Fibrosarcoma is a rare and aggressive type of cancer that begins in the connective tissues holding your body together, most often appearing as an unusual lump beneath the skin that may not hurt at first but can grow and cause problems over time.

Understanding Fibrosarcoma: A Rare Soft Tissue Cancer

Fibrosarcoma is a rare form of soft tissue cancer, also called a sarcoma, which is a tumor that starts in the tissues connecting and supporting different parts of the body. When someone develops fibrosarcoma, they have a tumor in their connective tissues, which include tendons (the cords attaching muscles to bones), ligaments (bands connecting bones to other bones), and fibrous tissue. In some cases, the tumor can also form inside bones or in the fibrous tissue covering bones.^[1]

This disease is uncommon in both adults and children, affecting only about 1 in 2 million people. The adult form represents roughly 10 percent of all soft tissue sarcoma cases. While fibrosarcoma can develop anywhere in the body, it most commonly appears deep inside the legs, especially around the shin bone or thigh bone, as well as in the upper arms, knees, or trunk. Less frequently, tumors start in the head or neck area.^[1]

There are two distinct types of fibrosarcoma that behave very differently. Infantile fibrosarcoma, also called congenital fibrosarcoma, usually appears at birth or shortly after in babies and young children. This form grows quickly but rarely spreads to other parts of the body, and is usually curable. The adult-type fibrosarcoma is most common in adults between ages 20 and 60, though it can also affect older children and adolescents. Unlike the infantile type, adult fibrosarcoma is typically more aggressive and harder to treat successfully.^[1]

Who Gets Fibrosarcoma: Patterns and Numbers

Fibrosarcoma can affect people of different ages, but the patterns vary between the two main types. The adult form is most common in middle-aged and older adults, with people between 30 and 55 years old most frequently affected. Men have a slightly higher chance of developing this cancer compared to women.^[2]

Infantile fibrosarcoma, while one of the most common sarcomas found in children under 5 years old, remains rare overall. It affects fewer than 5 out of every 1 million infants. Most children with this type are diagnosed within their first two years of life, often at birth or shortly afterward.^[1]

The frequency of fibrosarcoma diagnoses has actually decreased over recent decades. This isn’t because the disease has become less common, but rather because doctors have gotten better at distinguishing fibrosarcoma from other similar-looking tumors. Improved testing and understanding of tumor characteristics mean that some growths previously called fibrosarcoma are now more accurately identified as different types of cancer.^[2]

When looking at all soft tissue sarcomas together, about 13,520 new cases are diagnosed in the United States each year, with approximately 5,410 deaths. Fibrosarcoma represents a small fraction of these numbers, highlighting just how uncommon this particular cancer is.^[1]

What Causes Fibrosarcoma

The exact cause of fibrosarcoma remains unknown to researchers, but genetic changes in cells likely play an important role. Many fibrosarcomas show the same types of alterations in their cell DNA. These genetic changes can cause cells to multiply rapidly and uncontrollably, eventually forming cancerous tumors.^[1]

In infantile fibrosarcoma, scientists have identified a specific genetic pattern. About 90 percent of infantile cases involve problems with the NTRK gene family, which includes three separate NTRK genes. When these genes don’t function properly, tumors can develop. This discovery has been important for understanding the disease and developing targeted treatments.^[1]

While most fibrosarcomas occur randomly without a clear trigger, researchers believe that genetic mutations accumulate over time or occur during cell development. In the case of secondary fibrosarcoma, which develops from pre-existing conditions, the trigger may be damage to bone or tissue. However, even in these cases, the precise mechanism that transforms normal cells into cancer cells isn’t fully understood.^[3]

Some fibrosarcomas arise as secondary tumors, meaning they develop from a pre-existing condition or after treatment for another problem. For bone fibrosarcomas, prior damage to the bone—whether from injury or radiation therapy—may trigger the development of cancer. This form tends to be more aggressive and carries a poorer outlook than primary fibrosarcoma that develops on its own.^[2]

Risk Factors: Who Is More Likely to Develop Fibrosarcoma

Several inherited conditions can increase a person’s risk of developing fibrosarcoma. These genetic syndromes affect how cells grow and divide, making tumors more likely to form. People with Li-Fraumeni syndrome, caused by mutations in the TP53 gene, have an increased risk of various cancers including fibrosarcoma. Similarly, those with neurofibromatosis type 1, a condition causing multiple non-cancerous tumors on nerves, have about a 10 percent lifetime risk of developing fibrosarcoma or similar cancers.^[1]

Other inherited conditions associated with higher fibrosarcoma risk include familial adenomatous polyposis, nevoid basal cell carcinoma syndrome, retinoblastoma, tuberous sclerosis, and Werner syndrome. While these conditions are themselves rare, people diagnosed with them should be aware of the increased cancer risk and maintain regular medical monitoring.^[1]

Certain medical conditions affecting the bones can also increase risk. These include bone infarction (when bone tissue dies due to lack of blood flow), chronic osteomyelitis (long-lasting bone infection), fibrous dysplasia (abnormal bone development), and Paget’s disease of the bone (a disorder causing abnormal bone destruction and regrowth).^[1]

Environmental and treatment-related factors can also raise the likelihood of developing fibrosarcoma. Previous radiation therapy directed at an area of the body is a known risk factor. People who received radiation treatment for another cancer may develop fibrosarcoma in the treated area years or even decades later. Exposure to certain chemicals, including vinyl chloride and arsenic, has been linked to some types of sarcomas, though these associations are more firmly established for other sarcoma subtypes.^[1]

Recognizing the Symptoms of Fibrosarcoma

Fibrosarcoma symptoms often develop slowly and may be subtle at first. Because these tumors grow in deep soft tissues beneath the skin, many people don’t notice any changes until the tumor becomes larger. The most common first sign is discovering a lump or swelling somewhere on the body. This lump may feel soft or firm, and it can appear anywhere, though it’s most frequently found in the arms, legs, or trunk.^[1]

In the early stages, the lump may be completely painless. Some people discover it by accident while bathing or dressing. As the tumor grows, it may begin pressing on nearby nerves or blood vessels, which can cause additional symptoms. When a tumor presses on a nerve, it can create a tingling or “pins and needles” sensation, similar to when your foot falls asleep. Some people experience sharp, aching, or burning pain in the area around the tumor.^[1]

If the fibrosarcoma grows large enough to press on blood vessels, it can cause unusual swelling in the affected area. This happens because the compressed blood vessels can’t move blood efficiently, leading to fluid buildup. In the limbs, this might cause the arm or leg to appear larger than normal or feel heavy.^[1]

When fibrosarcoma develops in the abdomen or trunk, symptoms may not appear until the tumor reaches a significant size. As it grows, it can push on surrounding organs, muscles, nerves, or blood vessels. Depending on which structures are affected, this can lead to pain, tenderness, or breathing problems. A tumor in the chest area might make it difficult to take deep breaths, while one in the abdomen might cause digestive discomfort.^[4]

In infants with congenital fibrosarcoma, the tumor typically appears as a rapidly growing mass that parents or doctors notice at birth or shortly afterward. The symptoms in children depend on where the tumor is located but often include a visible or palpable lump, sometimes accompanied by swelling.^[7]

Preventing Fibrosarcoma: What You Can Do

Because the exact causes of fibrosarcoma aren’t fully understood, and many cases involve genetic factors that can’t be controlled, there’s no guaranteed way to prevent this disease. However, understanding risk factors can help people make informed decisions and seek appropriate monitoring when necessary.^[1]

For people with inherited genetic syndromes that increase cancer risk, regular medical check-ups and monitoring are crucial. While these screenings won’t prevent fibrosarcoma from developing, they can help catch it early when treatment is most likely to be successful. People with conditions like Li-Fraumeni syndrome or neurofibromatosis should work closely with their healthcare providers to establish an appropriate surveillance schedule.^[1]

Minimizing unnecessary radiation exposure is another consideration. While radiation therapy is often essential for treating certain cancers, doctors now use more targeted radiation techniques and lower doses when possible to reduce the risk of secondary cancers like fibrosarcoma. If you need radiation therapy, your medical team will carefully balance the treatment benefits against potential long-term risks.^[4]

Avoiding exposure to known cancer-causing chemicals can reduce overall cancer risk, though the link between specific chemicals and fibrosarcoma isn’t as well established as for some other cancers. Workers in industries using vinyl chloride or arsenic should follow proper safety procedures and use protective equipment.^[4]

Paying attention to your body and seeking medical evaluation for any unusual lumps or persistent symptoms is important. While most lumps turn out to be harmless, getting them checked ensures that any serious problems are caught early. Don’t wait to see a doctor if you notice a new lump that doesn’t go away, especially if it’s growing or causing pain.^[1]

How Fibrosarcoma Changes Normal Body Functions

Fibrosarcoma is a tumor composed of malignant fibroblasts, which are cells that normally produce collagen and other proteins forming the structural framework of connective tissue. In healthy tissue, fibroblasts work in an organized way to maintain and repair the body’s support structures. When these cells become cancerous in fibrosarcoma, they multiply uncontrollably and produce abnormal amounts of collagen, creating a tumor mass.^[2]

Under a microscope, fibrosarcoma cells often arrange themselves in a distinctive pattern called a “herringbone” pattern, named because it looks like the skeleton of a fish. These cells appear spindle-shaped (long and narrow) rather than round like normal cells. The tumor may contain varying amounts of collagen depending on how much the cancerous fibroblasts are producing. Some fibrosarcomas make lots of collagen and appear more organized, while others make less and look more chaotic.^[5]

Fibrosarcomas are classified by how differentiated they are, which describes how much they resemble normal tissue. Low-grade or well-differentiated fibrosarcomas look more like normal fibrous tissue, with cells that still resemble mature fibroblasts and produce regular collagen. These tumors grow more slowly and are less likely to spread. High-grade or poorly differentiated fibrosarcomas contain cells that look very abnormal, with irregular shapes, multiple nuclei, and frequent cell divisions. These aggressive tumors grow quickly and are more likely to spread to other parts of the body.^[2]

As fibrosarcoma grows, it disrupts normal tissue structure and function. The tumor mass physically displaces and damages surrounding healthy tissue. When it presses on nerves, it interferes with normal nerve signaling, causing pain, numbness, or tingling. Pressure on blood vessels can reduce blood flow, leading to swelling and potentially depriving nearby tissues of oxygen and nutrients.^[1]

In some cases, fibrosarcoma can form immature blood vessels within the tumor itself. These sarcomatous vessels lack the normal cell lining found in healthy blood vessels, making them leaky and inefficient. However, these abnormal vessels also provide a potential pathway for cancer cells to enter the bloodstream and spread to distant parts of the body, a process called metastasis.^[5]

In the infantile form of fibrosarcoma, the tumor grows rapidly but behaves differently from the adult type. Most infantile fibrosarcomas have a specific genetic change where parts of chromosomes 12 and 15 swap places, creating a fusion gene called ETV6-NTRK3. This genetic alteration drives the abnormal cell growth but, interestingly, these tumors rarely spread beyond their original location, which is why the prognosis for infants is generally much better than for adults.^[5]