Dermatofibrosarcoma protuberans (DFSP)

What is dermatofibrosarcoma protuberans?

Dermatofibrosarcoma protuberans (DFSP) is a type of skin cancer that starts in a part of the skin called the dermis. For most patients, the tumour is slow-growing and painless. Most tumours start on the chest or back but any part of the body can be affected.

Dermatofibrosarcoma protuberans can re-grow in the same location after surgery (local recurrence) especially if the tumour was not completely removed during the first surgery. However, unlike other types of cancer, dermatofibrosarcoma protuberans very rarely spread or metastasize to other parts of the body.

The skin

Skin is made up of three layers: epidermis, dermis, and subcutaneous fat. The surface and the part you can see when you look at your skin is called the epidermis. The cells that make up the epidermis include squamous cells, basal cells, melanocytes, Merkel cells, and cells of the immune system.

The dermis is directly below the epidermis. The dermis is separated from the epidermis by a thin layer of tissue called the basement membrane. The dermis contains blood vessels and nerves. Below the dermis is a layer of fat called subcutaneous adipose tissue.

Many different types of cells are normally found in the dermis including fibroblasts and histiocytes. Fibroblasts produce collagen and other substances that make the dermis strong. Histiocytes are a type of immune cell that protects the body from any infections or toxins that enter through the skin.

How do pathologists make this diagnosis?

The diagnosis is usually made after a small tissue sample is removed in a procedure called a biopsy. The diagnosis can also be made after the entire tumour is removed in a procedure called an excision. If the diagnosis is made after a biopsy, your doctor will probably recommend a second surgical procedure to remove the rest of the tumour.

Under the microscope, dermatofibrosarcoma protuberans is made up of long thin cells called spindle cells that look very similar to the fibroblasts normally found in the dermis. The spindle cells grow in a circular pattern that pathologists call “spoke and wheel” or “cartwheel” pattern. The tumour starts in the dermis, but cancer cells are often found growing into the subcutaneous fat below.

Your pathologist may perform a test called immunohistochemistry to help confirm the diagnosis. The cancer cells in dermatofibrosarcoma protuberans are usually positive or reactive for CD34 and negative or non-reactive for Factor 13A.

dermatofibrosarcoma protuberans

What to look for in your report after the tumour has been removed

Margins

A margin is any tissue that was cut by the surgeon in order to remove the tumour from your body. Whenever possible, surgeons will try to cut tissue outside of the tumour to reduce the risk that any cancer cells will be left behind after the tumour is removed.

Your pathologist will carefully examine all the margins in your tissue sample to see how close the cancer cells are to the edge of the cut tissue. Margins will only be described in your report after the entire tumour has been removed.

Dermatofibrosarcoma protuberans (DFSP) has the potential to re-grow (recur), especially if it is not completely removed. The border between the tumour and the surrounding normal tissue can be difficult to see because the cancer cells in a DFSP often extend as small groups of cells at the edge of the tumour into the surrounding normal tissue. This can make removing the tumour challenging as your surgeon may not know exactly where the tumour ends.

A negative margin means there were no cancer cells at the very edge of the cut tissue. If all the margins are negative, most pathology reports will say how far the closest cancer cells were to a margin. The distance is usually described in millimetres. A margin is considered positive when there are cancer cells at the very edge of the cut tissue. A positive margin is associated with a higher risk that the tumour will recur in the same site after treatment.

Margin

Molecular tests

Each cell in your body contains a set of instructions that tell the cell how to behave. These instructions are written in a language called DNA and the instructions are stored on 46 chromosomes in each cell. Because the instructions are very long, they are broken up into sections called genes and each gene tells the cell how to produce a piece of the machine called a protein.

Sometimes, a piece of DNA falls off one chromosome and becomes attached to a different chromosome. This is called translocation and it can result in the cell making a new and abnormal protein. If the new protein allows the cell to live longer than other cells or spread to other parts of the body, the cell can become cancer (a malignant tumour).

Dermatofibrosarcoma protuberans sometimes contains a translocation that combines the gene COL1A1 translocation with gene PDGFB. Pathologists test for this molecular change by performing either fluorescence in situ hybridization (FISH) or next-generation sequencing (NGS) on a piece of the tissue from the tumour. This type of testing is can be done on the biopsy specimen or when the tumor has been surgically removed.

A translocation involving the COL1A1 or PDGFB genes confirms the diagnosis of dermatofibrosarcoma protuberans and excludes other types of tumours that can look similar under the microscope.

by Glenda Wright MBBCh and Allison Osmond MD FRCPC (updated September 8, 2021)
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