Bowen's disease (squamous cell carcinoma in situ)

What it is. Bowen's disease is squamous cell carcinoma in situ of the skin. The keratinocytes of the epidermis have become malignant across its full thickness, but the basement membrane beneath them is intact, so there is nothing for the cancer to spread through: no blood vessels and no lymphatics reach into the epidermis. Cancer Research UK describes it as pre-invasive and as a very early form of non-melanoma skin cancer that grows very slowly. In the staging system UK reports use it is the pTis category, which is stage 0.

How it differs from its parent. Invasive cutaneous squamous cell carcinoma has broken through the basement membrane into the dermis and can therefore reach lymph nodes; Bowen's disease has not and cannot. That single structural difference is the whole of the difference in outlook, and it is why the treatments are different: a lesion that cannot spread can be treated with something that destroys the epidermis, such as a cream, cryotherapy or photodynamic therapy, where an invasive cancer needs the tissue beneath it removed and examined. It differs from actinic keratosis in the other direction: an actinic keratosis is dysplasia in sun-damaged skin and is not a neoplasm at all in ICD-10, while Bowen's disease is a carcinoma, coded in the neoplasm chapter.

How it looks and where. Usually red scaly patches; the skin may look red and sore, and as the patches enlarge they can become crusty or turn into an open sore. It most often appears on the head, neck, arms, trunk and the legs, particularly the lower legs, although it can occur anywhere. The lower leg in an older woman is the classic site and also the most awkward, because skin there heals slowly, which is one of the reasons treatment is often chosen to avoid surgery.

How it is treated and what happens next. Cancer Research UK lists topical fluorouracil or imiquimod, cryotherapy, curettage and cautery, surgical excision, Mohs surgery, photodynamic therapy, radiotherapy and laser, and says that all of these can work well, with high cure rates. It has a UK national guideline of its own, the British Association of Dermatologists guidelines for the management of people with cutaneous squamous cell carcinoma in situ, published in 2022, whose first line of definition is that Bowen disease is a form of intraepidermal, in situ, squamous cell carcinoma, originally described in 1912.

How often it becomes invasive, and why two very different numbers are both right. The figure most often quoted is per lesion: the earlier British guideline stated that most studies suggest a risk of invasive carcinoma of about 3 to 5 percent for typical squamous cell carcinoma in situ, and about 10 percent for erythroplasia of Queyrat, the form on the glans penis (Morton 2014); Cancer Research UK gives the same 3 to 5 in 100 and says that most Bowen's disease does not progress. The figure the 2022 guideline cites instead is per person and counts a cancer anywhere on the body: in a Dutch national cohort of 88,754 people with squamous cell carcinoma in situ, the cumulative risk of developing an invasive squamous cell carcinoma at any site within 5 years was 11.7 percent in men and 6.9 percent in women, and the excess over the general population was highest in the first year of follow-up (Tokez 2020). Those two statements are not in conflict. The patch on the leg is very unlikely to turn into a cancer; the person who grew that patch has skin that is likely to grow a cancer somewhere.