Carcinoma in situ
Carcinoma in situ describes a phase in the course of carcinoma before active invasion commences. Eventually, such a lesion will begin to infiltrate locally.
On histopathological examination, the tumour has not penetrated the basement membrane but shows cellular atypia characteristic of malignancy:
- cells and their nuclei show a wide variation in size and shape - pleomorphism
- disproportionately large nuclear size relative to the volume of cytoplasm surrounding it
- a greater number of cells in a state of mitosis than would be expected given the tissue of origin; an increased number of mitotic figures and nucleoli
- cells are disorganised relative to their normal histological relationship, e.g. sheets or branching structures
As a general rule, the more differentiated a tumour, the less likely it is to infiltrate and invade.
A good example of carcinoma in situ is cervical intraepithelial neoplasia - CIN.
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