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.

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

Core Tip: Acellular mucin in both neoplastic and non-neoplastic lower gastrointestinal tract conditions may complicate diagnosis, prognostication and management. In colorectal and appendiceal mucinous neoplasms, it plays a role in tumor classification and staging. In non-neoplastic conditions, the presence of acellular mucin and related epithelial changes may mimic neoplasm and lead to additional work-up. This review highlights the diagnostic and prognostic implications of acellular mucin in neoplastic and non-neoplastic conditions, emphasizing the need for standardized approaches to improve pathological interpretation and management.

INTRODUCTION

Acellular mucin, defined as pools of mucin devoid of viable epithelial cells, represents a distinctive histopathological feature observed across both neoplastic and non-neoplastic conditions of the gastrointestinal (GI) tract[1]. In neoplastic conditions such as colorectal mucinous adenocarcinoma and low-grade appendiceal mucinous neoplasms (LAMNs), mucin constitutes a substantial portion of the tumor[2]. This mucin factors into pathologic T staging for treatment-naïve colorectal cancer and appendiceal mucinous neoplasm. Current guidelines, however, do not offer staging recommendations pertaining to the presence of acellular mucin in regional lymph nodes in untreated colorectal adenocarcinoma[3], and associated clinical outcome data is limited. Interpretation of mucin in lymph nodes may therefore vary among pathologists, which highlights the need for consensus and standardization. Cancers following neoadjuvant therapy may also demonstrate acellular mucin, but this feature does not contribute to staging.

The prognostic relevance of mucin in mucinous colorectal adenocarcinoma remains controversial. Some studies documented worse oncologic outcome of mucinous adenocarcinoma compared to non-mucinous adenocarcinoma, especially in stage III/IV tumors[4,5], and in the rectum[6]. However other studies reported comparable prognosis between the mucinous and non-mucinous groups when tumor-node-metastasis stages were matched[7]. Similarly, inconsistent data exists regarding the survival outcome of patients with mucinous adenocarcinoma following adjuvant chemotherapy compared to those with non-mucinous adenocarcinoma, although survival benefit following chemotherapy has been clearly documented in the mucinous adenocarcinoma group[8,9]. In contrast, it is generally accepted that acellular mucin without viable tumor cells in post- neoadjuvant treatment setting does not portend adverse prognosis[10,11].

Acellular mucin is also encountered in non-neoplastic GI specimens, such as those from interval appendectomies and appendiceal diverticula, where it can mimic neoplastic lesions such as LAMNs[1]. Other inflammatory conditions of the GI tract associated with potential mural defects, such as colonic diverticulitis, volvulus and Crohn’s disease, also occasionally exhibit pools of mucin within the bowel wall. These mucin deposits are thought to originate from the lumen via mural defect, in the context of inflammation and increased luminal pressure. This phenomenon can complicate the diagnostic process by mimicking features of neoplastic lesions[12].

This work reviews the clinical and pathologic significance of acellular mucin in neoplastic and non-neoplastic conditions of the lower GI tract, focusing on its implications for diagnosis, staging, and relevance to clinical management. By integrating recent advances in molecular and histological characterization, we aim to provide a comprehensive understanding of this entity. Furthermore, we emphasize the need for standardized assessment criteria to ensure accurate prognostication and treatment planning.

ACELLULAR MUCIN IN NEOPLASTIC CONDITIONS OF THE LOWER GI TRACT

Mucinous adenocarcinoma and adenocarcinoma with mucinous features

Mucinous adenocarcinoma is defined as an adenocarcinoma in which at least 50% of the tumor volume is composed of extracellular mucin. This definition is endorsed by the American Joint Committee on Cancer (AJCC), the College of American Pathologists (CAP), and the World Health Organization classification[13-16]. Histologically, these tumors show clusters of malignant epithelial cells of varying grades, floating in abundant pools of extracellular mucin.

Although not recognized as an official subtype of colorectal adenocarcinoma, the term “adenocarcinoma with mucinous features/component” can be used when a tumor demonstrates extracellular mucin pools constituting less than 50% of tumor volume. Although this is a pure histomorphologic, descriptive entity, shared molecular profiles between mucinous adenocarcinoma and adenocarcinoma with mucinous features have been documented[17], suggesting these mucin-producing tumors may represent different phases of the same disease spectrum. Among known subtypes of colorectal adenocarcinoma, serrated adenocarcinoma and signet-ring cell carcinoma may fall into this category, as these can produce a significant amount of extracellular mucin and form acellular mucin pools[18,19]. In fact, it has been shown that 43% of serrated adenocarcinomas demonstrate mucinous features[20] (Figure 1). In routine clinical practice, adenocarcinoma with mucinous features is treated as conventional non-mucinous adenocarcinoma.

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).

Prevalence: In the colorectum, mucinous adenocarcinomas account for approximately 10%-15% of all cancer cases[6,10,21]. These tumors are more commonly located in the right colon than the left colon or rectum[21,22]. Its prevalence is even higher in the appendix, comprising about 40% of all appendiceal neoplasms[23,24]. Serrated adenocarcinoma accounts for approximately 7%-12% of colorectal cancers[25,26], and signet ring cell carcinoma comprises about 1% of colorectal cancers[27]. Demographically, mucinous adenocarcinomas are slightly more common in younger patients[28] and can be associated with certain risk factors, such as Lynch syndrome[29]. Tumors arising in inflammatory bowel disease and sporadic microsatellite instability (MSI)-high tumors can similarly be mucinous[30]. Regional and ethnic variations also exist, with higher incidences reported in Asian compared to Western populations, possibly reflecting genetic and environmental influences.

Molecular characteristics: Mutations in KRAS and GNAS are pervasive, particularly in mucinous tumors arising from the appendix[31,32]. Li et al[17] reported high frequencies of KRAS and BRAF mutations in colorectal cancer with any degree of mucinous differentiation. KRAS mutation activates aberrant signaling pathways and enhances cellular proliferation and survival, while GNAS mutation leads to constitutive activation of the cyclic AMP pathway, driving excessive mucin production[33]. In colorectal mucinous adenocarcinoma, alterations in the KRAS, BRAF and PI3K/AKT pathway are commonly observed, along with MSI and the CpG island methylator phenotype (CIMP)[10,34]. For example, defects in mismatch repair (MMR) genes are frequently observed in mucinous adenocarcinomas arising in Lynch syndrome[35], whereas sporadic MSI-high tumors with mucinous features often demonstrate methylation of the MLH1 gene promoter[36]. In contrast, despite their morphologic resemblance, mucinous adenocarcinomas arising in association with inflammatory bowel disease are not necessarily MSI-H[30]. Overexpression of mucin-specific genes, such as MUC2 and MUC5AC, further enhances mucin production and secretion in mucinous adenocarcinomas[10]. The molecular profile of mucinous adenocarcinomas has significant implications for targeted therapy, with KRAS mutations serving as predictors of resistance to anti-EGFR therapies[37].

Serrated adenocarcinomas which arise in association with traditional serrated adenomas more frequently feature microsatellite stable or MSI-low status. However, about 17% of serrated adenocarcinomas may arise in association with sessile serrated lesion and these carcinomas are MSI-H[25]. These observations imply that a subset of serrated adenocarcinomas are a product of the serrated neoplasia pathway with CIMP status[26]. However, recent studies have shown that serrated morphology per se is not uncommon in adenocarcinomas arising in association with conventional adenomas, and these carcinomas show a comparable rate of KRAS, BRAF mutation and MSI status with conventional colorectal adenocarcinoma[18,38]. The clinical and biologic significance of serrated morphology in colorectal cancer thus remains unclear.

Conflicting data exists regarding genetic alterations of colorectal signet ring cell carcinoma. Korphaisarn et al[39] found no significant association between colorectal signet ring cell carcinoma and MSI, NRAS, BRAF, SMAD4, TP53 or FBXW7 mutational status. In contrast, a literature review reported more frequent BRAF mutation and MSI-H and CIMP status in colorectal signet ring cell carcinoma compared to conventional adenocarcinoma[40].

Workup for limited sample: The malignant diagnosis in a hypocellular sample with a predominance of mucin can be particularly challenging. These specimens may result from, for example, minimally invasive image-guided biopsies for suspected pseudomyxoma peritonei (PMP) or locally recurrent, previously treated rectal cancer. In these cases, identifying malignant epithelial cells with potential atypia of varying degrees is crucial[11,41]. This assessment must additionally consider the possibility of reactive epithelial atypia, especially when the atypia exists in a background of marked inflammation or at the site of tissue damage. Only after confounding factors for atypia are addressed, can a diagnosis of neoplasm be rendered in spite of low cellularity. The origin of atypical epithelial cells can be confirmed by immunohistochemistry, when indicated. Immunomarkers such as CK20, SATB2 and CDX2 are commonly positive in colorectal tumors[42]. Although immunologic and structural differences in MUC glycoproteins between neoplastic and non-neoplastic conditions are well documented[43,44], mucin-specific markers are not presently used in routine tissue diagnosis. In a recent pilot study, García-Olmo et al[45] reported KRAS mutation in mucin cfDNA from mice injected with acellular mucin obtained from a patient with PMP, illustrating the potential utility of liquid biopsy for mucinous neoplasms.

Acellular mucin in pathologic staging

The presence of acellular mucin pools at the invasive front of colorectal adenocarcinoma, in regional lymph nodes, and at resection margins poses staging challenges due to high interobserver variability and conflicting guidelines.

Acellular mucin following neoadjuvant therapy: In colorectal adenocarcinoma following neoadjuvant therapy, acellular mucin pools without viable tumor cells do not contribute to the tumor (T) or nodal (N) staging categories[3,46]. Although conflicting data exists[41], several studies on clinical outcomes have shown that acellular mucin pools in treated rectal adenocarcinomas that achieved complete pathologic response are not prognostic[47-49]. In contrast, mucin pools containing viable tumor cells portend poorer prognosis with decreased survival[50]. One study divided treatment response in rectal cancer into three groups [(1) Downstaged; (2) Colloid (mucinous) response with or without viable tumor cells; and (3) No response], and found that the colloid response group showed intermediate oncologic outcome relative to the other two[51]. Further studies are needed to clarify any prospective clinical significance of acellular mucin encountered post neoadjuvant therapy.

Acellular mucin and challenges in staging: The histologic assessment of acellular mucin in colorectal cancer specimens is subject to considerable interobserver variation[46,52,53]. Karamchandani et al[52] highlighted the challenges faced by pathologists when measuring the depth of invasion in colonic endoscopic resection specimens with mucin pools. Discrepancies in the measurement of submucosal invasion were attributed to varying interpretations of the interface between acellular mucin and viable tumor cells. Similarly, an international survey of 118 surgical pathologists revealed discrepant reporting regarding depth of invasion (pathologic T stage), nodal involvement (pathologic N stage) and margin status when neoadjuvant-treated rectal cancer showed pools of hypocellular or acellular mucin[53].

Acellular mucin encountered in regional lymph nodes from treated colorectal cancer is not considered viable tumor, and accordingly does not contribute towards N stage. However, no formal consensus advises upon that found in treatment-naïve colorectal cancer. In a supplemental publication, the Union for International Cancer Control considers this entity as viable tumor, with a corresponding change in N disease[54]. CAP and AJCC have no formal recommendations at present. In a recent study, Lapinski et al[3] compared the clinicopathologic features of 16 treatment-naïve colorectal cancers exhibiting acellular mucin in regional lymph nodes to 59 without nodal acellular mucin. Those with nodal acellular mucin demonstrated survival outcomes more similar to conventional N0 disease than N1. This group of tumors were often right-sided with deficient MMR proteins. For prognostic and staging purposes, this study suggests that the presence of acellular mucin in lymph nodes may be considered distinct from metastatic deposits with viable tumor cells[3]. A larger, international study on the same topic is underway; its preliminary data was presented as an abstract at the 2024 United States and Canadian Academy of Pathology annual meeting in Baltimore, MD, United States.

Appendiceal mucinous neoplasm and PMP

Appendiceal mucinous neoplasms account for less than 1% of all GI cancers, and encompass a spectrum ranging from LAMNs to mucinous adenocarcinomas[55]. Histologically, these tumors show a proliferation of mucin-producing epithelial cells of varying grades associated with accumulation of abundant extracellular mucin, often dissecting the appendiceal wall.

LAMN: When the specimen is entirely submitted for microscopic examination, LAMNs are generally indolent when confined to the appendix or with extra-appendiceal acellular mucin limited to the right lower quadrant[2,56]. In addition, mucin with or without neoplastic epithelium at the appendectomy margin in organ-confined LAMN does not confer increased risk of recurrence or poor outcome without further surgery[57,58]. Delayed diagnosis and excessive mucin accumulation may result in appendiceal distention and rupture, leading to the dissemination of mucin into the peritoneal cavity, causing PMP[2]. Indeed, among variable appendiceal mucinous neoplasms, LAMN is the most common cause of PMP[59]. Ballentine et al[2] found that LAMN with acellular intraperitoneal mucin has a more favorable prognosis compared to those with cellular mucin collections. However, its prognosis was still worse than that of LAMN confined to the appendix[2]. Per current guidelines, LAMN is staged as pTis when the lesion or associated acellular mucin shows pushing invasion into the lamina propria, submucosa or muscularis propria, and as pT3/4 when the lesion or associated acellular mucin is beyond the muscularis propria. pT1 and pT2 do not apply to LAMN staging[14,60].

High grade appendiceal mucinous neoplasm: The term high grade appendiceal mucinous neoplasm (HAMN) was first introduced by Peritoneal Surface Oncology Group International (PSOGI) in 2012[61,62]. HAMN is rare compared to LAMN or mucinous adenocarcinoma of the appendix, thus outcome data is limited. The diagnosis can be made when a LAMN-like appendiceal lesion displays any amount of high grade cytologic features without infiltrative growth[59]. In a recent multi-institutional study, Gonzalez et al[62] documented favorable prognosis of organ-confined HAMN. However, thirteen (37%) of 35 patients had PMP either at the time of presentation or during the disease course, and the PMP typically showed high grade histology without signet ring cells[62]. Unlike LAMN, HAMN is staged using the conventional appendiceal adenocarcinoma scheme[14,60].

Appendiceal mucinous adenocarcinoma: An appendiceal neoplasm with infiltrative invasion and extracellular mucin constituting > 50% of the tumor volume is referred to as mucinous adenocarcinoma and graded similarly to its colorectal counterpart. Appendiceal mucinous adenocarcinoma can arise from precursor lesions such as serrated polyp, LAMN and HAMN, but must be differentiated from these precursors given its management and prognostic implications. Desmoplasia is considered the key histologic finding in mucinous adenocarcinoma. The presence of signet ring cells in mucinous adenocarcinoma needs to be clearly documented when present[15,59] (Table 1).

Table 1 Acellular mucin in neoplastic lower gastrointestinal tract conditions.

Condition	Key features	Diagnostic implications	Prognosis
Mucinous adenocarcinoma	≥ 50% extracellular mucin, malignant epithelial cells	Confirm site via IHC (CK20, CDX2, SATB2)	Varies by site and molecular alterations
LAMNs/HAMNs	Abundant mucin, no infiltrative growth	Risk of pseudomyxoma peritonei	Generally favorable when organ-confined
SRCC	Signet ring cells	May represent metastasis	Poor prognosis
Serrated adenocarcinoma	Epithelial serration, minimal necrosis, clear or eosinophilic cytoplasm	CIMP (subset)	Unclear

IHC: Immunohistochemistry; CIMP: CpG island methylator phenotype; SRCC: Signet-ring cell carcinoma; LAMNs/HAMNs: Low/high-grade appendiceal mucinous neoplasms.

PMP: PMP refers to mucinous deposits in the peritoneal cavity and surfaces associated with mucin-producing tumors of various organs to include, but not limited to, appendix, ovaries, small and large bowel, pancreas, lung, breast, stomach, and gallbladder[63]. The terminology and classification of PMP have undergone significant evolution over the years, reflecting advances in our understanding of its pathogenesis, histological features, and clinical behavior. In 1995, studies introduced the term “disseminated peritoneal adenomucinosis (DPAM)” to differentiate the mucin-rich deposits containing bland mucinous epithelial cells with favorable prognosis, from its hypercellular and overtly malignant counterpart with worse outcome, peritoneal mucinous carcinomatosis (PMCA). In their study of 109 PMP cases, 60% were classified as DPAM and 57% of DPAM were confirmed to have originated from LAMN[64,65]. However, as knowledge expanded, it became evident that PMP encompasses a spectrum of diseases, ranging from low-grade to high-grade histology with varying cellularity, with or without signet ring cells and tissue invasion, and with varying prognostic implications[66].

This was discussed at the 2012 World Congress of the PSOGI in Berlin in an effort to devise uniform terminology and a classification system for PMP. The participants of this consensus meeting acknowledged that PMP is most commonly associated with appendiceal neoplasm, and the presence or absence of tumor cells in PMP is prognostic. As such, four PMP categories were proposed: (1) Acellular mucin (mucin without epithelial cells); (2) Low-grade mucinous carcinoma peritonei or DPAM (PMP with low-grade histology); (3) High-grade mucinous carcinoma peritonei or PMCA (PMP with high-grade histology); and (4) High-grade mucinous carcinoma peritonei with signet ring cells or PMCA with signet ring cells (PMP with signet ring cells)[61]. Low-grade PMP, usually associated with LAMNs, is characterized by minimal cytologic atypia and abundant extracellular mucin, whereas high-grade PMP, often originating from mucinous adenocarcinomas, features more pronounced epithelial atypia, mitotic activity, and potential for invasive growth (Figure 1).

ACELLULAR MUCIN IN NON-NEOPLASTIC CONDITIONS OF THE LOWER GI TRACT

Interval appendectomy

Acellular mucin encountered in interval appendectomy specimens warrants a particular diagnostic scrutiny, as it can mimic LAMN. This distinction between interval appendectomy and LAMN is of special regard, not only to prevent subsequent overtreatment of the patient, but also to accurately evaluate the utility of interval appendectomy.

Interval appendectomy, typically performed following initial nonoperative management of complicated appendicitis (with phlegmon or abscess), is itself a subject of debate. In their large retrospective cohort study, Kaminski et al[67] argued against the practice of routine interval appendectomy given the low recurrence rate of appendicitis following nonoperative management. However, there has been a recent trend in the surgical literature towards recommending interval appendectomy due to the apparent increased risk of neoplasms in these specimens[68-73]. One retrospective study reported 3.6% of interval appendectomy specimens returned with a neoplasm, compared to 1.0% of acute (immediate) appendectomy specimens[71]. Similarly, Furman et al[68] demonstrate an even higher rate of neoplasm in interval appendectomy cases (29.4%), in contrast to immediate appendectomy cases (2.5%). All interval appendectomy neoplasms were notably found to be mucinous[68]. More recent studies reported the prevalence of neoplasms in interval appendectomy specimens ranging from 5% to 11%[74-77]. The concordant results between several single-institution studies propose that interval appendectomy be considered on the basis of increased neoplastic risk in the consequent specimens. Of note, the estimated incidence of periappendiceal or mural acellular mucin pools in interval appendectomy is around 16%[78].

A potential conundrum thus arises, as the above literature submits its conclusions based on the distinction between neoplastic vs non-neoplastic, which is itself a diagnostic dilemma in interval appendectomy specimens. In a multi-institutional study examining 43 appendiceal consults, 19 were initially submitted as LAMN, however only 9 of which were ultimately diagnosed as so by the consultant[79]. It is well described in the literature how post-inflammatory appendiceal changes may mimic neoplastic features[78-81]. Shared features include hypermucinous epithelium, extravasated pools of mucin, and mural alterations including fibrosis. In fact, this mimicry has been postulated to confound the high rate of mucinous neoplasia reported in interval appendectomy specimens[80]. Certain cases of mucinous neoplasia may represent misclassified benign, post-inflammatory processes.

Hissong et al[80] investigated 92 mucinous lesions of the right lower quadrant, of which 29 were products of interval appendectomies. Of note, 26 of these were non-neoplastic, and 3 were neoplastic. They did not find a significant difference in the rates of mural and extramural mucin between neoplastic and non-neoplastic lesions. However, extramural mucin in non-neoplastic cases were frequently accompanied by organizing abscesses, lymphoplasmacytic infiltrates, and frequent macrophages or multinucleated giant cells. In addition, non-neoplastic cases tended to exhibit mature goblet cells in the surface epithelium, goblet cell-rich crypts containing interspersed Paneth cells, intact lamina propria, muscularis mucosae of normal thickness, and preserved layers of the appendiceal wall distant from areas of perforation and diverticula. In contrast, mucinous neoplasms tended to exhibit circumferential involvement of the appendix, lack of normal crypts, decreased lamina propria with diffusely thickened muscularis mucosae, and lymphoid atrophy[80]. Carr[81] similarly reviewed distinguishing features between LAMN and reactive mucosal changes. In alignment with Hissong et al’s findings[80], non-neoplastic lesions differentially demonstrated intact lamina propria and muscularis mucosae without crowding of crypts. Moreover, reactive mucosal changes showed serration and hypermucinous epithelium confined to the superficial mucosa, frequent intramucosal neuromas, and absence of filiform villi[81] (Table 2).

Table 2 Histologic differences between low grade appendiceal mucinous neoplasm vs non-neoplastic mucin-rich conditions in the appendix.

Feature	LAMN	Non-neoplastic
Extent of disease	Extensive/circumferential involvement	Focal involvement
Crypt architecture	Loss of normal crypts	Preserved crypt architecture
Lamina propria	Attenuated lamina propria	Intact lamina propria
Muscularis mucosae	Diffusely thickened	Normal thickness
Epithelial villiform proliferation	Usually present	Absent
Lymphoid aggregates (lesional)	Usually absent	Usually present
Paneth cells (lesional)	Usually absent	Usually present
Inflammatory response	Usually sparse	Frequent periappendicitis and organized abscess

LAMN: Low-grade appendiceal mucinous neoplasm.

Appendiceal diverticulosis

Appendiceal diverticular disease represents another non-neoplastic condition which poses diagnostic challenges given its histologic similarities to LAMN. Like in interval appendectomy specimens, the presence of extravasated mucin does not differentiate between neoplastic and non-neoplastic processes in diverticular disease[82-85]. In fact, 55% to 64% of benign appendiceal diverticulosis cases examined demonstrated mural acellular mucin deposits[84,85]. In another study, of the consultation cases examined that were ultimately diagnosed as appendiceal diverticular disease, 71% had initially been submitted as LAMN by the referring pathologists[86]. Compared to LAMN, appendiceal diverticular disease tends to show relatively intact lamina propria and crypt architecture, without diffuse nuclear atypia, appendiceal wall fibrosis and hyalinosis, and extensive architectural distortion[80,84-86].

Of note, the diverticulum, as a single feature, has a well-documented association with LAMN[82,87-89]. Chan et al[88] conducted a retrospective review of 2711 appendectomy cases, and reported that patients with appendiceal diverticula were ten times more likely to have a neoplasm than those without. The same trend was observed by Dupre et al[89]. Indeed, it has been suggested that the diverticulum is the gateway by which neoplastic epithelium accesses peritoneum and leads to PMP[83]. This association only increases the significance of scrutinizing the appendix with diverticula in order to derive accurate diagnoses (Figure 2).

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).

Colonic diverticulitis, volvulus and Crohn’s disease

In addition to the well-documented instances of acellular mucin in appendiceal diseases, we have also studied the rare presence of mucin pools in other non-neoplastic GI diseases at our institution, including colonic diverticulitis, volvulus, and Crohn’s disease[12]. As anticipated, among 38 cases, acellular mucin was frequently observed in patients with appendicitis, with 71% having undergone interval appendectomy and 57% presenting with appendiceal diverticula[12]. The mucin was often mixed with inflammatory cells, indicating mucin translocation through diverticula or mural defects. In colonic diverticulitis, no significant differences were found between cases with and without acellular mucin with regards to patient demographics, symptom duration, or inflammation severity. However, additional histological workup, such as extra tissue sampling and immunohistochemical staining, were more common when acellular mucin was present. Acellular mucin pools in volvulus, Crohn’s disease and trauma were usually seen at the site of previous procedure (anastomosis site) in association with microscopic mural defect.

This observation is less reported in the formal literature thus the incidence/prevalence of acellular mucin deposits in these inflammatory conditions of lower GI tract is unknown. However, we suspect it is not limited to our institution and altogether unsurprising. Given their locoregional association with previous procedure site and diverticular disease, mucin pools appear to be of intraluminal origin which are mechanically displaced into the bowel wall via high luminal pressure and microscopic mural defect. Likewise, the etiology of the somewhat similar benign entities, gastritis/enteritis/colitis cystica profunda, has long been attributed to prior inflammation, trauma, and/or infection[90-93]. As such, acellular mucin pools in GI specimens do not necessarily imply occult malignancy, and can be seen in benign conditions. Our observations suggest that acellular mucin is an under-recognized phenomenon in benign lower GI conditions, and that recognizing this can help reduce unnecessary diagnostic procedures[12]. Further studies and collaborative documentation are required to examine the prevalence of acellular mucin in non-neoplastic GI diseases. Investigating the mechanisms behind the formation of acellular mucin pools and the associated histological changes would provide novel insights for future clinical management (Figure 2 and Table 3).

Table 3 Acellular mucin in non-neoplastic lower gastrointestinal tract conditions.

Condition	Etiology	Histological findings	Clinical considerations
Appendicitis	Mucin translocation via diverticula or mural defect, common in interval appendectomy	Mucin pools within inflammatory background without epithelial dysplasia	Can mimic LAMN
Colonic diverticulitis	Mural defect, high luminal pressure	Mucin pools following distribution of diverticular pouch	Additional work up to rule out mucinous neoplasia may be required
Crohn’s disease	Mural defect due to prior surgery or fistula	Mucin pools at the site of mural defect	Findings need to be interpreted in an appropriate clinical context
Volvulus	High luminal pressure and microscopic mural defect	Mucin displaced into bowel wall	Findings need to be interpreted in an appropriate clinical context

LAMN: Low-grade appendiceal mucinous neoplasm.

CONCLUSION

Acellular mucin can be seen in both neoplastic and non-neoplastic lower GI tract conditions. Its presence in neoplastic conditions can pose challenges in diagnosis, staging and prognostication. Outcome-based, standardized staging criteria are needed to minimize diagnostic variability among pathologists and elucidate its biological significance. In non-neoplastic conditions, the presence of acellular mucin can mimic neoplastic processes and lead to potential over-investigation and unnecessary treatments. Understanding the etiology of acellular mucin in the proper clinical context and performing thorough histological evaluation is crucial to improve diagnostic accuracy and optimize patient management.