Primary peritoneal carcinoma (PPC) is a rare malignant neoplasm originating from peritoneal mesothelial cells, clinicopathologically very similar to high-grade serous ovarian carcinoma. Minimal or no tumor in the ovaries distinguishes PPC from ovarian carcinoma — GOG (Gynecologic Oncology Group) criteria are used for definition. Shows marked female predominance and is usually diagnosed in the postmenopausal period. Increased risk exists in BRCA1/BRCA2 mutation carriers and may develop even after prophylactic bilateral salpingo-oophorectomy. Clinical presentation is characterized by abdominal pain, distension, ascites, and elevated CA-125. CT shows diffuse peritoneal thickening, nodular implants, omental cake, and ascites. Treatment consists of cytoreductive surgery plus platinum-based chemotherapy (carboplatin + paclitaxel); treatment response and prognosis are similar to stage III-IV serous ovarian carcinoma. Five-year survival is 25-40%.
Age Range
50-80
Peak Age
65
Gender
Female predominant
Prevalence
Rare
Primary peritoneal carcinoma originates from mesothelial cells lining the peritoneal cavity. These cells are embryologically derived from coelomic epithelium — the same source that gives rise to ovarian surface epithelium and fallopian tube epithelium; this common embryological origin explains the clinicopathological similarity between PPC and high-grade serous ovarian carcinoma. Recent studies have shown that many 'peritoneal' carcinomas may actually originate from serous tubal intraepithelial carcinoma (STIC) lesions arising from tubal fimbriae. TP53 mutation is found in nearly all cases and the genomic profile is equivalent to high-grade serous ovarian carcinoma. BRCA1/2 mutations facilitate DNA damage accumulation and carcinogenesis through defective homologous recombination repair (HRD). Tumor cells demonstrate transcelomic spread through peritoneal fluid flow; therefore diffuse peritoneal, omental, and diaphragmatic involvement is characteristic. Omental cake formation is explained by preferential implantation of tumor cells at omental milk spots (lymphovascular aggregates). On imaging, enhancement pattern reflects tumor neovascularization — VEGF overexpression creates rich vascular bed providing prominent enhancement in solid components; ascites results from increased peritoneal capillary permeability.
Omental cake is diffuse infiltration of the greater omentum by tumor tissue replacing normal fat with solid, plaque-like thickening. On CT, it appears as a smooth or nodularly contoured, enhancing soft tissue mass between the transverse colon and anterior abdominal wall. Enhancement is more prominent in delayed phase. This finding is the most characteristic CT finding of peritoneal malignancies and indicates widespread peritoneal spread.
Portal venous phase CT shows diffuse peritoneal thickening and nodular implants. Irregular, nodular thickening (>5 mm) along the parietal peritoneum is characteristic. Implants show enhancement — mild in arterial phase, more prominent in portal venous and delayed phases reflecting the fibrosis-rich stroma. Nodular involvement of diaphragmatic surfaces, pelvic peritoneal reflections, and paracolic gutters is common. Mesenteric leaf thickening and retraction may be seen. Ascites is usually prominent and may be complicated (septated).
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Diffuse peritoneal nodular thickening with enhancing peritoneal implants and prominent ascites consistent with primary peritoneal carcinoma or peritoneal carcinomatosis.
Delayed phase shows diffuse omental thickening and solid mass formation (omental cake). Infiltration of the greater omentum with solid tumor tissue replacing normal fat shows as homogeneously or heterogeneously enhancing thickened omental plaque (>2 cm thickness). Enhancement becomes more prominent in delayed phase as contrast continues to diffuse into fibrous stroma. The inferior edge of omental cake is usually irregular and nodular; located between transverse colon and anterior abdominal wall. Necrotic areas may appear as low-density foci. Calcification is rare but may be seen post-treatment (chemotherapy).
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Diffuse omental thickening and solid mass formation (omental cake) with progressive delayed enhancement consistent with peritoneal malignancy.
On T2-weighted MRI, peritoneal implants show intermediate signal intensity — fibrous stroma is T2 hypointense while tumoral cellular component shows T2 intermediate-to-hyperintense signal. This mixed signal allows recognition of solid implants as nodular or plaque-like lesions on peritoneal surfaces. Ascites is markedly T2 hyperintense and the signal contrast with implants increases implant detection sensitivity. Diaphragmatic implants, pelvic peritoneal implants, and omental cake can be clearly evaluated. On DWI, implants show diffusion restriction (high DWI signal, low ADC) — DWI is superior to conventional sequences for detecting small implants.
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Nodular implants with intermediate T2 signal on peritoneal surfaces with prominent ascites showing diffusion restriction on DWI.
Arterial phase shows mild-to-moderate enhancement of peritoneal implants. Solid implants at soft tissue density appear as nodular projections from basal peritoneal surfaces. Central necrosis in large implants appears as low-density areas. Implants adjacent to vascular structures require evaluation of vascular invasion. Calculation of Peritoneal Carcinomatosis Index (PCI) is critical for surgical planning — tumor burden in each of 13 abdominal regions is scored.
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Mildly to moderately enhancing nodular implants on peritoneal surfaces in arterial phase; progressive enhancement should be assessed in delayed phase.
FDG PET-CT shows increased metabolic activity (FDG uptake) in peritoneal implants. SUVmax values typically range 3-15, depending on tumor grade (high-grade serous carcinoma shows highest SUV). Omental cake, diaphragmatic implants, and pelvic peritoneal involvement appear FDG-avid. Small (<1 cm) implants may not be detected due to PET spatial resolution (partial volume effect). For treatment response assessment, baseline and post-treatment SUV change is critical — metabolic response precedes morphological response. PET-CT may reveal extraperitoneal involvement not seen on CT.
Report Sentence
FDG PET-CT shows increased metabolic activity (SUVmax: [value]) in peritoneal implants consistent with metabolically active peritoneal malignancy.
B-mode ultrasonography shows prominent ascites with nodular thickening on peritoneal surfaces. Ascites is usually seen as free fluid in all abdominal quadrants; thin internal septations or debris suggest complicated ascites. Nodular irregularity on parietal peritoneum and diaphragmatic surfaces may be seen. Omental thickening appears as solid echogenic mass replacing normal hyperechoic omental fat. Doppler may detect vascularity in peritoneal nodules. US plays primary role in diagnostic paracentesis guidance and ascites monitoring; however sensitivity for small peritoneal implants is lower than CT and MRI.
Report Sentence
Prominent ascites with nodular peritoneal thickening on ultrasonography; further imaging recommended for suspected peritoneal malignancy.
Criteria
Most common subtype (90%). Universal TP53 mutation. Histologically equivalent to high-grade serous ovarian carcinoma. Solid-papillary growth pattern, psammoma bodies, high mitotic index. Markedly elevated CA-125. CT triad of diffuse peritoneal implants + omental cake + ascites is typical.
Distinct Features
GOG criteria distinguish from ovarian carcinoma: ovaries must be normal-sized or with superficial involvement, peritoneal involvement must clearly exceed ovarian involvement. Good response to platinum-based chemotherapy (70-80% response rate). PARP inhibitors (olaparib) provide additional benefit in BRCA mutation carriers. Prognosis similar to stage III-IV serous ovarian carcinoma.
Criteria
Rare subtype (5-10%). Carries KRAS/BRAF mutation, TP53 wild-type. Low mitotic index, indolent course. Less responsive to chemotherapy than high-grade type. Hormonal therapy (letrozole) and MEK inhibitors (trametinib) are treatment options.
Distinct Features
CT shows calcified peritoneal implants more frequently (due to psammoma bodies) — calcification is rare in high-grade type. Slower growth may result in prolonged stable disease. Prognosis better than high-grade type but late relapse rate is high.
Criteria
Rare clinical scenario. PPC with diffuse peritoneal thickening pattern mimicking peritoneal mesothelioma. PPC more likely without asbestos exposure history. Immunohistochemistry critical for differential: PPC is PAX8+, WT1+, calretinin-; mesothelioma is calretinin+, PAX8-, D2-40+.
Distinct Features
Mesothelioma typically starts with pleural thickening with secondary peritoneal involvement; PPC has primary peritoneal involvement. CA-125 is usually normal or mildly elevated in mesothelioma vs markedly elevated in PPC. CT may show pleural plaques and calcifications in mesothelioma; pleural involvement in PPC is a late finding. Asbestos exposure history strongly supports mesothelioma.
Distinguishing Feature
Peritoneal carcinomatosis represents secondary peritoneal spread from a known primary tumor (ovarian, gastric, colorectal, pancreatic). PPC is distinguished by minimal/no ovarian tumor and absence of known extraperitoneal primary. In carcinomatosis, characteristic imaging findings of the primary tumor are present. Both conditions show similar peritoneal implants, omental cake, and ascites.
Distinguishing Feature
Peritoneal mesothelioma is associated with asbestos exposure and shows diffuse peritoneal thickening. Differences from PPC: CA-125 usually normal in mesothelioma vs markedly elevated in PPC; pleural plaques may be seen in mesothelioma; immunohistochemistry shows calretinin+/PAX8- (mesothelioma) vs PAX8+/calretinin- (PPC). Both may show diffuse peritoneal thickening but omental cake and ascites are more prominent in PPC.
Distinguishing Feature
Peritoneal lymphomatosis is peritoneal spread of lymphoma showing diffuse peritoneal thickening. Differences from PPC: lymphomatosis has more prominent mesenteric and retroperitoneal lymphadenopathy; CA-125 usually normal; very high FDG uptake on PET-CT (SUVmax >10); elevated LDH and beta-2 microglobulin support lymphoma. Lymphomatosis tends to show more homogeneous and smooth peritoneal thickening; PPC shows more nodular and irregular thickening.
Distinguishing Feature
Peritoneal tuberculosis shows peritoneal thickening, ascites, and omental involvement due to granulomatous inflammation — may mimic PPC. TB ascites has high protein and ADA (adenosine deaminase); ADA is normal in PPC. TB peritoneal thickening is smoother and more uniform; PPC is more nodular and irregular. Central necrosis in mesenteric lymphadenopathy (rim enhancement) is characteristic of TB. Migration from endemic region and immunosuppression support TB.
Urgency
urgentManagement
surgicalBiopsy
NeededFollow-up
3-monthPrimary peritoneal carcinoma is diagnosed as advanced-stage malignancy requiring aggressive multidisciplinary treatment. Diagnosis is confirmed by peritoneal biopsy or pathological examination during cytoreductive surgery — GOG criteria distinguish from ovarian carcinoma. Treatment: (1) Primary cytoreductive surgery — optimal cytoreduction (residual tumor <1 cm) significantly improves survival; PCI score guides surgical planning. (2) Platinum-based chemotherapy (carboplatin + paclitaxel 6 cycles) — 70-80% response rate. (3) Neoadjuvant chemotherapy for suboptimal cytoreduction candidates. (4) PARP inhibitors (olaparib, niraparib) as maintenance in BRCA carriers. (5) Bevacizumab may be combined. Follow-up: CA-125 and CT at 3-month intervals. Five-year survival 25-40%.
Primary peritoneal carcinoma is treated with the same protocol as ovarian cancer (debulking surgery + cisplatin-based chemotherapy). Prognosis is similar to stage IV ovarian cancer. CA-125 monitoring assesses treatment response. Increased risk in BRCA mutation carriers.