Fig. 6.13 A Tubulovillous adenoma showing invasive adenocarcinoma within the core of the polyp. B Adenocarcinoma arising in a villous adenoma.

Fig. 6.14 Villous adenoma of rectum and invasive adenocarcinoma. Two of four lymph nodes in perirectal tissue have metastasis.

Fig. 6.15 Adenocarcinoma within lymphatic vessel.
Fig. 6.16 Metastatic adenocarcinoma in regional lymph node.

Medullary carcinoma

This rare variant is characterized by sheets of malignant cells with vesicular nuclei, prominent nucleoli and abundant pink cytoplasm exhibiting prominent infiltration by intraepithelial lymphocytes {856}. It is invariably associated with MSI-H and has a favourable prognosis when compared to other poorly differen tiated and undifferentiated colorectal carcinomas.

Undifferentiated carcinoma

These rare tumours lack morphological evidence of differentiation beyond that of an epithelial tumour and have variable histological features {1946}. Despite their undifferentiated appearances, these tumours are genetically distinct and typically associated with MSI-H.

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