Sinonasal Undifferentiated Carcinoma (SNUC): Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
July 10, 2026

Sinonasal undifferentiated carcinoma (SNUC) is a rare, fast-growing cancer that starts from cells that line the inside of the nasal cavity and paranasal sinuses, an area of the body called the sinonasal tract. The paranasal sinuses are air-filled spaces in the bones around the nose. The term “undifferentiated” means the cancer cells are so abnormal that they no longer resemble any specific normal cell type, making this a high-grade cancer that often grows quickly and is frequently found at an advanced stage. Although the outlook has historically been poor, treatment has improved with modern combined approaches. This article will help you understand the findings in your pathology report, what each term means, and why it matters for your care.

What causes sinonasal undifferentiated carcinoma?

Compared with other head and neck cancers, very little is known about what causes sinonasal undifferentiated carcinoma. Most cases occur without any clear reason. In a small number of people, the cancer develops in an area that received radiation therapy in the past for a different cancer. Unlike some other sinonasal cancers, it is not clearly linked to workplace dust exposure or to a virus.

What are the symptoms?

The symptoms of sinonasal undifferentiated carcinoma often develop quickly because the tumor tends to grow rapidly. Common symptoms include a blocked or stuffy nose, frequent nosebleeds, facial pain or pressure, and headaches. Because the tumor can grow toward the eye socket or the base of the skull, some people also notice vision changes, double vision, or swelling around the eye. These symptoms can overlap with common, noncancerous conditions, but their rapid onset often prompts imaging and a biopsy.

How is the diagnosis made?

The diagnosis of sinonasal undifferentiated carcinoma is made after a tissue sample is examined under the microscope by a pathologist. The sample is usually obtained through a biopsy, in which a small piece of the tumor is removed, typically via an endoscope through the nose. Under the microscope, sinonasal undifferentiated carcinoma consists of cells that do not resemble any normal, healthy cell type. Large numbers of mitotic figures (cells caught in the act of dividing) and areas of necrosis (cell death) are commonly seen.

Sinonasal undifferentiated carcinoma is a “diagnosis of exclusion,” which means it is made only after other sinonasal cancers that can look similar under the microscope have been ruled out. To do this, the pathologist performs immunohistochemistry, a test that uses specially labeled antibodies to detect proteins in the tumor cells. In sinonasal undifferentiated carcinoma, the tumor cells are usually positive for a group of proteins called cytokeratins (including CK8/18), confirming that the tumor is a carcinoma. The cells are typically negative, or only focally positive, for markers of other specific tumors, such as p40 (squamous cell carcinoma), synaptophysin and chromogranin (neuroendocrine tumors), p16 and EBER (virus-associated cancers), and S100. Additional tests are used to exclude specific look-alike tumors, including NUT carcinoma, SMARCB1-deficient sinonasal carcinoma, neuroendocrine tumors, and olfactory neuroblastoma.

Molecular testing may also be performed. Some tumors once diagnosed as sinonasal undifferentiated carcinoma now carry an IDH2 mutation and are recognized as a separate tumor type, IDH2-mutant sinonasal carcinoma. Identifying an IDH2 mutation can affect how the tumor is classified and may make a patient eligible for targeted drug clinical trials. Once the diagnosis is confirmed, imaging studies such as CT and MRI are used to determine the size of the tumor and the extent of its spread.

Histologic grade

Sinonasal undifferentiated carcinoma is not assigned a histologic grade in the usual way, because grading is based on how closely the tumor cells resemble normal cells, and by definition the cells of this tumor show no resemblance to any normal cell type. For this reason, sinonasal undifferentiated carcinoma is considered a high-grade cancer by definition. Your pathology report will not include a grade number, and this is expected. Being high grade means the tumor tends to grow quickly and is more likely to spread, which is why treatment usually begins soon after the diagnosis is made.

Perineural invasion

In sinonasal undifferentiated carcinoma, the pathologist looks for perineural invasion, which means cancer cells were seen attached to or growing along the outside of a nerve. Nerves run throughout the head and neck, carrying signals such as temperature, pressure, and pain between the body and the brain. Perineural invasion matters because cancer cells can use nerves as a pathway to travel into surrounding tissues, which raises the risk of the tumor returning after treatment. If perineural invasion is present, it will be described in your pathology report.

Lymphovascular invasion

Lymphovascular invasion means that cancer cells from the sinonasal undifferentiated carcinoma were seen within a blood or lymphatic vessel. Blood vessels carry blood throughout the body, and lymphatic vessels carry a fluid called lymph. Both types of vessels connect to other parts of the body, so cancer cells that enter them can travel to distant sites such as lymph nodes or the lungs. If lymphovascular invasion is present, it will be included in your pathology report.

Surgical margins

A surgical margin is the edge of the tissue that the surgeon cuts through when removing the tumor. Margins are assessed after a procedure that removes the entire tumor, such as an excision or resection, and are usually not evaluated after a biopsy, which removes only part of the tumor. Because sinonasal undifferentiated carcinoma is often removed in more than one piece, the pathologist may not be able to reliably assess the margins, and many reports for this tumor do not include margin information.

Negative margin — No cancer cells are present at the cut edge of the tissue. This suggests the tumor was completely removed.
Close margin — Cancer cells are near the cut edge but do not reach it. The distance from the nearest cancer cells to the edge may be measured and reported, because a very close margin can be relevant to decisions about additional treatment.
Positive margin — Cancer cells are present at the cut edge. This means some tumor may remain in the body, and the treatment team will use this finding when considering whether additional surgery or radiation therapy is appropriate.

Lymph nodes

Lymph nodes are small immune organs found throughout the head and neck. Sinonasal undifferentiated carcinoma can spread through lymphatic vessels to reach these nodes, and lymph node involvement is relatively common with this tumor. For this reason, lymph nodes in the neck are sometimes removed at the same time as the main tumor, in a procedure called a neck dissection. When lymph nodes are removed, they are examined under the microscope and the results are described in your pathology report.

Your report will include the total number of lymph nodes examined, the number that contain cancer cells, and the size of the largest deposit of cancer cells (often called a “focus” or “deposit”). A node that contains cancer cells is described as “positive,” and a node with no cancer cells is described as “negative.” The pathologist also checks for extranodal extension, which means cancer cells have broken through the outer capsule of a lymph node and spread into the surrounding tissue. Lymph node findings are used to determine the pathologic nodal stage (pN) and, along with evidence of cancer cells spreading to other parts of the body (metastasis), may influence decisions about additional treatment, such as chemotherapy, radiation therapy, or immunotherapy.

Pathologic stage (pTNM)

The pathologic stage for sinonasal undifferentiated carcinoma is based on the TNM staging system, as defined in the American Joint Committee on Cancer (AJCC) Cancer Staging Manual, 8th edition. This system describes the tumor using three categories: the primary tumor (pT), the regional lymph nodes (pN), and distant spread (pM). In general, a higher stage reflects more advanced disease. The metastatic stage (pM) is determined by imaging and clinical evaluation, not by the pathologist examining the surgical specimen. Because the tumor stage depends on where the cancer began, the criteria differ for tumors that start in the nasal cavity or ethmoid sinus versus those that start in the maxillary sinus.

Tumor stage (pT) — nasal cavity and ethmoid sinus

pT1 — The tumor is limited to one area (subsite) of the nasal cavity or ethmoid sinus, with or without involvement of the surrounding bone.
pT2 — The tumor involves two subsites within the nasal cavity or ethmoid sinus, or extends into an adjacent area within this region, with or without involvement of the surrounding bone.
pT3 — The tumor has invaded the floor or inner wall of the orbit (the socket that holds the eye), the maxillary sinus, the palate (the roof of the mouth), or the cribriform plate (a bony shelf at the top of the nasal cavity).
pT4a — The tumor has grown into the front part of the eye socket, the skin of the nose or cheek, a limited area at the base of the skull, the pterygoid plates (wing-shaped bones at the base of the skull), or the sphenoid or frontal sinuses.
pT4b — The tumor has grown into the deepest part of the eye socket, the coverings of the brain, the brain itself, the middle cranial fossa, specific cranial nerves, the upper throat behind the nose (nasopharynx), or a bony area at the base of the skull (clivus).

Tumor stage (pT) — maxillary sinus

pT1 — The tumor is limited to the lining (mucosa) of the maxillary sinus and has not damaged the surrounding bone.
pT2 — The tumor has eroded or destroyed bone, possibly including the hard palate or the middle nasal passage, but has not reached the back wall of the maxillary sinus or the pterygoid plates.
pT3 — The tumor has invaded the back wall of the maxillary sinus, the tissue beneath the skin, the floor or inner wall of the orbit, the pterygoid fossa (a depression at the side of the skull), or the ethmoid sinuses.
pT4a — The tumor has grown into the front part of the eye socket, the skin of the cheek, the pterygoid plates, the infratemporal fossa (a space at the side of the skull), the cribriform plate, or the sphenoid or frontal sinuses.
pT4b — The tumor has grown into the deepest part of the eye socket, the coverings of the brain, the brain itself, the middle cranial fossa, specific cranial nerves, the nasopharynx, or the clivus.

Nodal stage (pN)

pNX — The lymph nodes could not be assessed.
pN0 — No cancer cells were found in any of the lymph nodes examined.
pN1 — Cancer cells were found in a single lymph node on the same side of the neck as the tumor. The node is 3 cm or smaller and shows no extranodal extension.
pN2a — Cancer cells were found in a single lymph node on the same side of the neck that is either 3 cm or smaller with extranodal extension, or larger than 3 cm but no larger than 6 cm without extranodal extension.
pN2b — Cancer cells were found in more than one lymph node on the same side of the neck. None is larger than 6 cm, and none shows extranodal extension.
pN2c — Cancer cells were found in lymph nodes on both sides of the neck, or on the opposite side from the tumor. None is larger than 6 cm, and none shows extranodal extension.
pN3a — A lymph node containing cancer cells is larger than 6 cm and shows no extranodal extension.
pN3b — A lymph node with extranodal extension is present, or multiple involved nodes show extranodal extension.

What is the prognosis?

Prognosis refers to the likely long-term outcome after a diagnosis. Sinonasal undifferentiated carcinoma is a high-grade cancer that tends to grow quickly and is often found after it has spread beyond the sinonasal tract, so the outlook has historically been guarded. With modern treatment that combines chemotherapy, radiation, and sometimes surgery, five-year survival is now in the range of roughly 35 to 50 percent, and outcomes depend heavily on the stage of the tumor and how well it responds to initial treatment.

Stage — Tumors that have grown into the orbit, skull base, or brain, or that have spread to lymph nodes or distant sites, carry a higher risk.
Response to initial chemotherapy — Tumors that shrink well with chemotherapy given before other treatment tend to have a better outlook.
Margin status — When surgery is performed, positive or close margins are associated with a higher risk of the tumor returning in the same area.
Perineural and lymphovascular invasion — Either finding is associated with a higher chance that the tumor will spread or return.

What happens after the diagnosis?

Treatment for sinonasal undifferentiated carcinoma is planned by a multidisciplinary team that may include ear, nose, and throat (ENT) surgeons, neurosurgeons for tumors near the skull base, radiation oncologists, and medical oncologists. Because this tumor grows quickly and is often advanced at diagnosis, treatment usually combines more than one approach.

Many patients receive chemotherapy first, before other treatment. How the tumor responds to this initial chemotherapy helps the team decide what comes next: tumors that respond well are often treated with chemotherapy and radiation together, while in other cases surgery to remove the tumor is followed by radiation. When surgery is performed, the pathology findings, including margin status, perineural invasion, and lymph node involvement, directly inform whether additional radiation or chemotherapy is added. For tumors with an IDH2 mutation, targeted drug clinical trials may be an option. After treatment, close follow-up with imaging and physical examination is important, because this tumor can return.

Questions to ask your doctor

Where exactly did my cancer start — the nasal cavity, ethmoid sinus, or maxillary sinus?
How was the diagnosis confirmed, and were other sinonasal cancers ruled out?
Was molecular testing performed, and did my tumor have an IDH2 mutation?
What is my pathologic stage (pT and pN), and what does that mean for my treatment?
Were lymph nodes examined, and did any contain cancer cells? Was extranodal extension present?
Was perineural or lymphovascular invasion present in my tumor?
Will my treatment begin with chemotherapy, and how will my response guide the next steps?
If surgery is done, what will the margin results mean for whether I need more treatment?
What signs of recurrence should I watch for, and how will I be monitored after treatment?
Are there any clinical trials available for my type of cancer?