Squamous cell carcinoma in situ: An early stage of skin cancer. Also known as Bowen's disease.

This is a tumor that develops from the squamous cells which are flat, scalelike cells in the outer layer of the skin (the epithelium). The term "in situ" (borrowed from the Romans) means "in the natural or normal place" and, in the case of cancer, it says that the tumor cells are still confined to the site where they originated and they have neither invaded neighboring tissues nor metastasized afar.

The hallmark of squamous cell carcinoma in situ (Bowen's disease) is a persistent, progressive, slightly raised, red, scaly or crusted plaque. Bowen's disease may occur anywhere on the skin surface (or on mucosal surfaces such as the mouth).

Under the microscope, atypical squamous cells are seen to have proliferated through the whole thickness of the epidermis (the outer layer of the skin) but to have gone no farther.

The cause of Bowen's disease classically was prolonged exposure to arsenic. Today, Bowen's disease occurs most often in the sun-exposed areas of the skin in "older" white males.

Treatment options include freezing with liquid nitrogen, cauterization (burning), surgical removal, and chemosurgery.

Bowen's disease is named after the American dermatologist John Templeton Bowen (1857-1941).