# Appendiceal Neoplasms and Pseudomyxoma Peritonei

## Introduction

Appendiceal neoplasms are uncommon tumors found in approximately 1% of appendectomy specimens. However, their clinical significance is substantial, as they encompass a spectrum from benign mucoceles to aggressive adenocarcinomas and can be associated with pseudomyxoma peritonei (PMP), a condition characterized by mucinous ascites and peritoneal implants. Accurate pathologic classification is essential, as it dictates the need for completion right hemicolectomy, cytoreductive surgery, and hyperthermic intraperitoneal chemotherapy (HIPEC).

## Classification of Appendiceal Neoplasms

### Epithelial Neoplasms

Mucinous neoplasms represent the most common category of appendiceal neoplasms and exist along a spectrum of increasing aggressiveness. Serrated polyps and sessile serrated lesions are low-risk and cured by appendectomy alone. Low-grade appendiceal mucinous neoplasm (LAMN) is characterized by pushing invasion rather than infiltrative growth, with low-grade cellular atypia, and may present as a mucocele. When confined to the appendix with negative margins, appendectomy is curative. High-grade appendiceal mucinous neoplasm (HAMN) demonstrates high-grade cytologic atypia without destructive invasion but carries a higher risk of peritoneal dissemination. Mucinous adenocarcinoma shows infiltrative invasion through the appendiceal wall and may be well, moderately, or poorly differentiated. A signet ring cell component exceeding 50% confers a significantly worse prognosis.

Non-mucinous adenocarcinoma is a colonic-type adenocarcinoma treated similarly to colon cancer, with right hemicolectomy as the standard operation. Goblet cell adenocarcinoma (formerly goblet cell carcinoid) demonstrates mixed neuroendocrine and glandular differentiation and is biologically aggressive. The Tang classification (A, B, C) predicts prognosis, and right hemicolectomy is recommended due to its high propensity for peritoneal spread.

### Neuroendocrine Neoplasms

Well-differentiated neuroendocrine tumors (carcinoids) are the most common appendiceal tumor overall and are typically found incidentally at the tip of the appendix. | Size | Frequency | Management |
| --- | --- | --- | --- |
| <1 cm | 80% | Appendectomy alone (curative) |  |
| 1–2 cm (low risk) | — | Appendectomy if tip location, clear margins, no LVI, low Ki-67 |  |
| 1–2 cm (high risk) or >2 cm | — | Right hemicolectomy |  |

Management is based on size. Tumors smaller than 1 cm, which account for 80% of cases, are cured by appendectomy alone with essentially zero recurrence rate. Tumors measuring 1 to 2 cm are adequately treated by appendectomy if they are located at the tip with no mesoappendix invasion, no lymphovascular invasion, clear margins, and low Ki-67 index. Tumors larger than 2 cm, or those with base involvement, positive margins, mesoappendix invasion greater than 3 mm, high Ki-67, or lymphovascular invasion, warrant right hemicolectomy. High-grade neuroendocrine carcinoma is rare and aggressive, treated with right hemicolectomy and systemic platinum-based chemotherapy.

<image>Classification diagram of appendiceal neoplasms showing the histologic spectrum from serrated polyp through LAMN, HAMN, and mucinous adenocarcinoma with corresponding cross-sectional illustrations of the appendiceal wall demonstrating pushing versus infiltrative invasion patterns, and a separate panel for neuroendocrine tumors with size-based management algorithm</image>

## Appendiceal Mucocele

An appendiceal mucocele is defined as obstructive dilatation of the appendix with mucin accumulation. It is a descriptive term rather than a pathologic diagnosis, and the underlying cause may be a retention cyst (simple mucocele), LAMN, HAMN, mucinous adenocarcinoma, or rarely endometriosis or mucosal hyperplasia. On CT imaging, it appears as a well-defined, encapsulated cystic mass in the right lower quadrant, sometimes with mural calcification described as a "porcelain appendix."

Surgical management of an intact mucocele involves appendectomy with care to avoid rupture, as gentle handling is critical to prevent peritoneal dissemination. If rupture occurs, peritoneal surfaces should be inspected and biopsied. When the cecum is involved, cecectomy or right hemicolectomy may be required. The laparoscopic approach is feasible for small, intact mucoceles using a retrieval bag and avoiding direct grasping of the mucocele, though some surgeons advocate an open approach for large mucoceles due to rupture risk. An important clinical association is that synchronous ovarian mucinous tumors occur in 10 to 20% of women with appendiceal mucinous neoplasms, and the appendix should be considered the primary site.

## Pseudomyxoma Peritonei (PMP)

### Pathophysiology

Pseudomyxoma peritonei is defined by the progressive accumulation of mucinous ascites and peritoneal mucinous implants, typically originating from a ruptured appendiceal mucinous neoplasm. The redistribution phenomenon describes how mucin accumulates in dependent areas and at sites of peritoneal fluid reabsorption, including the right hemidiaphragm, greater omentum, pelvis, and paracolic gutters. The Ronnett classification divides PMP into three categories: disseminated peritoneal adenomucinosis (DPAM), a low-grade form with abundant extracellular mucin and scant low-grade epithelium carrying a favorable prognosis; peritoneal mucinous carcinomatosis (PMCA), a high-grade form with abundant epithelium showing high-grade atypia and signet ring cells carrying a poor prognosis; and an intermediate category (PMCA-Intermediate) with features of both.

### Clinical Presentation

PMP often presents insidiously with increasing abdominal girth, a new-onset hernia containing mucin (sometimes called "jelly belly"), or is discovered incidentally at appendectomy or other abdominal surgery. Advanced disease may cause bowel obstruction, ureteral obstruction, and malnutrition. CT findings include scalloping of hepatic and splenic surfaces by mucinous material, omental caking, mucinous ascites, and calcifications.

### Treatment: Cytoreductive Surgery (CRS) and HIPEC

Cytoreductive surgery aims for complete removal of all visible peritoneal disease and includes peritonectomy procedures of the parietal, diaphragmatic, and pelvic peritoneum, omentectomy, and resection of involved viscera such as right hemicolectomy, anterior resection, cholecystectomy, splenectomy, and hysterectomy with bilateral salpingo-oophorectomy as needed.

The Peritoneal Cancer Index (PCI), developed by Sugarbaker, divides the abdomen into 13 regions and scores each from 0 to 3 based on tumor volume, yielding a total score of 0 to 39. A PCI exceeding 20 for high-grade tumors or exceeding 25 for low-grade tumors is associated with poor outcomes. The Completeness of Cytoreduction (CC) score ranges from CC-0 (no visible residual) through CC-1 (less than 2.5 mm residual), CC-2 (2.5 mm to 2.5 cm), to CC-3 (greater than 2.5 cm). Achieving CC-0 or CC-1 is the surgical goal and is the strongest predictor of survival.

HIPEC is delivered intraoperatively after cytoreduction, most commonly using heated mitomycin C at 42 degrees Celsius for 90 minutes. The chemotherapy penetrates 1 to 2 mm into tissue. Alternative agents include oxaliplatin and cisplatin.

<image>Surgical illustration of cytoreductive surgery for pseudomyxoma peritonei showing the Sugarbaker peritoneal cancer index scoring regions overlaid on an abdominal diagram, the peritonectomy technique stripping mucinous tumor from the diaphragm and parietal peritoneum, and the HIPEC circuit setup with inflow and outflow catheters, heated perfusion system, and temperature monitoring probes</image>

### Outcomes

Low-grade PMP treated with CRS and HIPEC achieves 10-year survival rates of 63 to 70% when complete cytoreduction is accomplished. High-grade PMP and signet ring cell variants have 10-year survival rates of only 10 to 20%. Completeness of cytoreduction is the single most important prognostic factor. Recurrence is common, and repeat CRS/HIPEC may be considered in select patients.

## Management Algorithm After Incidental Appendiceal Neoplasm

For carcinoid tumors smaller than 1 cm confined to the tip, no further surgery is needed and surveillance is not required. Carcinoid tumors of 1 to 2 cm with high-risk features or larger than 2 cm warrant right hemicolectomy. LAMN with negative margins and no perforation is cured by appendectomy, with baseline CT and tumor markers (CEA, CA 19-9, CA 125) considered at 6 months. LAMN with positive margins, perforation, or mucin on the serosa should be referred for CRS evaluation with surveillance CT and tumor markers. Mucinous adenocarcinoma requires right hemicolectomy, staging CT, and consideration of CRS/HIPEC if peritoneal disease is present. Goblet cell adenocarcinoma requires right hemicolectomy with staging workup, as peritoneal disease is common. Non-mucinous adenocarcinoma is treated with right hemicolectomy with lymphadenectomy and standard colon cancer staging and adjuvant therapy.

<image>Management algorithm flowchart for incidental appendiceal neoplasms discovered at appendectomy, branching by histologic subtype (carcinoid, LAMN, HAMN, mucinous adenocarcinoma, goblet cell adenocarcinoma, non-mucinous adenocarcinoma) with decision points for additional surgery, surveillance, and referral for CRS/HIPEC</image>

## Clinical Pearls

All appendectomy specimens should undergo thorough histopathologic examination, as 1% harbor neoplasms that may change management. Appendiceal mucoceles must be handled gently during surgery because rupture can convert a localized LAMN into potential pseudomyxoma peritonei. The term "mucocele" is a clinical and radiologic descriptor rather than a pathologic diagnosis, and final histology determines the subsequent management plan. For pseudomyxoma peritonei, completeness of cytoreduction to CC-0 or CC-1 is the strongest predictor of long-term survival. Synchronous ovarian mucinous tumors in women with appendiceal mucinous neoplasms are usually metastatic from the appendix rather than primary ovarian tumors.

## References

1. Carr NJ, Cecil TD, Mohamed F, et al. A consensus for classification and pathologic reporting of pseudomyxoma peritonei and associated appendiceal neoplasia: the results of the Peritoneal Surface Oncology Group International (PSOGI) modified Delphi process. *Am J Surg Pathol*. 2016;40(1):14-26.
2. Sugarbaker PH. Cytoreductive surgery and perioperative intraperitoneal chemotherapy as a curative approach to pseudomyxoma peritonei syndrome. *Eur J Surg Oncol*. 2001;27(3):239-243.
3. Chua TC, Moran BJ, Sugarbaker PH, et al. Early- and long-term outcome data of patients with pseudomyxoma peritonei from appendiceal origin treated by a strategy of cytoreductive surgery and hyperthermic intraperitoneal chemotherapy. *J Clin Oncol*. 2012;30(20):2449-2456.
4. Glasgow SC, Gaertner W, Stewart D, et al. The American Society of Colon and Rectal Surgeons Clinical Practice Guidelines for the Management of Appendiceal Neoplasms. *Dis Colon Rectum*. 2019;62(12):1425-1438.