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Carcinoma in situ

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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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