Hi everyone. I'd like to tell you about various skin cancers and where they arise in the skin. Specifically, I want to clarify why one skin cancer cannot turn into another type of skin cancer. Because many people know that basal cell carcinoma is not as dangerous as melanoma, patients often ask if a basal cell carcinoma would eventually turn into a deadly melanoma if it were to go untreated. Because these are two different skin cancers, one cannot not become the other. I hope to more clearly discuss this and give a better understanding of skin cancers in this post.
The skin is divided into different layers, with different types of cells populating these layers. Without getting very technical, the top of the skin is the "epidermis" and is made up of different types of cells. Below the epidermis is the "dermis", which is comprised of collagen. This gives the skin its durability and elasticity. At the very base or bottom of the epidermis are basal cells, who get their name from being at the base of the epidermis. These cells usually line the bottom of the epidermis and are very uniform in shape and appearance. If these basal cells become abnormal and start to appear deeper down in the dermis (where they should not be), then these cells have become cancerous and are then called basal cell carcinoma. Despite being cancerous, basal cell carcinoma tends not to go very deep into the dermis and does not spread to other parts of the body.
Above the basal cells of the epidermis are cells called keratinocytes, also known as the "squamous epithelium", which make up the majority of the epidermis. When these cells become abnormal and cancerous, it is known as squamous cell carcinoma. If these cells are confined to the epidermis, then they are termed as being "in-situ", which signifies a relatively superficial skin cancer. If these cancerous cells invade deeper down into the dermis then it is called invasive squamous cell carcinoma. Invasive squamous cell carcinoma is more likely to metastasize than basal cell carcinoma, but this fortunately does not happen often. "Squamous cells" are not confined to the skin so a person could also have squamous cell carcinoma of the lung, for example.
While the majority of the cells of the epidermis are keratinocytes and basal cells, there are other cell types that are fewer in number. Melanocytes are the skin cells that make melanin, which gives skin color (brown). When a group of melanocytes come together in the skin, they form what is known as a "melanocytic nevus", also known more commonly as a "mole." If the melanocytes of the mole are very abnormal and cancerous, then it is termed melanoma. When melanoma is diagnosed, the thickness is often mentioned and this is a measurement of how deep into the dermis these cancerous melanocytes penetrate. This is very important because, unlike the previously mentioned cancers, melanoma has a high tendency to become metastatic and spread to the brain or liver or other parts of the body.
Each different type of skin cell can unfortunately "go bad" and become cancerous, but I hope that now you can see that while they are all skin cancers, the different types stem from different cells of the skin. As a result, a cancer does not turn into another type of cancer. Also, it is not common to have two different cancers on the same site. However it is common for one person to have different types of skin cancer on different parts of the body. This is why prevention and yearly skin exams by your dermatologist are an important part of a healthy lifestyle.