Squamous cell carcinoma in situ is an early form of skin cancer where abnormal cells are found only in the outermost layer of skin (the epidermis) and have not spread deeper. It is also called Bowen disease. Because the abnormal cells haven’t broken through the basement membrane, the boundary between the epidermis and the tissue beneath it, this is considered a pre-invasive cancer that is highly treatable.
How It Differs From Invasive Skin Cancer
Your skin is built in layers. The outermost layer, the epidermis, sits on top of the dermis, separated by a thin structural boundary called the basal layer. In squamous cell carcinoma in situ, abnormal squamous cells (the flat cells that make up most of the epidermis) fill the entire thickness of the epidermis but stop at that boundary. Under a microscope, pathologists often see what’s called the “eyeliner sign,” where the basal layer remains visibly intact.
Invasive squamous cell carcinoma, by contrast, has broken through that boundary and penetrated into the dermis or deeper. That distinction matters enormously. Once cancer cells reach the dermis, they gain access to blood vessels and lymphatic channels, which creates a potential path for the cancer to spread. In situ disease has none of that risk, which is why treatment outcomes are so favorable.
What It Looks Like
A typical lesion appears as a slowly enlarging reddish, scaly patch or plaque on the skin. The borders are usually well defined, and the surface can look rough or crusty. Lesions are often flat or only slightly raised. Most patches range from a few millimeters to a couple of centimeters across, though they can grow larger if left alone for years. Occasionally, a lesion takes on a warty or bumpy appearance rather than the classic flat patch.
These spots show up most often on sun-exposed areas: the lower legs (especially in women), forearms, hands, head, and neck. They can easily be mistaken for eczema, psoriasis, or a fungal infection because of their scaly, red appearance, which is one reason diagnosis sometimes takes a while.
Causes and Risk Factors
Cumulative sun exposure is the primary driver. Years of ultraviolet radiation damage the DNA in skin cells, eventually causing them to grow abnormally. People in high-exposure occupations like farming, construction, landscaping, fishing, and military service face elevated risk.
Other established risk factors include fair skin, a history of sunburns, prior radiation therapy to the skin, a weakened immune system (from organ transplant medications or other causes), and exposure to arsenic. Certain strains of human papillomavirus (HPV) are also linked to squamous cell carcinoma in situ, particularly on the genitals and around the nails.
How It Relates to Actinic Keratosis
Actinic keratosis is another UV-driven skin change that involves abnormal cells in the epidermis, and many people wonder where the line falls between the two. The key difference is depth of involvement within the epidermis. In actinic keratosis, the abnormal cells occupy only part of the epidermal thickness. In squamous cell carcinoma in situ, atypical cells span the full thickness. Pathologists can further classify in situ lesions into subtypes, including a “hypertrophic actinic keratosis” type that sits at the borderline between the two diagnoses. This is why a biopsy, not just a visual exam, is necessary to distinguish them.
How It Is Diagnosed
Diagnosis requires a skin biopsy. Your dermatologist will remove a small piece of the suspicious patch (or sometimes the entire lesion) and send it to a pathologist. Under the microscope, the pathologist looks for full-thickness atypia, meaning the abnormal cells extend from the bottom of the epidermis to the top, while the basement membrane below remains intact. If those criteria are met, the diagnosis is squamous cell carcinoma in situ, classified as stage 0 (pTis) in the pathological staging system.
Risk of Becoming Invasive
Left untreated, squamous cell carcinoma in situ progresses to invasive squamous cell carcinoma in roughly 3 to 10 percent of cases. That percentage may sound low, but it means that about 1 in 10 untreated lesions could eventually become a cancer capable of spreading. Since treatment at the in situ stage is straightforward and highly effective, there is little reason to watch and wait.
Treatment Options
Several effective treatments exist, and the best choice depends on the size and location of the lesion, your overall health, and whether you’ve had prior skin cancers.
Surgical Removal
Standard surgical excision involves cutting out the lesion along with a margin of normal-looking skin. Five-year recurrence rates after excision are about 3.5 percent. Mohs micrographic surgery, where the surgeon removes tissue in thin layers and examines each one under a microscope before proceeding, has the lowest recurrence rate at roughly 2.1 percent over five years. Mohs is particularly useful for lesions on the face or other areas where preserving healthy tissue matters.
Electrodessication and curettage (ED&C), a technique where the lesion is scraped away and the base is cauterized, is a quicker in-office procedure. Its five-year recurrence rate is higher at about 4.9 percent, making it a reasonable option for smaller, low-risk lesions on the trunk or limbs.
Topical Treatments
For patients who prefer to avoid surgery or have lesions in locations where surgery is difficult, topical creams can be applied at home over several weeks. Imiquimod, a cream that stimulates the immune system to attack abnormal cells, has clearance rates between 73 and 88 percent for in situ lesions. Fluorouracil, a topical chemotherapy cream, has more variable results with clearance rates ranging from 27 to 85 percent depending on the treatment regimen. Both are generally recommended for small tumors in low-risk locations rather than as first-line treatment for every case.
Photodynamic Therapy
Photodynamic therapy (PDT) uses a light-sensitizing solution applied to the skin, followed by exposure to a specific wavelength of light. The combination selectively destroys abnormal cells. It works well for larger or multiple lesions and tends to produce good cosmetic results. Effectiveness can vary depending on the lesion’s location, size, and how long the sensitizing agent is left on before light exposure.
Follow-Up After Treatment
After treatment for an in situ lesion, ongoing skin surveillance is important because having one such lesion increases the likelihood of developing others. The National Comprehensive Cancer Network recommends a physical exam and skin check at least once a year for life for stage 0 skin cancers. Monthly self-exams at home, checking for new or changing spots, are also part of long-term management. Your dermatologist may want to see you more frequently in the first year or two, especially if you have additional risk factors like a history of multiple skin cancers or immune suppression.