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:
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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