Code 0: In Situ

  • Translates as “in place”
  • Can only be made microscopically (histologically confirmed)
  • Presence of malignant cells within cell group from which they arose
  • Fulfills pathological criteria for malignancy
  • No invasion of basement membrane
  • No invasion of supporting structures of the organ on which it arose

Pathologists have many ways of describing in situ cancer

  • Intracystic
  • Intraepithelial
  • No penetration of/or below the basement membrane
  • No stromal invasion
  • Non-infiltrating
  • Noninvasive
  • Pre-invasive

Organs and tissues that have no epithelial layer cannot be staged as in situ, since they do not have a basement membrane (e.g., sarcomas). The SS2018 manual lists the Chapters that cannot have in situ (see page 10).

Illustration showing in situ tumor with arrows identifying an In Situ cancer cell, epithelium of organ, and the basement membrane where there is no penetration of the basement membrane of the tissue and no stromal invasion.

Figure 1. In situ tumor.
Source: Adapted from an illustration by Brian Shellito of Scientific American, as printed in Cancer in Michigan, The Detroit News, Nov. 1-2, 1998.

In situ means “in place.” The technical definition of in situ is the presence of malignant cells within the cell group from which they arose. There is no penetration of the basement membrane of the tissue and no stromal invasion. Generally, a cancer begins in the rapidly dividing cells of the epithelium or lining of an organ and grows from the inside to the outside of the organ. An in situ cancer fulfills all pathologic criteria for malignancy except that it has not invaded the supporting structure of organ on which it arose.

Figure 2. In situ stage.
Source: Young JL Jr, Roffers SD, Ries LAG, Fritz AG, Hurlbut AA (eds). SEER Summary Staging Manual - 2000: Codes and Coding Instructions, National Cancer Institute, NIH Pub. No. 01-4969, Bethesda, MD, 2001.

Updated: November 5, 2024

Suggested Citation

SEER Training Modules: Code 0: In Situ. U.S. National Institutes of Health, National Cancer Institute. Cited 20 December 2025. Available from: https://training.seer.cancer.gov.