**3. Histology**

Carcinoma "in situ", intramucosal carcinoma, high displasia or intraepithelial carcinoma is the stage at which there is no involvement of *the muscularis mucosa.* In general, this tumour stage does not cause metastasis. It is classified as pTis or Stage 0 in the TNM staging system.These terms are defined as non-invasive high grade neoplasia in the Vienna classification [12].Carci‐ noma in situ or severe displasia or intraepithelial carcinoma corresponds to a carcinoma that is restricted to the epithelial layer without invasion into the lamina propria. Intramucosal car‐ cinoma is a carcinoma characterized by the invasion into the lamina propria.

When the carcinoma spreads to the submucosa, the polyp is considered to have become ma‐ lignant, being able to spread to lymph nodes or distant sites. The tumours that affect the submucosa are classified as T1 and correspond to Stage I of the TNM staging system. This term is defined as submucosal carcinoma in the classification of Vienna [12].

The term pseudoinvasion refers to the presence of glandular epithelium of the mucosa be‐ neath the muscularis mucosa in colonic polyps. These lesions have no malignant potential and should be management in a similar way to adenomas [13]. However, an inexperienced pathologist can mistake this phenomenon for invasive carcinoma. Pseudoinvasion usually occurs in large polyps (>1 cm), especially those with long stalks, and is most commonly found in polyps of the sigmoid colon. Islands of adenomatous epithelium are displaced through the muscularis mucosa and are found within the submucosa of the stalk. The dis‐ placed glandular tissue usually has rounded not infiltrative, contours, carries with it a small amount of lamina propria, and is cytologically identical to the overlying adenomatous com‐ ponent. Hemorrhage and hemosiderin depositions, are commonly seen and are a clue to di‐ agnosis. In addition, inflammation and granulation tissue, can be found.Cystic dilatation of the displaced glands with mucin distention is also not uncommon in pseudoinvasion be‐ cause mucin produced by the entrapped glands has no means of reaching the lumen. Occa‐ sionally, rupture of dilated glands occurs with acellularmucin extravasation and there is a subsequent inflammatory response. Distinction from mucinous (colloid) carcinoma is im‐ portant and can be difficult. Specifically, in mucinous carcinoma, the mucin pools contain malignant cells, a feature lacking in pseudoinvasion.

For these reasons, it is highly recommended that level sections and second opinions, are ob‐ tained in cases of polyps with potential pseudoinvasion [14].

All adenomas have some degree of dysplasia. However, low and high grade dysplasias are artificial subdivisions of a spectrum. There is no definition of "high-grade". Indeed, the WHO book on tumors of the digestive system, does not contain a list of criteria for highgrade dysplasia in adenomas [15,16]. However in general, high-grade dysplasia entails more substantial changes and includes carcinoma "in situ". Among these changes we consider ar‐ chitectural alteration, often resembling the glandular arrangement of adenomas and cyto‐ logic abnormalities, principally cellular and nuclear pleomorphism, nuclear hyperchromatism, loss of nuclear polarity, and marked stratification of nuclei. Other au‐ thors have considered as features of high grade displasia: loss of normal glandular architec‐ ture, hyperchromatic cells with multilayered irregular nuclei and loss of mucin, high nuclear/cytoplasmic ratio, marked nuclear atypia with prominent nuclei and focal cribri‐ form patterns. Not all these features are necessarily present to the same degree in all dys‐ plastic epithelia, while low-grade dysplasia manifests these same changes but to a lesser degree [15,16].
