What is Carcinoma In Situ and What Does it Mean?

Carcinoma In Situ Stage, Microscopic Appearance, and Ability to Spread

pathology slide showing carcinoma in situ
What is carcinoma in situ and what does it mean for you?. Flickr.com/Ed Uthman/Creative Commons

Definition: Carcinoma In Situ

Carcinoma in situ is a term used to describe a cancer that is only present in the cells where it started, and has not spread to any nearby tissues. Carcinoma in situ is the earliest stage of a cancer, and is, at this stage, considered "non-invasive."

Microscopic Appearance

The diagnosis of carcinoma in situ must be made is under the microscope, where it defined by the presence of cancer cells.

 Cancer cells differ from normal cells in many ways, not just the ability to spread.     

How is Carcinoma In Situ Different Than "Full Blown" Carcinoma?

In contrast to carcinoma - or invasive cancer, carcinoma in situ has not yet invaded the basement membrane, and there is no stromal invasion. Other than this - the fact that the cells have not yet broken through the supporting structure from which they began - the cells appear the same as invasive cancer cells would appear under the microscope.  

What Does Carcinoma Mean?

In many ways, the term "carcinoma" is simply equated with cancer. Roughly 85 percent of cancers are carcinomas. Carcinomas are composed of epithelial cells - the type of cells that line the skin, breast ducts, and other surfaces of organs in the body. The subtypes of carcinomas include adenocarcinoma, squamous cell carcinoma, transitional cell carcinoma, and basal cell carcinoma.


Carcinoma in situ can be further defined by the tissue type in which cancer is beginning. For example, squamous cell carcinoma in situ of the cervix would represent a cancer that had started in squamous cells which line the cervix and have not yet become invasive.

Tumors such as sarcomas arise in tissues which do not have a basement membrane, so that for these types of cancer there is not a stage of carcinoma in situ.

Is Carcinoma In Situ (CIN) Cancer?

A million dollar question lately, especially with controversy over the treatment of ductal carcinoma in situ (DCIS) or in situ breast cancer, is whether or not carcinoma in situ is really cancer. The answer is that it depends on who you talk to. Some physicians classify carcinoma in situ as non-invasive cancer and others may prefer calling it pre-cancer. The distinction would lie in knowing whether or not the CIN would progress to invasive cancer, and that, by definition, is not known. Nor are we apt to get answers soon, as it wouldn't be ethical to simply watch large numbers of carcinoma in situ's to see if they became invasive cancer and spread.

Carcinoma In Situ vs Precancerous Cells vs Dysplasia

The many terms describing the spectrum between normal cells and invasive cancer cells. One of these is dysplasia. Dysplasia can run the spectrum from mild dysplasia in which the cells are barely abnormal appearing to carcinoma in situ, which some pathologists describe as severe dysplasia involving the full thickness of the epithelium.

Stage of Cancer with Carcinoma In Situ

A common question is, "What stage of cancer is carcinoma in situ?" Carcinoma in situ is referred to as stage 0 cancer.

At this stage, cancer is considered non invasive. Stage 1 cancers and beyond are considered invasive. Carcinoma in situ of the lungs would be considered stage 0 lung cancer.​ Other terms that may be used in defining the same thing as carcinoma in situ or stage 0 cancer include non-infiltrating, non-invasive, and intra-epithelial.

Will Carcinoma In Situ Become Cancer?

The answer to this question is maybe, but it can vary depending upon the tissue in which it is present. For many abnormalities classified as carcinoma in situ there is a serious risk that the cancer cells will become invasive - that is, full blown cancer.


Carcinoma in situ is usually removed via surgery, though the type of surgery will depend on the location, and even within particular locations there is significant controversy.

Also Known As: stage 0 cancer

Examples: After his bronchoscopy, Ted was told that he had carcinoma in situ of his lungs, the earliest stage of the disease.


National Cancer Institute. SEER Training Modules. In Situ (Stage = 0). Accessed 02/26/16. https://training.seer.cancer.gov/ss2k/staging/categories/insitu.html

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