Carcinomain situ refers to a group of abnormal cells that have not spread from the location where they first formed, although they may later spread into normal tissue and become cancer.

Carcinoma in situ, also called in situ cancer, is different from invasive carcinoma, which has spread to surrounding tissue, and frommetastatic carcinoma, which has spread throughout the body to other tissues and organs. In general, carcinoma in situ is the earliest form of cancer, and is considered stage 0.

An example of carcinoma in situ is ductal carcinoma in situ, or DCIS, which is considered anearly form of breast cancerand occurs when abnormal cells form a breast’s milk duct. According to theAmerican Cancer Society, an estimated one in five new breast cancers each year will be diagnosed as DCIS.