Carcinoma in situ is present when a tumor has been detected that has the characteristics of malignancy but has not invaded other tissues. Many forms of cancer are initially carcinomas in situ, but are not detected early enough.

Many clinicians would not refer to carcinoma in situ as "cancer" when relating results to a patient. Conversely, although many are treated much the same way as a proven malignancy, there may be important differences in the therapeutical management.

Examples of carcinomas in situ are: -

• Many bladder cancers are CISes.
• Cervical cancer is often predated by cervical intraepithelial neoplasia (CIN).
• Similarly, vaginal tumors are VaIN (vaginal intraepithelial neoplasia) when they are still at in situ stage; vulval tumors begin as VIN (vulval intraepithelial neoplasia).