory condition than a cancer, particularly on small or superficial biopsies. The surface lining may show ulceration and reactive changes that can superficially resemble squamous cell carcinoma, and pathologists are trained to look through these changes to identify the underlying lymphoma cells.

Because microscopic appearance alone is insufficient, additional tests are essential. Immunohistochemistry (IHC), a test that detects specific proteins in cells, is used to confirm the diagnosis and determine what type of cell the lymphoma came from. In situ hybridization for EBER (small RNA molecules produced in EBV-infected cells) confirms EBV infection in lymphoma cells. EBER positivity is present in virtually all cases and is one of the essential diagnostic criteria; a negative EBER result would cast serious doubt on this diagnosis and prompt the pathologist to reconsider other lymphoma types. Flow cytometry may also be performed on fresh tissue when available, and molecular testing of T cell receptor gene rearrangements can help confirm that the lymphoma originated from T cells. Together, these tests distinguish extranodal NK/T cell lymphoma from other lymphomas that can look similar, and from inflammatory or infectious conditions that can mimic lymphoma in the nasal cavity. Your report will typically describe the following markers.

CD2 — Positive. A surface protein found on both NK cells and T cells. Its presence confirms the immune cell origin of the lymphoma but does not, by itself, distinguish NK from T cell lineages.
CD3 — Positive in the cytoplasm, negative or absent on the cell surface. This is one of the most important and sometimes confusing findings in this disease. CD3 is normally displayed on the surface of T cells. In NK cells, a component of CD3 is present inside the cell but not on its surface. Your report may describe this as “CD3 cytoplasmic positive,” or note that CD3 is positive by IHC but negative by flow cytometry, which measures only surface proteins. This pattern supports NK cell origin.
CD56 — Positive in most cases. The most reliable marker of NK cells, positive in roughly 80-90% of cases. Some cases lack CD56, typically those arising from cytotoxic T cells rather than NK cells, and these are identified through other markers.
TIA1 and granzyme B — Positive. These are cytotoxic granule proteins, the molecular weapons NK cells and cytotoxic T cells use to kill infected or abnormal cells. Your report may describe them as markers of cytotoxic differentiation.
CD5, CD4, and CD8 — Variable. Some cases express these T cell markers, particularly those arising from cytotoxic T cells. These markers help determine whether the lymphoma arose from an NK cell or a T cell, although this distinction does not currently change treatment.
CD20 and CD79a — Negative. These B cell markers are absent, confirming the lymphoma is not of B cell origin.
Ki-67 — High. Typically 40-80% or higher, reflecting how rapidly this lymphoma divides.

Once the diagnosis is established, the staging evaluation includes whole-body PET/CT imaging, blood tests including a complete blood count, LDH, and EBV DNA level, and a bone marrow biopsy to assess for marrow involvement.

Biomarker and molecular testing

A biomarker is a feature of the disease that provides information beyond the diagnosis itself, such as how likely the lymphoma is to respond to treatment or whether it is coming back. In extranodal NK/T cell lymphoma, the EBER test described above confirms the diagnosis, while the biomarkers below guide treatment decisions and monitoring.

Plasma EBV DNA

Plasma EBV DNA measures the amount of EBV genetic material circulating in the blood, using a technique called PCR. It reflects the amount of lymphoma present in the body, and it is one of the most useful tests in this disease. A high level at diagnosis is associated with a less favorable outlook and is built into the prognostic scoring systems described below. During treatment, a falling level indicates the lymphoma is responding, while a level that fails to fall suggests it is not. After treatment, a rising level can be an early sign of relapse, sometimes weeks before symptoms or imaging show any change. For this reason, plasma EBV DNA is checked at diagnosis and monitored throughout treatment and follow-up.

PD-L1

PD-L1 is a protein that allows cancer cells to avoid being attacked by the immune system. Drugs called immune checkpoint inhibitors block this pathway, helping the immune system recognize and attack the lymphoma. These drugs have shown meaningful activity in extranodal NK/T cell lymphoma that has come back or has not responded to initial treatment, and PD-L1 expression may provide additional information about the likelihood of response. PD-L1 is measured by immunohistochemistry on tumor tissue. You can read more in the Biomarkers and Molecular Testing section.

Staging

Extranodal NK/T cell lymphoma is staged using the Ann Arbor/Lugano classification, though this system was designed primarily for lymphomas that start in the lymph nodes and is less well suited to this predominantly extranodal disease. Staging is based on PET/CT imaging, bone marrow biopsy, and cerebrospinal fluid examination in selected cases.

Stage I — The lymphoma is limited to a single extranodal site (for example, the nasal cavity alone) without regional lymph node involvement.
Stage II — The lymphoma involves one extranodal site with regional lymph node involvement, or extends to adjacent structures (such as the nasopharynx or palate) beyond the primary site.
Stage III — Lymph node regions on both sides of the diaphragm are involved.
Stage IV — The lymphoma has spread to one or more distant extranodal organs such as the bone marrow, liver, lung, or skin, or involves extensive dissemination.

The majority of patients with nasal disease are diagnosed with limited-stage (stage I or II) disease, which carries a significantly better outlook than advanced-stage disease. Lymphomas starting outside the nasal area tend to be diagnosed at a more advanced stage. The EBV DNA level in the blood at diagnosis is incorporated into several prognostic scoring systems alongside clinical stage.

What is the prognosis?

Prognosis refers to the likely long-term outcome after a diagnosis. The outlook in extranodal NK/T cell lymphoma varies considerably by stage, location, and treatment approach. For limited-stage nasal disease (stage I–II) treated with concurrent chemoradiation, five-year overall survival rates of 60 to 80 percent are achievable with modern regimens. For advanced-stage disease, five-year survival is lower, in the range of 20 to 40 percent with standard treatment, though outcomes continue to improve as newer regimens are developed.

Prognostic scoring systems developed specifically for this disease, including the PINK score (Prognostic Index for Natural Killer Lymphoma) and its updated version PINK-E, which incorporates the EBV DNA level, divide patients into low-, intermediate-, and high-risk groups. PINK-E is based on age, stage, the location of disease, and the EBV DNA level in the blood. Patients classified as low risk have a five-year overall survival of approximately 80 percent, while high-risk patients have a survival of approximately 25 to 30 percent with standard treatment. Your care team will calculate your PINK or PINK-E score to guide treatment decisions and provide a more individualized estimate of your prognosis.

Several findings are associated with a less favorable outlook.

Advanced stage — Stage III or IV disease at diagnosis.
High EBV DNA level — A high level in the blood at diagnosis.
Extranasal primary site — Lymphomas starting outside the nasal area tend to do worse, even compared with advanced-stage nasal disease.
Elevated LDH — A blood test that reflects rapid cell turnover.
Bone marrow or multiple site involvement — Disease in the marrow or at several extranodal sites.

Response to initial treatment, assessed by PET/CT and EBV DNA levels, is one of the strongest predictors of long-term outcome.

What happens after the diagnosis?

Because extranodal NK/T cell lymphoma grows quickly and requires prompt treatment, therapy typically begins within one to two weeks of diagnosis. Most patients are referred to a hematologist or lymphoma oncologist experienced in T-cell and NK-cell lymphomas.

A critical aspect of treatment planning is that this lymphoma is resistant to standard CHOP-based chemotherapy, the backbone used for most B-cell lymphomas. This is partly because the lymphoma cells overexpress a protein called P-glycoprotein that pumps many standard chemotherapy drugs out of the cells before they can work. Treatment regimens for this disease therefore avoid CHOP and instead use drugs not affected by this resistance mechanism.

For limited-stage nasal disease (stage I–II), concurrent chemoradiation, combining radiation to the nasal region with asparaginase-based chemotherapy, is the standard of care. Asparaginase is an enzyme that depletes asparagine, an amino acid that lymphoma cells depend on but cannot produce for themselves. Radiation doses to the nasal region are typically 50 Gy or higher and are delivered concurrently with chemotherapy. This approach achieves complete response rates of 70 to 85 percent in limited-stage disease.

For advanced-stage disease, or disease that has come back or not responded, asparaginase-based combination regimens are used. The most established include SMILE (steroid, methotrexate, ifosfamide, L-asparaginase, and etoposide) and P-GEMOX (pegaspargase, gemcitabine, and oxaliplatin), along with modified versions of these regimens. Response rates of 60 to 80 percent have been reported in previously untreated advanced-stage disease. For patients who achieve a complete response with intensive chemotherapy, consolidation with autologous or allogeneic stem cell transplantation may be considered in those who are eligible.

Immune checkpoint inhibitors, which block the PD-1/PD-L1 pathway, are showing increasing promise in relapsed or refractory disease, and pembrolizumab and sintilimab have each shown activity. Throughout treatment and follow-up, EBV DNA levels in the blood are monitored as a sensitive marker of disease activity and early relapse.

Questions to ask your doctor

Was EBER testing performed on my biopsy, and was the result positive?
Did my biopsy contain adequate viable tissue, or will a repeat biopsy be needed?
What stage is my lymphoma, and what is the primary site — nasal or extranasal?
What is my EBV DNA level in the blood, and what does that tell you about my prognosis?
What is my PINK or PINK-E risk score?
What treatment regimen are you recommending, and why is it different from the standard CHOP chemotherapy used in other lymphomas?
Will I receive radiation therapy, and to which area?
How will you monitor my response to treatment — with PET/CT, EBV DNA levels, or both?
Is stem cell transplantation being considered as part of my treatment plan?
What symptoms should prompt me to contact you urgently during treatment?
What are the options if the lymphoma does not respond to or comes back after initial treatment?
Are there clinical trials available for my stage and risk group?