by Glenda Wright MBBCh and Allison Osmond MD FRCPC
October 23, 2022
Fibrous histiocytoma is a very common type of non-cancerous skin tumour. The tumour is made up of a combination of fibroblastic cells, macrophages, and a type of connective tissue called collagen. The tumour develops in a layer of skin called the dermis. Another name for a fibrous histiocytoma is dermatofibroma.
Most fibrous histiocytomas grow slowly, have a pink to brown colour, and are usually small and painless.
Some fibrous histiocytomas appear to develop after trauma, an insect bite, or prolonged inflammation of the skin. However, most tumours develop without any preceding event and the cause remains unknown.
No. A fibrous histiocytoma is a non-cancerous tumour that will not turn into cancer over time. However, tumours that are not completely removed can re-grow.
Pathologists divide fibrous histiocytomas into types based on how the tumours look when examined under a microscope. The most common types include aneurysmal, cellular, deep, clear cell, lipidized, palisaded, and myxoid. Unlike other types of fibrous histiocytomas, the epithelioid type contains a genetic change involving the ALK gene and as a result, maybe a separate entity.
The diagnosis is usually made after the entire tumour has been surgically removed and sent to a pathologist for examination under a microscope. Sometimes, your doctor will remove a small piece of the tumour in a procedure called a biopsy before removing the entire tumour. Under the microscope, fibrous histiocytomas are made up of long thin cells that pathologists call spindle cells. These spindle cells look very similar to the fibroblasts and histiocytic cells normally found in a layer of the skin called the dermis. Giant cells, which are histiocytes that have joined together to form one large cell, are also seen. It can be very challenging for a pathologist to make the diagnosis of fibrous histiocytoma after only examining a small piece of the tumour because there are other types of tumours that can look very similar to fibrous histiocytoma. In this case, your pathologist may offer a list of possible diagnoses which includes fibrous histiocytoma. A final diagnosis will be given after the entire tumour is removed and examined.