A Spitz nevus (more than one are called nevi) is a relatively rare non-cancerous type of skin tumour. Spitz nevus is part of a group of skin tumours that are commonly called moles. Spitz nevi tend to grow quickly at first, forming pink or brown coloured bumps on the surface of the skin. They are typically found on the head, neck, or legs in children and young adults. The tumour is named after Dr. Sophie Spitz, a pathologist who first described Spitz nevi.
Your skin is the largest organ in your body. It 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 squamous cells in the epidermis produce a material called keratin which makes the skin waterproof and strong and protects us from toxins and injuries.
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.
Melanocytes are normally found in the bottom layer of the epidermis. They are responsible for producing a dark pigment called melanin that helps protect our skin from the sun’s ultraviolet light. The amount of melanin in a person’s skin determines their skin colour – people with light skin produce little melanin and people with darker skin produce more melanin.
Most Spitz nevi are diagnosed after the entire tumour has been removed and sent to the pathologist to examine under the microscope. If the tumour shows any unusual features, the tissue may be sent to a pathologist who specializes in diagnosing skin lesions, called a dermatopathologist, to give a final diagnosis.
Spitz nevi are made up of melanocytes that are found in the epidermis, dermis, or both. In the epidermis, melanocytes in the tumour form groups called nests. The nests are often separated from each other and from the epidermis by open spaces called clefts.
As the nevus grows, melanocytes eventually enter into the dermis. At the top of the dermis, melanocytes can be seen in nests, single cells, or both. As melanocytes go deeper into the dermis, the melanocytes get smaller in size. This process is called maturation. Maturation is an important feature that pathologists look for when examining a Spitz nevus under the microscope.
Two types of melanocytes can be found in a Spitz nevus. The first type of cells are called epithelioid melanocytes and they are large round cells. The second type of cells are called spindle cell melanocytes and they are long thin cells. Spitz nevi can contain either epithelioid melanocytes, spindle cell melanocytes, or both.
Round pink structures called Kamino bodies are often seen in a Spitz nevus. Kamino bodies are found in the epidermis and can be a helpful clue under the microscope in recognizing a Spitz nevus.
Most Spitz nevi are symmetrical which means that when examined under the microscope, the right half and the left half of the tumour are similar in shape and size. This is typical for a non-cancerous tumour. In contrast, melanoma, a type of cancer, often grows more in one direction than the other. This type of growth is called asymmetrical.
Tumour cells divide to create new tumour cells. This process is called mitosis and a cell that is undergoing mitosis is called a mitotic figure. Pathologists frequently count the number of mitotic figures in a tumour and the result is called the mitotic rate. In non-cancerous tumours made of melanocytes, including Spitz nevi, the mitotic rate is low and any mitotic figures are usually seen towards the surface of the tumour.
Atypical is a word pathologists use to describe cells that look abnormal when examined under the microscope. Pathologists use the diagnosis atypical Spitz nevus to describe a tumour that has some features that are not commonly seen in a typical Spitz nevus. For example, a tumour may be called atypical if it is asymmetrical or if the mitotic rate is higher than normal. An atypical Spitz nevus does not mean cancer, but your doctor should follow you more closely to make sure the tumour does not grow back.
Your pathology report may also include a description of these other non-cancerous microscopic changes that are often seen in the background skin associated with a Spitz nevus.
Other non-cancerous changes seen with a Spitz nevus include:
Each cell in your body contains a set of instructions that tell the cell how to behave written in a language called DNA. The DNA is very long and is stored on 46 chromosomes in each cell, divided into sections called genes. Each gene tells the cell how to produce important parts of its machinery, such as proteins.
When a tumour is formed, there are changes in the DNA. Pathologists sometimes perform a test to analyze the DNA in a tumour that looks like a Spitz nevus in order to confirm the diagnosis. This test also helps to make sure the tumour is not a type of melanoma that can look very similar to a Spitz nevus under the microscope. This type of test is more commonly performed on tumours with atypical features.