In a medical position statement on the management of Barrett’s esophagus by the American Gastroenterological Association, Barrett’s esophagus is defined as the condition in which any extent of metaplastic columnar epithelium that predisposes to cancer development replaces the stratified squamous epithelium that normally lines the distal esophagus; presently, intestinal metaplasia is required for the diagnosis of Barrett’s esophagus as it is the only type of esophageal columnar epithelium that predisposes to malignancy [11]. However, not all professional organizations (including the British Society of Gastroenterology) require the histologic documentation of intestinal metaplasia [12].

Macroscopically, Barrett’s esophagus appears as a well-demarcated area of erythematous or “velvety” mucosa within the squamous-lined tubular esophagus. Histologically, three different types of columnar or glandular metaplasia can be seen on biopsies from patients with “Barrett’s” esophagus: (1) gastric cardia-type mucosa (composed of mucin-secreting glands); (2) gastric oxyntic (or fundus)-type mucosa composed of parietal cells, chief cells, and mucus-secreting cells; and (3) specialized columnar epithelium or intestinal metaplasia containing round, bluish, barrel-shaped cells called goblet cells, often admixed with gastric cardia-type glands (Fig. 3.2). From the pathologist’s perspective, the gastric-type mucosa ± goblet cells seen in the tubular esophagus is indistinguishable from the true gastric mucosa seen in the cardia. Hence, if the pathologist does not see submucosal glands on a biopsy and does not know what the endoscopist observed at the time of biopsy, a diagnosis of “gastric-type mucosa with intestinal metaplasia” may be rendered instead of a diagnosis of “Barrett’s esophagus.” Goblet cells produce acid-rich mucins and thus stain intensely blue with an Alcian blue stain at a pH of 2.5. In addition, the intestinal mucosa stains positive for immunohistochemical stains such as CDX2, villin, and MUC-2 (markers of intestinal differentiation) [13–16]. Although ancillary stains can detect goblet cells and intestinalized epithelium, these stains are not required for a diagnosis of intestinal metaplasia, as the goblet cells can often be readily identified on the routine hematoxylin and eosin (H&E) stain (Fig. 3.2).

It is worth noting that in addition to designating the presence and/or absence of intestinal metaplasia, pathologic reports may detail several other histologic mimics of metaplasia. “Pseudogoblet cells” refer to barrel-shaped gastric foveolar cells that may look like goblet cells on low magnification, but have an eosinophilic tinge and do not stain for Alcian blue due to a production of neutral mucins. The term “columnar blues” refers to the identification of mucus cells that contain bluish mucin on the H&E stain and stain positive for Alcian blue but lack goblet cell morphology. The term “multilayered epithelium” may be noted in reports. This finding refers to the identification of an epithelium that contains flattened squamous-appearing cells in the basal layers with an overlying columnar mucus cell layer. Studies have shown that this epithelium can show immunohistochemical features similar to intestinal-type epithelium, indicating that it may represent an early or intermediate phase in the development of intestinal metaplasia and Barrett’s esophagus [17]. Finally, pathologists may report the presence of “subsquamous intestinal metaplasia” (also called “buried Barrett’s” or “squamous overgrowth”), particularly in endoscopic mucosal resection specimens. Although subsquamous intestinal metaplasia is present in the majority of endoscopic mucosal resection specimens and is no longer felt to be a post-ablative phenomenon, the long-term clinical significance is uncertain, and histologic evaluation of squamous-lined mucosa adjacent to areas of BE may be indicated [18, 19].

Once a diagnosis of Barrett’s esophagus has been rendered, increased surveillance is indicated in order to monitor for the presence of dysplasia and/or adenocarcinoma. Studies have shown that 0.12–0.38 % of patients with nondysplastic Barrett’s esophagus will develop adenocarcinoma each year [20]; thus, screening efforts have focused largely on identifying metaplasia in at-risk individuals. Definitive dysplastic change falls into one of two general categories: low-grade or high-grade dysplasia. Features of low-grade dysplasia (Fig. 3.3a) include increased epithelial proliferation characterized by nuclear crowding and minimal distortion of glandular architecture. Mild cytologic atypia including nuclear enlargement, hyperchromasia, stratification, and irregular nuclear contours is also observed. Nuclear-to-cytoplasmic ratios and mitotic counts remain low, basal nuclear polarity is preserved, and in general, the glandular architecture is retained, though there may be increased glandular crowding. Low-grade dysplasia in Barrett’s esophagus often resembles the epithelium of a colonic-type adenoma. For a diagnosis of low-grade dysplasia, the cytologic atypia should extend to the surface. When surface involvement is not present or cannot be evaluated (e.g., due to squamous overgrowth), the term “indefinite for dysplasia” may be used.

In contrast to low-grade dysplasia, high-grade dysplasia (Fig. 3.3b) is characterized by increased cellular atypia and complex architecture. Cells have high nuclear-to-cytoplasmic ratios, and there is a loss of basal nuclear polarity with the large often rounded nuclei containing prominent nucleoli. There is also distortion of the normal glandular architecture, often with intraglandular bridging or cribriforming. Cellular or apoptotic debris may be found within glandular lumens. Mitotic activity is evident and often extends beyond the deep proliferative zone and may reach the luminal surface. Deep glands with dilated lumens lined by attenuated epithelium may also indicate the presence of high-grade dysplasia or a lesion of greater clinical significance (Fig. 3.4). In the 7th ed of the AJCC staging manual, the term “Tis” (or “carcinoma in situ”) was removed from all epithelial neoplasia of the gastrointestinal tract and was replaced by “high-grade dysplasia” [21]. Since there are some cases of intraepithelial neoplasia that are more atypical than expected for high-grade dysplasia, or cases where definitive invasion cannot be documented, some pathologists may use the term “carcinoma in situ” or “at least high-grade dysplasia.” If these terms are noted in the pathology report without an explanatory comment, a discussion with the pathologist may help clarify its meaning in individual cases.

Once adenocarcinoma develops, it can be managed by different modalities. Small early lesions may be amenable to endoscopic mucosal resection. Surgery ± neoadjuvant therapy is the treatment of choice for more deeply invasive or more advanced lesions. Grossly, esophageal adenocarcinoma often appears as an infiltrative mass in the distal third of the esophagus, although fungating, polypoid, and flat growths can be seen (Fig. 3.5a). Histologically, adenocarcinoma is characterized by invasion beyond the basement membrane. Invasion beyond the basement membrane into the lamina propria of the esophagus is a significant development, as it allows access to lymphatic channels that are not found in other areas of the alimentary track, such as the colon. Hence, as per the AJCC staging recommendations, early esophageal adenocarcinoma is staged as either T1a (intramucosal invasion) or T1b (submucosal invasion) [21].

In pathology, most early adenocarcinomas are evaluated on either small biopsies or endoscopic mucosal resection (EMR) specimens. Determining the depth of invasion can be challenging on such specimens because of a phenomenon known as duplication of the muscularis mucosae. For unknown mechanistic reasons, in areas of intestinal metaplasia (Barrett’s esophagus), a new internal layer of the muscularis mucosa is created [22, 23] (Fig. 3.5b). In order to become a T1b lesion, an adenocarcinoma arising in these areas needs to invade through three layers: the “new” superficial muscularis mucosae, the loose connective tissue between the two layers, and then the deep or true muscularis mucosae. Hence, true submucosal invasion is difficult to determine on small biopsies or superficial EMR specimens.

Since EMR specimens are small but contain pertinent information, recommendations for the handling of such specimens have been reported [24]. Ideally, fresh specimens should be pinned to cork or foam prior to formalin fixation. Photographic documentation is recommended. Then the entire specimen should be submitted for histologic evaluation. In addition to reporting the main pathology (dysplasia or intramucosal carcinoma), pathologists must comment on the status of the lateral (mucosal) and deep margins. If an adenocarcinoma is present, the depth of invasion must be assessed. As noted above, the pitfall of overcalling a T1a lesion as a T1b tumor due to duplicated muscularis mucosae must be avoided. The AJCC further subdivides both intramucosal (m1, m2, and m3) and submucosal (sm1, sm2, sm3) invasion [25]. Although the AJCC does not take into account the different layers of the duplicated MM, other studies have [26]. Hence, pathologists should attempt to describe the specific depth of invasion, for example, “intramucosal adenocarcinoma, invasive into the superficial muscularis mucosal layer.” However, only the depth of mucosal invasion may be discernable on biopsies or small EMR specimens. Determining the different levels of submucosal invasion (i.e., sm1, sm2, sm3) is not practical in biopsies or small EMR specimens where the outer limit of the measurement (border of the muscularis propria) is absent. However, given the high risk of nodal involvement with submucosal invasion, the precise depth of invasion on these types of specimens should not alter patient management.

Grading of adenocarcinoma has prognostic significance and falls into one of three categories: well (G1), moderately (G2), or poorly (G3) differentiated; undifferentiated tumors are uncommon, often cannot be subtyped as squamous or glandular, and are considered to be grade 4 (G4) tumors. The importance of accurate tumor grading is critical for clinical management, as AJCC guidelines for clinical staging of adenocarcinoma integrate tumor grade into the algorithm for determining clinical stage [21]. For example, a well- or moderately differentiated T2 N0 M0 tumor is stage IB, while a poorly differentiated T2 N0 M0 tumor is stage IIA. For adenocarcinoma, grading involves determining the percent of tumor that is composed of glands: well-differentiated tumors contain >95 % glands, moderately differentiated tumors contain 50–95 % glands, and poorly differentiated tumors display <50 % glandular architecture [21]. When tumors contain areas of multiple grades, the highest grade is documented. Histologically, well-differentiated tumors are composed of glands with irregular shapes or profiles (often with focal cribriform formation) lined by cuboidal to columnar cells with mild to moderate atypia. Not all adenocarcinomas illicit a desmoplastic response. In the absence of desmoplasia, a well-differentiated adenocarcinoma could be easily misdiagnosed as dysplasia if assessed out of context. Hence, even the most bland-appearing glands are adenocarcinoma if they are present in the submucosa or muscularis propria (Fig. 3.5c). Esophageal adenocarcinomas are often well to moderately differentiated (Fig. 3.5d). As tumors become more poorly differentiated, a more sheetlike appearance is identified. Signet ring cells, single infiltrating cells, and/or wildly atypical tumor cells may be present. Often, poorly differentiated tumors elicit a strong desmoplastic response from neighboring stromal cells. When dealing with a poorly differentiated adenocarcinoma on biopsy specimens, metastatic disease should be considered and excluded, either clinically or immunohistochemically (e.g., immunostains for breast markers could help diagnose metastatic breast carcinoma). Unfortunately, there are no immunomarkers that are diagnostic of esophageal adenocarcinoma, although most tumors express CK7 and may express CDX2 and CK20.