The defining feature of colorectal adeno-carcinoma is invasion through the muscu-laris mucosae into the submucosa. Lesions with the morphological characteristics of adenocarcinoma that are confined to the epithelium or invade the lamina propria alone and lack invasion through the muscularis mucosae into the submucosa have virtually no risk of metastasis. Therefore, 'high-grade intraepithelial neoplasia'is a more appropriate term than 'adenocarcinoma in-situ', and 'intramucosal neoplasia' is more appropriate than 'intramucosal adenocarcinoma'. Use of these proposed terms helps to avoid overtreatment.

Most colorectal adenocarcinomas are gland-forming, with variability in the size and configuration of the glandular structures. In well and moderately differentiated adenocarcinomas, the epithelial cells are usually large and tall, and the gland lumina often contain cellular debris.

Mucinous adenocarcinoma

This designation is used if > 50% of the lesion is composed of mucin. This variant is characterized by pools of extracellular mucin that contain malignant epithelium as acinar structures, strips of cells or single cells. Many high-frequency micro-satellite instability (MSI-H) carcinomas are of this histopathological type.

Signet-ring cell carcinoma

This variant of adenocarcinoma is defined by the presence of > 50% of tumour cells with prominent intracytoplas-mic mucin {1672}.

The typical signet-ring cell has a large mucin vacuole that fills the cytoplasm and displaces the nucleus. Signet-ring cells can occur in the mucin pools of mucinous adenocarcinoma or in a diffusely infiltrative process with minimal extracellular mucin. Some MSI-H carcinomas are of this type.

Adenosquamous carcinoma

These unusual tumours show features of both squamous carcinoma and adeno-carcinoma, either as separate areas within the tumour or admixed. For a lesion to be classified as adenosquamous, there should be more than just occasional small foci of squamous differentiation. Pure squamous cell carcinoma is very rare in the large bowel.

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

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