Squamous cell carcinoma in situ

by Jason Wasserman MD PhD FRCPC
May 29, 2023

What is squamous cell carcinoma in situ?

Squamous cell carcinoma in situ is a non-invasive type of cancer made up of squamous cells. It is called non-invasive because the abnormal cells have not spread into any of the surrounding tissues. If left untreated, this condition can turn into a type of invasive cancer called squamous cell carcinoma.

Where in the body does squamous cell carcinoma in situ develop?

Squamous cell carcinoma in situ can develop anywhere in the body where squamous cells are normally found including the skin, oral cavity, esophagus, and lungs. This condition can also develop in areas of the body where squamous cells are not normally found but where they arise after periods of long-standing inflammation (for example in the bladder).

Where does squamous cell carcinoma in situ start?

For skin tumours, squamous cell carcinoma in situ starts in a thin layer of tissue called the epidermis which is the outermost layer of the skin. For tumours in other parts of the body such as the oral cavity, esophagus, or lungs, squamous cell carcinoma in situ starts in a thin layer of tissue called the epithelium which covers the inside surface of these organs.

What causes squamous cell carcinoma in situ?

The cause of squamous cell carcinoma in situ depends on where the tumour starts. In the skin, most tumours are associated with long-term and excessive exposure to UV radiation, typically from the sun. Tumours in the oral cavity, larynx, esophagus, and lungs are associated with tobacco smoke. Immune suppression related to medications or immune disorders increases the risk of developing squamous cell carcinoma in situ in both the skin and internal sites.

Is squamous cell carcinoma in situ benign or malignant?

Squamous cell carcinoma in situ is made up of malignant cells, however, because the tumour is non-invasive, the cells are unable to spread to other parts of the body. For this reason, treated early, squamous cell carcinoma in situ behaves like a benign disease.

What is the difference between squamous cell carcinoma in situ and squamous cell carcinoma?

These two conditions are very similar and squamous cell carcinoma in situ can be thought of as an early stage of squamous cell carcinoma. Squamous cell carcinoma in situ is a non-invasive type of cancer which means the tumour cells are only found in the epidermis or epithelium and have not yet spread into the surrounding tissues. In contrast, squamous cell carcinoma is an invasive type of cancer which means that the tumour cells have spread beyond the epidermis or epithelium into the surrounding tissues.

How is the diagnosis of squamous cell carcinoma in situ made?

The diagnosis of squamous cell carcinoma in situ can only be made after a sample of the tumour is examined under the microscope by a pathologist.

What does squamous cell carcinoma in situ look like under the microscope?

When examined under the microscope, squamous cell carcinoma in situ is made up of large abnormal-looking squamous cells that replace the healthy squamous cells normally found in the epidermis (for skin tumours) or the epithelium (for tumours in other parts of the body). The tumour cells often have large atypical (abnormal) shaped nuclei. Mitotic figures (tumour cells dividing to create new tumour cells) are also commonly seen.