Occasionally, cells of the upper epidermis will undergo vacuolization, demonstrating an abundant and strongly eosinophilic cytoplasm. There may be a mild to moderate lymphohistiocytic infiltrate detected in the upper dermis. In situ disease
Bowens disease is essentially equivalent to and used interchangeably with SCC in situ, when not having invaded through the basement membrane. Depending on source, it is classified as precancerous or SCC in situ (technically cancerous but non-invasive). In SCC in situ (Bowens disease), atypical squamous cells proliferate through the whole thickness of the epidermis. The entire tumor is confined to the epidermis and does not invade into the dermis. The cells are often highly atypical under the microscope, and may in fact look more unusual than the cells of some invasive squamous cell carcinomas. Erythroplasia of Queyrat is a particular type of Bowens disease that can arise on the glans or prepuce in males,: 733 : 656  and the vulvae in females. It mainly occurs in uncircumcised males, over the age of 40. It is named for French dermatologist Louis Queyrat (1856–1933),  who was head of the dermatology service of lHôpital Ricord, a venereal hospital in Paris, now Hôpital Cochin. Invasive disease
In invasive SCC, tumor cells infiltrate through the basement membrane. The infiltrate can be somewhat difficult to detect in the early stages of invasion: however, additional indicators such as full thickness epidermal atypia and the involvement of hair follicles can be used to facilitate the diagnosis.
Once keratinocytes begin to grow uncontrollably, they have the potential to become cancerous and produce squamous cell carcinoma. The lesion caused by SCC is often asymptomatic
Ulcer or reddish skin plaque that is slow growing
Intermittent bleeding from the tumor, especially on the lip
The clinical appearance is highly variable
Usually the tumor presents as an ulcerated lesion with hard, raised edges
The tumor may be in the form of a hard plaque or a papule, often with an opalescent quality, with tiny blood vessels
The tumor can lie below the level of the surrounding skin, and eventually ulcerates and invades the underlying tissue
The tumor commonly presents on sun-exposed areas (e.g. back of the hand, scalp, lip, and superior surface of pinna)
On the lip, the tumor forms a small ulcer, which fails to heal and bleeds intermittently
Evidence of chronic skin photodamage, as in multiple actinic keratoses (solar keratoses)
The tumor grows relatively slowly

Spread
Unlike basal-cell carcinoma (BCC), squamous cell carcinoma (SCC) has a higher risk of metastasis. Risk of metastasis is higher clinically in SCC arising in scars, on the lower lips or mucosa, and occurring in immunosuppressed and solid organ transplant patients. Risk of metastasis is also higher in SCC that are >2 cm in diameter, growth into the fat layer and along nerves, thickness >6mm. Causes
Squamous cell carcinoma is the second-most common cancer of the skin (after basal-cell carcinoma but more common than melanoma). It usually occurs in areas exposed to the sun.