CIC-DUX4 Sarcoma (CIC-rearranged sarcoma): Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
September 11, 2026

CIC-rearranged sarcoma is an uncommon, fast-growing cancer that develops most often in the deep soft tissues. It belongs to a group called undifferentiated round cell sarcomas, meaning the tumor cells look very immature and do not resemble any normal tissue under the microscope.

Your report may use either of two names. CIC-rearranged sarcoma is the current name and covers the whole group. CIC-DUX4 sarcoma names the commonest form, in which the CIC gene is joined to a gene called DUX4, and this accounts for roughly 95 percent of cases. Both refer to the same diagnosis.

This article explains what appears on the pathology report, why confirming this diagnosis is unusually difficult, and why the distinction from Ewing sarcoma matters for treatment.

Where does CIC-rearranged sarcoma develop, and who gets it?

Most CIC-rearranged sarcomas arise in the deep soft tissues of the arms, legs, or trunk. Less commonly, they occur in the head and neck, pelvis, or retroperitoneum, the space behind the abdomen’s organs. About 1 in 10 arise in an internal organ such as the kidney, the digestive tract, or the brain. Tumors starting in bone are uncommon, which is one difference from Ewing sarcoma.

This tumor can occur at any age but has a strong tendency to affect young adults, most often between about 25 and 35. Fewer than a quarter of cases occur in children, and males have a slight predominance.

What are the symptoms of CIC-rearranged sarcoma?

The usual presentation is a rapidly growing mass that may or may not be painful. Because these tumors grow fast and spread early, a large proportion of patients already have metastatic disease when the diagnosis is made.

Symptoms depend on where the cancer has spread. Spread to the lungs, the most common site, can cause breathlessness or a cough. A large tumor can cause pressure or discomfort in nearby tissues.

What causes CIC-rearranged sarcoma?

CIC-rearranged sarcoma is caused by a genetic change acquired during life within the tumor cells. It is not inherited, is not passed to children, and carries no implications for relatives.

A piece of the CIC gene becomes abnormally joined to a second gene, creating a fusion gene that produces an abnormal protein. That protein switches on growth-promoting genes that should stay off, driving the cells to divide rapidly. DUX4 is by far the commonest partner. Less often, CIC joins NUTM1, FOXO4, LEUTX, or another gene, and those tumors behave in much the same way.

How is the diagnosis made?

Diagnosing CIC-rearranged sarcoma requires microscopic examination, immunohistochemistry, and molecular testing, and it is one of the more difficult diagnoses in sarcoma pathology.

CT or MRI locates the tumor, measures it, and looks for spread. These tumors are often large when first found and may show areas of bleeding or dead tissue on imaging.

Under the microscope, the tumor is made of sheets of round, immature-looking cells. They are fairly uniform but show more variation in size and shape than Ewing sarcoma does. The nuclei often have open chromatin and prominent nucleoli. The tumor commonly grows in lobules separated by bands of fibrous tissue, and small areas of spindle-shaped or epithelioid cells may be present.

Necrosis (dead tumor tissue) is common, and mitotic figures (dividing cells) are numerous. In about a third of cases, the supporting tissue has a gel-like, or myxoid, quality, and the tumor cells form delicate net-like or gland-like patterns within it.

Immunohistochemistry

Immunohistochemistry uses antibodies to detect proteins in the tumor cells. In CIC-rearranged sarcoma, it does more work than usual because the molecular tests are unreliable, as explained in the next section.

ETV4. Positive in more than 9 out of 10 cases. The fusion protein switches this gene on, so the stain acts as a stand-in for the genetic change itself and is the most useful single marker.
WT1. Positive in roughly 90 to 95 percent of cases.
CD99. Usually patchy and uneven, in contrast to the strong, uniform staining seen in Ewing sarcoma.
NKX2-2. Typically negative, which is another point of separation from Ewing sarcoma.

Keratins, S100, and muscle markers are usually absent. The combination of patchy CD99 with positive ETV4 and WT1, and negative NKX2-2, points to this diagnosis.

Molecular testing, and why it is difficult

Molecular testing looks directly for the CIC rearrangement. Confirming it is harder than for most fusion-driven sarcomas, and understanding why explains what your report says.

FISH for a break in the CIC gene misses a meaningful number of genuine cases. Partly because DUX4 sits within a highly repetitive stretch of DNA that laboratory probes and sequencing software both struggle with. Tumors that carry the fusion have been reported as negative by FISH.

RNA sequencing, usually part of a next-generation sequencing panel, detects more cases and identifies the partner gene, but it can also miss the fusion for the same reason.

Because no single test finds every case, the diagnosis rests on the whole picture: the appearance under the microscope, the immunohistochemistry pattern, and any available molecular result. A negative FISH result does not by itself rule out this diagnosis. A strongly positive ETV4 stain in a tumor with the right appearance carries real weight even without a confirmed fusion.

The round cell sarcoma group

CIC-rearranged sarcoma is one of four diseases that look similar under the microscope and were once grouped as Ewing-like sarcomas. The World Health Organization classification of soft tissue and bone tumors, 5th edition, published in 2020, separated them.

Ewing sarcoma. Defined by a fusion joining EWSR1 or a related gene to a gene in the ETS family.
CIC-rearranged sarcoma. The disease this article describes.
Sarcoma with BCOR genetic alterations. Seen most often in adolescent boys, usually in bone.
Round cell sarcoma with an EWSR1 fusion to a non-ETS partner.

This separation is not a technicality. CIC-rearranged sarcoma responds less well to the chemotherapy used for Ewing sarcoma and carries a different outlook. Being placed in the right category therefore changes what a patient is told and what treatment is considered.

Surgical margins

A margin is the edge of tissue the surgeon cuts to remove the tumor. The pathologist inks these edges and examines them. A negative margin means no tumor cells were seen at the edge. A positive margin means the tumor reaches the edge and some may have been left behind.

Negative margins matter here because the tumor has a high risk of returning in the same place. Margin status influences whether radiation or further surgery is recommended.

Two further findings may be reported. Lymphovascular invasion means tumor cells are seen inside small blood or lymphatic vessels, and it points to a higher risk of spread. Perineural invasion, meaning tumor cells around a nerve, is uncommon in this tumor and is noted when present.

Stage and grade

CIC-rearranged sarcoma is staged using the soft tissue sarcoma system, which takes account of the size and depth of the tumor, lymph node involvement, and spread to distant sites. The lungs are the commonest site of spread.

Grade is handled differently from most soft tissue sarcomas. The usual grading system does not apply to round cell sarcomas because these tumors are high grade by definition, in the same way that Ewing sarcoma is not graded. If your report gives no grade, that is expected rather than an omission.

As with Ewing sarcoma, the distinction that matters most is not the stage number but whether the disease is confined to one place or has already spread.

What is the prognosis?

CIC-rearranged sarcoma is a difficult cancer. It grows quickly, spreads early, and responds less well to chemotherapy than Ewing sarcoma does. Reported five-year survival ranges from roughly 17 to 43 percent. These figures come from small series collected over a period when many of these tumors were still being diagnosed as something else.

Features associated with a worse outcome include spread at the time of diagnosis, a larger tumor, and a positive surgical margin. Complete surgical removal of a tumor that has not spread offers the best chance.

Two things are worth keeping in mind alongside those numbers. This entity was only recognized in the last decade, so the published outcomes largely predate any treatment designed specifically for it. And research into this tumor is active, including work on the specific machinery the fusion protein depends on, so clinical trial options are worth asking about early rather than late.

What happens after the diagnosis?

A sarcoma team at a center that treats these tumors regularly should guide care. Because the diagnosis is difficult and uncommon, a sarcoma-specialist pathologist usually reviews the slides, and it is reasonable to ask whether that has happened.

Treatment combines chemotherapy, surgery, and sometimes radiation. Chemotherapy regimens are generally modeled on those used for Ewing sarcoma or for soft tissue sarcoma, because no regimen has been developed specifically for this tumor. Responses are often shorter than in Ewing sarcoma, which is part of why clinical trial participation is raised early.

Follow-up is close, with imaging of the original site and the chest, because the lungs are the most common site of spread and recurrence can happen early.

Questions to ask your doctor

Where is my tumor, and has it spread?
Was a CIC rearrangement confirmed, and which partner gene was involved?
Which tests were used, and were both FISH and sequencing performed?
If molecular testing was negative, what supports the diagnosis?
Were the ETV4, WT1, CD99, and NKX2-2 stains done, and what did they show?
Has a sarcoma pathologist reviewed the slides?
Could this be Ewing sarcoma or another round cell sarcoma instead?
What chemotherapy is planned, and what is it based on?
Will surgery be possible, and were the margins negative?
Is radiation part of the plan?
Are there clinical trials open for this diagnosis?
Should I be treated at a specialized sarcoma center?
What follow-up imaging will I need, and how often?