Extranodal NK/T Cell Lymphoma: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
April 16, 2026

Extranodal NK/T cell lymphoma is a rare and aggressive 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.

What are the symptoms of extranodal NK/T cell lymphoma?

Symptoms depend on where in the body the lymphoma has developed. In the most common form — called nasal-type extranodal NK/T cell lymphoma — the lymphoma 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 tissue, leading to facial swelling, destruction of the palate (the roof of the mouth), loss of sensation in the face, or visible ulceration inside 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 present 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 requires urgent recognition and treatment.

What causes extranodal NK/T cell lymphoma?

The disease 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 instead of B cells, and within those cells it activates genes that drive the cells 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.

Extranodal NK/T cell 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 strains of EBV circulating in these populations and in genetic susceptibility. In Western countries, including Canada and the United States, the disease is rare but 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.

How is the diagnosis made?

The diagnosis of extranodal NK/T cell lymphoma requires tissue examination under the microscope. 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 tissue 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.

The pathologist examines the tissue under the microscope and then performs immunohistochemistry (IHC) — a test that detects specific proteins in cells — and in situ hybridization for EBER (EBV-encoded RNA molecules) to confirm EBV infection in the lymphoma cells. Flow cytometry may also be performed on fresh tissue when available. Molecular testing for T cell receptor gene rearrangements can help confirm whether the lymphoma originated from T cells specifically. Taken together, these tests confirm the diagnosis and distinguish extranodal NK/T cell lymphoma from other lymphomas that can look similar, particularly other T cell and NK cell lymphomas, and from inflammatory or infectious conditions that can mimic lymphoma in the nasal cavity.

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

What does extranodal NK/T cell lymphoma look like under the microscope?

The microscopic appearance has several characteristic features that, taken together, help identify this lymphoma — though the combination of these features with IHC and EBER testing is essential for a definitive diagnosis.

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 the malignant cells look alike, extranodal NK/T cell lymphoma shows a spectrum of cell sizes, which can initially be confusing. The cells divide frequently, and pathologists 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, causing extensive necrosis (tissue death). The large areas of necrosis visible both clinically (as ulceration and tissue destruction) and under the microscope are directly explained by this angioinvasive pattern. In biopsy tissue, pathologists will specifically look for lymphoma cells within blood vessel walls as a key diagnostic clue.

The background tissue contains a mixed inflammatory infiltrate — normal-looking small lymphocytes, plasma cells, histiocytes, and eosinophils — that can make the tissue look more like an infection or inflammatory condition than a cancer, particularly on small or superficial biopsies. This reactive background can obscure the malignant cells and is one reason adequate tissue sampling is so important.

The surface epithelium (the lining layer of the nose, skin, or gastrointestinal tract) may show ulceration and secondary reactive changes that can superficially resemble squamous cell carcinoma. Pathologists are trained to look through this reactive surface change to identify the underlying lymphoma cells.

Immunohistochemistry and EBER testing

Immunohistochemistry (IHC) is essential for confirming the diagnosis and characterizing the lymphoma cells. The protein profile of extranodal NK/T cell lymphoma is described below.

CD2 — Positive. CD2 is a surface protein expressed 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 lineage.
CD3 — Positive in the cytoplasm; negative or absent on the cell surface. This is one of the most important and sometimes confusing IHC findings in this disease. CD3 is a protein complex that is normally expressed on the surface of T cells. In NK cells, a component of CD3 is present inside the cell (cytoplasm) but not displayed on the cell surface. Most extranodal NK/T cell lymphomas show cytoplasmic CD3 positivity — meaning the stain shows positive inside the cell but not at its membrane. Your pathology report may describe this as “CD3 cytoplasmic positive” or note that CD3 is positive on IHC but negative by flow cytometry, which measures only surface proteins. This pattern supports NK cell origin for most cases.
CD56 — Positive in most cases. CD56 is the most reliable marker of NK cells and is positive in approximately 80–90% of extranodal NK/T cell lymphomas. Its expression confirms NK or NK-like lineage. Some cases lack CD56 expression, typically those arising from cytotoxic T cells rather than NK cells, and these can be identified through other markers.
TIA1 and Granzyme B — Positive. These are cytotoxic granule proteins — molecular weapons that NK cells and cytotoxic T cells use to kill infected or abnormal cells. Their expression in the lymphoma cells reflects the cytotoxic (cell-killing) nature of the cell from which this lymphoma arises and is an important diagnostic feature. Your report may describe these as markers of cytotoxic differentiation.
CD5, CD4, CD8 — Variable. Some cases express these T cell markers, particularly those arising from cytotoxic T cells rather than NK cells. The presence or absence of these markers helps the pathologist determine whether the lymphoma arose from an NK cell or a T cell, though this distinction does not currently change treatment.
CD20, 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 the aggressive and rapidly proliferating nature of this lymphoma.

EBER testing

EBER in situ hybridization is a test that detects EBER — small RNA molecules produced inside cells that are infected with Epstein-Barr virus — by applying a labeled probe to the tissue section. A positive EBER result means that EBV is present inside the lymphoma cells. EBER positivity in lymphoma cells is present in virtually all cases of extranodal NK/T cell lymphoma and is one of the essential diagnostic criteria. A negative EBER result would cast serious doubt on this diagnosis and would prompt the pathologist to reconsider other lymphoma types.

In addition to EBER testing on the biopsy tissue, EBV DNA levels in the blood (measured by a blood test called plasma EBV DNA, using PCR) are routinely checked at diagnosis and during treatment. Elevated EBV DNA levels in the blood at diagnosis are associated with a worse prognosis, and declining EBV DNA during treatment is used as a marker of treatment response. Rising EBV DNA after treatment can be an early sign of relapse, sometimes weeks before clinical or imaging signs appear.

Staging

Extranodal NK/T cell lymphoma is staged using the Ann Arbor/Lugano classification, though this system was designed primarily for nodal lymphomas 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 (e.g., 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-type disease present with limited-stage (stage I or II) disease, which carries a significantly better prognosis than advanced-stage disease. Extranasal presentations tend to present 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?

The prognosis of extranodal NK/T cell lymphoma varies considerably by stage, location, and treatment approach. For limited-stage nasal-type disease (stage I–II) treated with concurrent chemoradiation, five-year overall survival rates of 60–80% are achievable with modern regimens. For advanced-stage disease, five-year survival rates are lower, in the range of 20–40% with standard treatment, though outcomes continue to improve as newer regimens are developed.

Prognostic scoring systems specifically developed for this disease — including the PINK score (Prognostic Index for Natural Killer Lymphoma) and its updated version PINK-E, which incorporates 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%, while high-risk patients have a survival of approximately 25–30% 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.

Key factors associated with a worse prognosis include advanced stage, high EBV DNA level in the blood at diagnosis, extranasal primary site (non-nasal presentations tend to fare worse), elevated LDH, and bone marrow or multiple extranodal site involvement. 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 is aggressive and requires prompt treatment, therapy typically begins within one to two weeks of diagnosis. Most patients are referred to a hematologist or lymphoma oncologist with experience in T cell and NK cell lymphomas.

A critically important aspect of treatment planning is that extranodal NK/T cell lymphoma is resistant to standard CHOP-based chemotherapy (the backbone used in most B cell lymphomas), in part because the lymphoma cells overexpress a protein called P-glycoprotein that pumps many standard chemotherapy drugs out of the cells before they can work. This is why treatment regimens for this disease specifically avoid CHOP and instead use drugs not affected by this resistance mechanism.

For limited-stage nasal-type 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, delivered concurrently with chemotherapy. This approach achieves complete response rates of 70–85% in limited-stage disease.

For advanced-stage disease or relapsed/refractory disease, 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 60–80% have been reported with these regimens in previously untreated advanced-stage disease. For patients who achieve a complete response with intensive chemotherapy, consolidation with autologous or allogeneic stem cell transplantation is considered in eligible patients.

Immune checkpoint inhibitors — drugs that block the PD-1/PD-L1 pathway and help the immune system recognize and attack lymphoma cells — are showing increasing promise in relapsed or refractory extranodal NK/T cell lymphoma. Pembrolizumab and sintilimab have each shown activity in this disease. EBV DNA levels in the blood are monitored throughout treatment and follow-up as a sensitive marker of disease activity and early relapse detection.

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 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?