An adenoma (at least in the tubular-to-villous family) is defined as a polyp with low-grade dysplasia. Low-grade dysplasia in the colon indicates a cytologic change and stands out from the normal colon as looking blue on the slide. The cells lining the crypts and the surface become tall and dark (because of depleted mucin) and have cigar-shaped and/or pseudostratified hyperchromatic nuclei (Figure 8.2). Mitoses may be present but are generally confined to the base of the crypts.

The dysplasia must extend all the way to the surface epithelium to qualify as an adenoma ; if the epithelium shows signs of maturing (progressing to more normal-appearing nuclei) as it ascends, it is more likely to be reactive changes.

Adenomas are subdivided by their architecture. The most common type is the tubular adenoma , which has a smooth surface and parallel crypts, similar to normal epithelium. A villous adenoma is covered in fingerlike projections, whereas a tubulovillous adenoma has features of both. These are soft and subjective distinctions, in practice.

Important considerations for sign-out include the following:

Hyperplastic polyps are those in which the epithelial cells, although benign, begin to outgrow their available space. The glands have an increased number of goblet cells and therefore look pale or cleared out next to normal epithelium, and there tends to be a thickened or “sclerotic-appearing” basement membrane under the neoplastic cells. Furthermore, because the surface area is outgrowing the lamina propria, hyperplastic polyps take on a distinctive frilly (like a skirt) or lacy appearance (Figure 8.5). An imaginary cast of the inside of the crypts might look like a flaring fluted vase, narrow at the base. Crypts cut in cross section have a distinctive star-shaped lumen, and this architecture is called “serrated.”

A polyp with adenomatous-looking cells at the base of the crypts, and frilly hyperplastic cells at the surface, is still a hyperplastic polyp. Remember that surface maturation is not consistent with a tubular adenoma . However, a true adenoma with a serrated surface profile may be in the serrated adenoma category, below.

Historically, a “serrated adenoma ” was a polyp with low-grade dysplasia extending to the surface, as seen in tubular adenoma , but with serrated architecture. Now a subset of what were once lumped in with hyperplastic polyps, especially in the right colon, are recognized as a distinct type of polyp with malignant potential, associated with the microsatellite instability (MSI) cancer pathway. These polyps, often broad and sessile, are called either sessile serrated adenomas (SSA ) or sessile serrated polyps (SSP ). The crypts have characteristic widening and horizontal branching at the base (“duck feet”), and the epithelial cells may be more eosinophilic (less mucin) and pseudostratified than the usual hyperplastic polyp (Figure 8.6). In addition, the thickened basement membrane seen near the surface in hyperplastic polyps is absent in SSAs. However, mature goblet cells and the serrated profile are still evident.