Pseudomyxoma peritonei (PMP) is a rare clinical entity characterized by diffuse mucinous ascites and mucinous implants on peritoneal surfaces. Incidence is 1-2/million/year. Over 90% of cases originate from appendiceal mucinous neoplasms (low-grade appendiceal mucinous neoplasm — LAMN, or appendiceal mucinous adenocarcinoma); rarely ovarian mucinous tumors, colorectal, or urachal neoplasms may be the source. Pathophysiologically, mucin-producing tumor cells spill into the peritoneal cavity and accumulate in specific anatomical sites (right lower quadrant, greater omentum, left subdiaphragmatic area, pelvis) through redistribution phenomenon. CT shows low-density mucinous ascites, peritoneal implants, omental cake, and scalloping of liver/spleen surfaces as pathognomonic findings. Treatment is cytoreductive surgery (CRS) + hyperthermic intraperitoneal chemotherapy (HIPEC) — the Sugarbaker procedure — as gold standard with 10-year survival reaching 63-70% in optimal cases.
Age Range
35-75
Peak Age
55
Gender
Female predominant
Prevalence
Rare
PMP typically originates from appendiceal mucinous neoplasm. LAMN or mucinous adenocarcinoma fills the appendiceal lumen with mucin and perforates the wall (appendiceal mucocele rupture) or causes perforation by obstructing the orifice. Mucin-producing tumor cells spill into the peritoneal cavity and accumulate in specific sites through redistribution phenomenon. Redistribution depends on peritoneal fluid circulation pattern — fluid flows from right paracolic gutter to right subdiaphragmatic area, then left subdiaphragmatic area and pelvic cavity in a 'clockwise' circulation. Mucinous material accumulates by gravitational pooling mostly in right lower quadrant, greater omentum, left subdiaphragmatic area, and pouch of Douglas. Scalloping mechanism involves chronic pressure of mucinous material on liver and spleen capsules causing indentation of Glisson's/splenic capsule — solid organ capsules resist invasive penetration but change surface shape under chronic pressure. Mucin CT density is near water but slightly higher (5-20 HU); protein and glycoprotein content slightly increases X-ray attenuation compared to water. Calcifications result from dystrophic calcification of mucinous material in chronic cases.
Scalloping is the pattern of surface indentation of liver and spleen capsular surfaces from chronic pressure of mucinous material. Normal smooth convex organ surfaces become concave with wave-like contour irregularity. This finding is the pathognomonic CT sign of PMP distinguishing it from peritoneal carcinomatosis, simple ascites, and other peritoneal diseases. Scalloping mechanism is non-invasive — mucinous material cannot penetrate the capsule, only changing surface shape through chronic pressure effect.
Portal venous phase CT shows diffuse low-density (5-20 HU) mucinous ascites accumulation in the peritoneal cavity. Mucinous ascites shows slightly higher density than simple transudative ascites (0-5 HU) — due to glycoprotein content. Scalloping of liver and spleen capsular surfaces from mucinous material pressure is pathognomonic — smooth convex surfaces of solid organs become concave. Calcifications (dystrophic), septations, and solid nodular implants may be seen within mucinous ascites. Following redistribution pattern, right lower quadrant, greater omentum, left subdiaphragmatic area, and pelvis are sites of concentrated involvement. Calcified or low-density cystic mass in appendiceal region (mucocele/primary tumor) should be carefully sought.
Report Sentence
Diffuse low-density mucinous ascites with scalloping of liver and spleen surfaces and omental implants consistent with pseudomyxoma peritonei.
Non-contrast CT shows appendiceal mucocele as low-density (5-15 HU) dilated tubular or cystic structure at the cecal base in right lower quadrant — the primary source of PMP. Mucocele usually appears as dilated appendix >15 mm in diameter; wall calcification (curvilinear or ring-shaped) is present in chronic cases. Mucinous material leakage around ruptured mucocele (periappendiceal mucinous collection) may be seen. Normal-appearing appendix does not exclude PMP — primary tumor may be small or may have disappeared after rupture. Non-contrast phase best evaluates calcifications.
Report Sentence
Dilated, low-density tubular structure at the cecal base in right lower quadrant (appendiceal mucocele) should be evaluated as pseudomyxoma peritonei source.
T2-weighted MRI shows diffuse hyperintense mucinous ascites in the peritoneal cavity. Mucinous ascites may not be as bright as simple ascites — glycoprotein content slightly shortens T2 producing intermediate-to-hyperintense signal lower than free water. Mucinous implants show intermediate T2 signal contrasting with ascites background. Scalloping of liver and spleen surfaces is clearly seen on T2. Septations appear as T2 hypointense thin bands. On DWI, mucinous implants may show mild diffusion restriction; mucinous fluid generally shows free diffusion.
Report Sentence
Diffuse intermediate-to-hyperintense mucinous ascites with liver/spleen scalloping and septations on MRI consistent with PMP.
Delayed phase shows omental cake with mucinous infiltration and solid implants. Omental cake in PMP differs from carcinomatosis — more heterogeneous with low-density mucinous components mixed with enhancing solid implants. Omental calcifications seen in chronic cases. Solid implants show progressive delayed enhancement; mucinous components show no enhancement.
Report Sentence
Heterogeneous omental cake with low-density mucinous components and enhancing solid implants consistent with omental involvement of PMP.
T1-weighted MRI shows mucinous ascites as mildly hyperintense or intermediate signal — simple ascites is T1 hypointense while mucinous ascites shows higher signal due to protein/glycoprotein content. This T1 signal difference helps distinguish from simple ascites. Mucinous implants show intermediate T1 signal. On contrast-enhanced T1 fat-sat sequences, solid implants enhance while mucinous fluid does not — this distinction is critical for tumor burden assessment.
Report Sentence
Mildly T1-hyperintense mucinous ascites with solid implants showing enhancement on contrast-enhanced sequences.
B-mode ultrasonography shows low-echogenic or echogenic ascites in peritoneal cavity — unlike simple anechoic ascites, mucinous ascites shows internal echoes and septations. Scalloping of liver and spleen surfaces may be detected on US. Omental thickening appears as heterogeneous tissue replacing normal hyperechoic omental fat. Dilated mucinous structure in appendiceal region (mucocele) may appear as hypoechoic or anechoic cystic mass. US is useful for paracentesis guidance and ascites monitoring.
Report Sentence
Echogenic ascites with septations and liver surface scalloping on ultrasonography consistent with PMP.
Criteria
Most common type (80%). Originates from low-grade appendiceal mucinous neoplasm (LAMN). Mucin production dominant, few tumor cells. CT shows dominant low-density mucinous ascites with minimal solid components. CRS + HIPEC achieves 10-year survival of 63-70%.
Distinct Features
Indolent course, slow progression. Mucinous ascites dominant with small, few solid implants. Histology shows low cellularity and bland cytology. KRAS mutation common (50-70%). Optimal cytoreduction (CC-0/CC-1) determines prognosis. Peritoneal mucinous carcinomatosis nomenclature used (PSOGI classification).
Criteria
Originates from appendiceal mucinous adenocarcinoma. More prominent solid implants, higher tumor cell density. CT shows enhancing solid implants, peritoneal nodules, and necrosis more frequently. More aggressive course, 5-year survival 20-40%.
Distinct Features
Faster progression and recurrence. Solid components show prominent CT enhancement. TP53 mutation and high Ki-67 index. Systemic chemotherapy (FOLFOX) added to CRS + HIPEC. Signet ring cell differentiation is worst prognostic factor.
Criteria
Rare (<5%). Originating from ovarian mucinous tumor. Appendix must be histologically normal (confirmed by appendectomy and pathology). Ovarian mucinous borderline tumor or mucinous carcinoma. CA-125 may be elevated.
Distinct Features
May present with bilateral ovarian masses in women. If appendix is tumor-free with ovarian mucinous tumor present, ovarian-origin PMP is considered. Treatment: CRS + HIPEC + bilateral salpingo-oophorectomy. Prognosis worse than low-grade appendiceal-origin PMP.
Distinguishing Feature
In peritoneal carcinomatosis, ascites is simple transudative (0-5 HU) or exudative (5-15 HU) without mucinous character. PMP shows mucinous ascites (5-20 HU) which may be septated and calcified. Scalloping is pathognomonic for PMP — rare in carcinomatosis. PMP omental cake is heterogeneous (mucinous + solid); carcinomatosis is homogeneous solid. Appendiceal mucocele may be identified in PMP.
Distinguishing Feature
Peritoneal mesothelioma shows diffuse peritoneal thickening and ascites but without mucinous character. Scalloping is very rare in mesothelioma. Asbestos exposure history, pleural plaques, and calcifications may be seen. Appendiceal mucocele in PMP guides diagnosis. Immunohistochemistry provides definitive differentiation.
Distinguishing Feature
TB ascites is high-protein exudative but not mucinous. TB peritoneal thickening may be smooth; PMP shows scalloping and mucinous implants. TB mesenteric lymphadenopathy shows central necrosis; lymphadenopathy not prominent in PMP. ADA elevated in TB, normal in PMP.
Distinguishing Feature
Chylous ascites is triglyceride-rich (milky-white) fluid from lymphatic leak showing negative or low CT density (-10 to +10 HU). PMP mucinous ascites is 5-20 HU, may be septated and calcified. Scalloping absent in chylous ascites. No appendiceal pathology expected. Investigation for chylous ascites causes (lymphoma, trauma, surgery).
Urgency
urgentManagement
surgicalBiopsy
NeededFollow-up
6-monthMultidisciplinary approach is critical in PMP. Diagnosis: CT/MRI imaging → diagnostic paracentesis (mucinous character, cytology) → laparoscopic biopsy (histological confirmation, PSOGI classification). Treatment: (1) CRS + HIPEC (Mitomycin C or Oxaliplatin, 42°C, 60-90 min) — Sugarbaker procedure. Optimal cytoreduction (CC-0: no visible tumor; CC-1: residual <2.5 mm) is the most important prognostic factor. (2) PCI ≤20 suitable for surgery; >20 has poor prognosis. (3) Systemic chemotherapy (FOLFOX) added for high-grade PMP. (4) Appendectomy (source control) in all cases. Follow-up: CT at 6-month intervals, tumor markers (CEA, CA-19-9, CA-125). Low-grade PMP 10-year survival 63-70%; high-grade 5-year survival 20-40%.
Pseudomyxoma peritonei is a slow-growing but recurrent disease. Cytoreductive surgery with HIPEC is standard treatment. Identifying appendiceal origin is critical. Low-grade (DPAM) and high-grade (PMCA) forms have different prognoses.