Carcinoma is the most common category of skin cancer, accounting for the vast majority of skin cancer diagnoses. About 5.4 million basal cell and squamous cell carcinomas are diagnosed each year in the United States alone, occurring in roughly 3.3 million people. Unlike melanoma, which starts in pigment-producing cells, carcinomas begin in the outer layer of skin called the epidermis. They are highly treatable when caught early, but the two main types behave quite differently.
Basal Cell vs. Squamous Cell Carcinoma
The word “carcinoma” simply means a cancer that starts in the cells lining surfaces of the body. In the skin, that means the epidermis. The two common types are named after the specific cells where they originate.
Basal cell carcinoma (BCC) starts in the deepest part of the epidermis, in small round cells that continually produce new skin cells. It makes up about 8 out of every 10 skin carcinomas, making it the single most common cancer in humans. BCC grows slowly and almost never spreads to other parts of the body. That said, it can be locally destructive, growing into nearby tissue, cartilage, or bone if left untreated for years.
Squamous cell carcinoma (SCC) arises in the flat cells that form the upper and middle layers of the epidermis. It is less common than BCC but more aggressive. Metastasis is still rare, occurring in roughly 3 to 9 percent of cases, typically one to two years after the initial diagnosis. When SCC does spread, it most often reaches the lymph nodes near the original tumor: about 74 percent of metastases are found in nearby lymph nodes. Distant spread to organs is uncommon.
What They Look Like
Both types tend to appear on sun-exposed skin, especially the face, head, neck, and arms, though they can develop anywhere. Recognizing them early is the single most important factor in treatment success.
Basal cell carcinomas often show up as small, pink or red bumps that may look shiny or slightly translucent with raised edges. They are often fragile and bleed easily after a minor injury or shaving. A sore that won’t heal within a week or two is a classic warning sign. In people with darker skin tones, BCCs can share these same features but appear darker in color.
Squamous cell carcinomas tend to look rougher. Typical signs include scaly red patches that may crust or bleed, raised lumps sometimes with a depressed center, open sores that ooze or crust and don’t heal (or heal and return), and wart-like growths. Both types can also appear as a flat area that looks only slightly different from normal skin, which makes them easy to miss.
How UV Damage Leads to Carcinoma
Ultraviolet radiation from sunlight is the primary driver of skin carcinomas. UV rays damage DNA in skin cells, and the body’s repair systems usually fix these errors. But in slowly dividing cells like those in human skin, a specific type of damage accumulates over time. UV light causes chemical bonds to form between neighboring DNA bases, creating structures called pyrimidine dimers. Within these damaged sites, a chemical building block of DNA called cytosine becomes unstable and undergoes a reaction with water, converting it to a different molecule. When the cell copies its DNA during division, this altered molecule is misread, locking in a permanent mutation.
This process is especially significant in skin cells because they divide slowly, giving the chemical conversion more time to occur before repair enzymes can intervene. Over years and decades of sun exposure, these mutations accumulate in genes that control cell growth, eventually pushing a cell toward uncontrolled division.
Risk Factors Beyond Sunlight
While UV exposure is the dominant cause, several other factors raise the risk of developing skin carcinomas.
- Immunosuppression: Organ transplant recipients who take anti-rejection medications face dramatically higher risk. For squamous cell carcinoma specifically, their risk is 65 to 100 times greater than the general population. Other immunosuppressive medications used for autoimmune conditions can also increase risk.
- Chemical exposure: Arsenic, found in some contaminated water supplies and industrial settings, acts as a co-carcinogen by amplifying the DNA damage caused by UV radiation. Polycyclic aromatic hydrocarbons from vehicle exhaust, industrial processes, and cigarette smoke also play a role.
- Smoking: Cigarette smoking has been linked to higher rates of squamous cell carcinoma, particularly on the lips, ears, and genital areas.
- Certain medications: Hydrochlorothiazide, a common blood pressure medication, has been associated with increased SCC risk. Methotrexate, used for autoimmune conditions, raises the risk of both melanoma and non-melanoma skin cancers through its immunosuppressive and sun-sensitizing effects.
Merkel Cell Carcinoma: A Rarer Type
Beyond BCC and SCC, a much rarer skin carcinoma called Merkel cell carcinoma (MCC) develops from specialized nerve-related cells in the skin. It is far more dangerous than the common types, spreading early to lymph nodes and carrying a high mortality rate. Its incidence has been rising, partly because the population is aging and more people are taking immunosuppressive therapies. MCC is uncommon enough that most people will never encounter it, but it underscores why any new, rapidly growing skin bump warrants attention.
How Skin Carcinomas Are Treated
Treatment depends on the type, size, location, and whether the cancer has come back after a prior treatment. Most skin carcinomas are treated with some form of surgery, and the prognosis is excellent.
Mohs micrographic surgery offers the highest cure rates available. A surgeon removes thin layers of tissue and examines each layer under a microscope during the procedure, continuing until no cancer cells remain. For new basal cell carcinomas, cure rates reach up to 99 percent. For new squamous cell carcinomas, they range from 95 to 99 percent. Even cancers that have recurred after previous treatment respond well, with cure rates of about 94 percent for BCC and 90 percent for SCC. Mohs surgery is especially useful for cancers on the face, where preserving as much healthy tissue as possible matters most.
Standard surgical excision, where the cancer and a margin of surrounding tissue are removed in one piece, is another common approach and works well for less complex cases. For very superficial basal cell carcinomas, a prescription cream that stimulates the immune system to attack abnormal cells can be applied at home over several weeks. This topical approach is limited to early, surface-level tumors and is not appropriate for deeper or more aggressive cancers.
Reducing Your Risk
Most skin carcinomas are preventable. UV protection is the foundation. Use a broad-spectrum sunscreen with an SPF of 15 or higher, applied 15 minutes before going outside and reapplied at least every two hours. If you’re swimming or sweating, reapply more frequently. Sunscreen works best as one layer of a broader strategy that includes seeking shade during peak sun hours, wearing hats, and covering exposed skin with clothing.
Regular skin self-checks matter too. Because both BCC and SCC can appear as subtle flat patches or slow-growing bumps, paying attention to any spot that changes, bleeds without clear cause, or won’t heal is the most reliable way to catch a carcinoma early, when treatment is simplest and cure rates are highest.