EHE, or epithelioid hemangioendothelioma, is an ultra-rare cancer that develops from the cells lining blood vessels. It affects roughly one in every million people worldwide, making it one of the rarest cancers known. EHE sits in a gray zone between benign vascular tumors and aggressive cancers like angiosarcoma, behaving unpredictably: some tumors remain stable for years, while others slowly spread to distant organs.
Where EHE Develops
Because EHE arises from blood vessel cells, it can technically appear almost anywhere in the body. The most common primary sites are the liver, lungs, and bone, though it also occurs in soft tissue, skin, and occasionally the brain or other organs. Many people are diagnosed after imaging reveals multiple nodules in the liver or lungs, sometimes found incidentally during scans for unrelated issues. The tumors often appear as small, round lesions scattered through the affected organ rather than a single large mass.
Symptoms depend on where the tumors grow. Liver EHE may cause upper abdominal pain, an enlarged liver, or abnormal liver function tests. Lung involvement can lead to shortness of breath, cough, or chest pain. Some people have no symptoms at all, and the cancer is discovered only on routine imaging. When EHE involves more than one organ at the time of diagnosis, which is common, doctors consider it multifocal disease.
What Drives It Genetically
Unlike many cancers caused by accumulated mutations over time, EHE is driven by a single defining genetic event: a chromosomal translocation that fuses two genes together, creating an abnormal protein that pushes cells to grow. About 90% of EHE tumors carry a fusion between genes called WWTR1 and CAMTA1, resulting from pieces of chromosomes 1 and 3 swapping places. The remaining 10% carry a different fusion involving YAP1 and TFE3.
Both of these fusion proteins hijack a growth-control system called the Hippo pathway, which normally acts as a brake on cell division. The WWTR1-CAMTA1 fusion protein is particularly disruptive because it forces its way into the cell nucleus and resists the normal “off” signals the Hippo pathway sends. Once inside the nucleus, it opens up regions of DNA that should stay closed and triggers a state of excessive gene activity, causing significant DNA damage in the process. This is what transforms normal blood vessel lining cells into tumor cells.
The 10% of cases with the YAP1-TFE3 fusion tend to look slightly different under a microscope, with more obvious blood vessel formation within the tumor. Early evidence suggests these tumors generally behave in an indolent (slow-growing) way, though they still carry a long-term risk of spreading to distant sites.
How EHE Is Diagnosed
Diagnosing EHE is notoriously difficult because it can mimic other conditions. Liver lesions are sometimes mistaken for metastatic cancer from another organ, while lung nodules can look like infections or other tumors on imaging. A biopsy is essential. Under the microscope, EHE cells have a characteristic appearance: rounded (epithelioid) cells with glassy cytoplasm, some containing small internal spaces called intracytoplasmic lumina, sometimes described as “blister cells.”
Pathologists confirm the vascular origin of the tumor using protein markers that highlight blood vessel cells. These include CD31 and transcription factors called ERG and FLI1. However, since other vascular tumors also express these markers, the most definitive step is testing for the WWTR1-CAMTA1 or YAP1-TFE3 gene fusions. This molecular testing separates EHE from its more aggressive cousin, epithelioid angiosarcoma, which shares some visual features but has higher rates of cell division, more abnormal-looking cells, and areas of tissue death. Getting the distinction right matters enormously because the treatment approach and expected outcomes differ significantly.
Prognosis and What Affects It
EHE generally carries a better prognosis than most sarcomas, though outcomes vary widely depending on individual factors. A large study of liver EHE found 5-year overall survival of about 85%, with 3-year survival near 88%. These are encouraging numbers for a cancer diagnosis, but they reflect the full spectrum from slow-growing tumors to more aggressive disease.
One of the most important prognostic factors is the presence of serosal effusions, which means fluid accumulation around the lungs or in the abdomen. Patients without effusions have dramatically better outcomes. In studies of drug therapy, those without effusions had a median time before the disease worsened of nearly 48 months, compared to under 5 months for those with effusions. Whether the tumor is confined to one site or spread across multiple organs also shapes the outlook, as does how quickly tumors are growing at the time of diagnosis.
Treatment Approaches
There is no single standard treatment for EHE, and management is highly individualized. The approach depends on how many tumors are present, where they are, and whether they’re actively growing.
Active Surveillance
Because some EHE tumors grow extremely slowly, not every case requires immediate treatment. Active surveillance, meaning regular imaging to monitor tumor size and behavior, is the recommended initial approach when the disease has already spread but isn’t causing symptoms. The logic is straightforward: if the cancer isn’t progressing, the side effects of treatment may cause more harm than the disease itself. For single-site EHE, however, experts generally recommend against prolonged watching and waiting because local treatments may offer a chance at cure, and delaying carries a risk of spread.
Surgery and Local Treatments
When EHE is confined to one location and can be completely removed, surgery is the primary option. For liver EHE, this can range from removing part of the liver to liver transplantation in select cases. Interestingly, one large study found that long-term survival for liver EHE patients who had surgery was similar to those who did not, with 5-year survival around 87-88% in both groups. This likely reflects the slow-growing nature of many cases rather than surgery being unnecessary; it underscores how variable this cancer’s behavior can be.
Drug Therapy
For progressive or multifocal EHE that can’t be surgically removed, sirolimus (a drug that blocks a cell growth pathway called mTOR) has the strongest evidence of any medication. The biological rationale is direct: the same growth signals hijacked by EHE’s fusion proteins feed into the mTOR pathway, so blocking it can slow tumor growth. In the largest study to date, 88% of patients without serosal effusions experienced clinical benefit, with most tumors stabilizing rather than shrinking. True tumor shrinkage occurred in about 8% of patients, but stable disease in a slow-growing cancer is considered a meaningful win.
Clinical trials are also evaluating drugs that target other pathways involved in EHE’s growth, including MEK inhibitors. Because the cancer is so rare, clinical trials are small and move slowly, but the clear genetic drivers of EHE make it an appealing target for precision therapies.
Living With an Ultra-Rare Cancer
One of the biggest challenges with EHE isn’t the biology but the rarity. Many oncologists will never see a case in their entire career, which can lead to misdiagnosis or treatment plans borrowed from more common cancers that may not apply. Seeking care at a sarcoma center or a hospital with experience in rare vascular tumors can make a meaningful difference in both diagnosis and management. Patient advocacy groups, particularly the EHE Foundation, connect patients with specialists and maintain registries that help advance understanding of the disease.
The unpredictable course of EHE creates a unique psychological burden. Some patients live with stable disease for a decade or more, while others experience steady progression. This uncertainty makes regular follow-up imaging essential, typically every three to six months depending on the tumor’s behavior, so that any change in pace can be caught early and the treatment plan adjusted.