Acellular mucin in neoplastic and non-neoplastic conditions of the lower gastrointestinal tract

Abstract

Acellular mucin refers to pools of mucin without epithelial component, oftentimes harboring inflammatory cells. Acellular mucin can be observed in both neoplastic and non-neoplastic lower gastrointestinal (GI) conditions. While mucinous neoplasms are classified and staged using established guidelines, interobserver variability occurs when acellular mucin pools are encountered, leading to inconsistent interpretation and staging. In particular, acellular mucin found in regional lymph nodes of colorectal adenocarcinoma patients who have not received treatment presents a diagnostic challenge, as its prognostic implication is not clearly defined. Acellular mucin is also commonly seen in treated colorectal adenocarcinoma, post neoadjuvant therapy. Although acellular mucin is not counted toward T or N staging in this setting, variation in how pathologists report and stage these cases persists. Acellular mucin can also be seen in non-neoplastic specimens, such as those from interval appendectomies, appendiceal diverticula, colonic diverticulitis, volvulus, and Crohn's disease where it may mimic a neoplastic lesion. Acellular mucin in this setting is often a byproduct of inflammation, increased luminal pressure, and mural defect. This review highlights the clinical relevance and diagnostic complexity of acellular mucin in pathologic conditions of the lower GI tract. Further studies are needed to clarify its prognostic value and develop standardized guidelines.

Keywords: Acellular mucin; Interval appendectomy; Low-grade appendiceal mucinous neoplasm; Mucinous adenocarcinoma; Pseudomyxoma peritonei.

Figure 1

Acellular mucin in neoplastic conditions. A: Well-differentiated mucinous adenocarcinoma of the transverse colon, inset: Higher magnification view of the tumor [hematoxylin and eosin (H&E)]; B: Serrated adenocarcinoma of the cecum with abundant extracellular mucin, inset: Higher magnification view of the tumoral epithelium with eosinophilic cytoplasm and low nuclear to cytoplasmic ratio (H&E); C: Acellular mucin involving a lymph node in treatment-naïve adenocarcinoma of the right colon, the tumor was microsatellite-high (H&E); D: Acellular mucin pools in the tumor bed of treated rectal adenocarcinoma (H&E), inset shows rare viable tumor cells; E: Low-grade appendiceal mucinous neoplasm of the appendix with acellular mucin extending to the serosal surface through rupture (pT4a), inset: Higher magnification view of low grade epithelium (H&E); F: Pseudomyxoma peritonei of the same patient (E) showing mucin and low-grade epithelium (inset) in the omentum (H&E).

Figure 2

Acellular mucin in non-neoplastic conditions. A: Interval appendectomy with subserosal acellular mucin mixed with inflammatory cells [hematoxylin and eosin (H&E)], inset: Higher magnification view of acellular mucin pools; B: Appendiceal diverticular disease with luminal acellular mucin (triangle) in the diverticulum (H&E); C: Ileum with acellular mucin pool at previous anastomosis site in a patient with Crohn’s disease (H&E), inset: Higher magnification view of acellular mucin pool; D: Acellular mucin pool (triangle) at previous diverticular pouch in colonic diverticulitis (H&E).