Fibromyxoid Tumour: Definition

The term fibromyxoid tumour is not a final diagnosis but a descriptive term that pathologists sometimes use when looking at tissue under the microscope. It refers to tumours made up of a mix of fibrous tissue (tough, scar-like tissue) and myxoid tissue (a gelatin-like material). These tumours can be benign (noncancerous) or malignant (cancerous). They may arise in soft tissues, such as muscle or fat, or in bone. The behaviour of the tumour—how it grows, spreads, or recurs—depends on its exact type.

Types of fibromyxoid tumours

Fibromyxoid tumours are a diverse group. They are divided into those that arise in soft tissues and those that arise in bone. Some are slow growing and harmless, while others are aggressive cancers that can spread.

Soft tissue fibromyxoid tumours

• Low grade fibromyxoid sarcoma (LGFMS): A rare cancer that grows slowly in deep soft tissue, usually in the trunk, arms, or legs. It may return after surgery and, in some cases, spread to other parts of the body.

• Myxofibrosarcoma: More common in older adults, this cancer often arises in the limbs. Its behaviour can vary, but it has a high chance of coming back after treatment.

• Ossifying fibromyxoid tumour: Usually appears as a painless lump under the skin or in soft tissue. Most are noncancerous, but cancerous types do occur. These tumours often have a shell of bone-like material around them.

• Desmoid-type fibromatosis: A noncancerous tumour that does not spread to other organs but can grow into nearby tissues. It may have both fibrous and myxoid features.

• Solitary fibrous tumour: Can be found in many parts of the body and sometimes shows myxoid areas. Most are noncancerous, but some act aggressively.

• Intramuscular myxoma: A benign tumour made entirely of myxoid tissue. It usually grows slowly within muscle.

• Spindle cell lipoma: A benign fatty tumour usually seen in the neck, shoulders, or back. Some areas of the tumour may look myxoid under the microscope.

Bone fibromyxoid tumours

• Fibromyxoma: A benign tumour most often found in the jawbones (mandible or maxilla) but can also appear in other bones. It grows slowly and rarely causes serious problems.

How are fibromyxoid tumours diagnosed?

Because “fibromyxoid tumour” is a descriptive, preliminary term, further tests are almost always required to make a final diagnosis.

• Imaging tests: MRI or CT scans help show whether the tumour comes from soft tissue or bone and how it relates to nearby structures.

• Biopsy and microscopic examination: A pathologist examines a small piece of the tumour under the microscope, looking for features that suggest whether it is benign or malignant.

• Immunohistochemistry (IHC): Special stains are used to detect proteins in tumour cells. For example, LGFMS usually shows positivity for a protein called MUC4.

• Genetic tests: Tests such as next-generation sequencing (NGS) can identify specific genetic changes. LGFMS often shows a fusion gene created by a translocation between chromosomes 7 and 16.

• Fluorescence in situ hybridization (FISH): This test can detect translocations and other genetic abnormalities that are linked to certain fibromyxoid tumours.

Why is the diagnosis important?

The exact type of fibromyxoid tumour determines how it is treated and what the outlook will be.

• Benign tumours may only need surgery and rarely come back once removed.

• Malignant tumours may require surgery, radiation, or chemotherapy, and some may spread to other parts of the body.

Because fibromyxoid tumours can look similar under the microscope, additional testing is often needed to make sure the right diagnosis is made.

Questions to ask your doctor

• What type of fibromyxoid tumour do I have?

• Is it benign or malignant?

• Do I need further tests such as IHC or genetic testing?

• What are the treatment options for my type of tumour?

• What is the chance that this tumour will come back or spread?