Definition:
The presence of malignant cells within the cell group from
which they arose; no penetration of basement membrane of the
tissue; no stromal invasion.
In situ means "in place" It applies to epithelial tissue
only (no such thing as "sarcoma in situ") The tumor must be
microscopically diagnosed in order to visualize the basement
membrane. If the tumor is shown to be microinvasive, case
is considered localized.
Synonyms:
non- or pre-invasive, non-infiltrating, intra-epithelial,
Stage 0, intraductal, intracystic, no stromal invasion, no
penetration below the basement membrane.
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