Peritoneal Carcinomatosis
Peritoneal carcinomatosis describes the spread of cancer to the peritoneum, the thin membrane lining the abdominal cavity and covering the organs within it. Rather than travelling through the bloodstream to form a single distant deposit, tumour cells shed from a primary cancer circulate within peritoneal fluid and implant across peritoneal surfaces, producing widely distributed nodules, plaques, and sheets of tumour. It arises most often from ovarian, colorectal, gastric, appendiceal, and pancreatic cancers, and less commonly from primary peritoneal tumours such as mesothelioma. Once regarded almost uniformly as end-stage disease, it is now approached differently depending on the tumour type and the extent of spread, with selected patients treated aggressively and with intent to achieve long-term control.
What is it?
The peritoneum is a thin, continuous membrane that lines the inside of the abdominal wall and folds over to cover the abdominal organs. Between its two layers lies a small volume of lubricating fluid that allows the bowel to move freely. The mesentery and omentum are specialised extensions of this same membrane. In total the peritoneum represents a surface area comparable to that of the skin, which is part of why disease affecting it can become so extensive.
Peritoneal carcinomatosis occurs when cancer cells establish themselves across this surface. The mechanism is distinct from the more familiar pattern of metastasis through the bloodstream. Cells shed from a primary tumour that has reached the outer surface of an organ enter the peritoneal fluid, are carried along predictable circulation pathways driven by gravity, respiration, and bowel movement, and settle at sites where fluid pools or is absorbed—the pelvis, the right upper abdomen beneath the diaphragm, the paracolic gutters, and above all the omentum, whose immune-rich structure and fat-dense environment favour tumour establishment. This is why deposits appear in characteristic distributions rather than randomly.
The term itself deserves brief comment. Peritoneal carcinomatosis is widely used and appears in most reports and literature, though peritoneal metastases and peritoneal surface malignancy are increasingly preferred, partly because carcinomatosis has historically carried an implication of untreatable terminal disease that no longer fits every situation. Patients encountering the word should know that its prognostic meaning now varies considerably by tumour type and extent.
The primary sources differ in both frequency and behaviour.
Ovarian cancer is the most characteristic cause. It spreads across peritoneal surfaces early and extensively, and because early ovarian cancer produces few specific symptoms, the majority of women are diagnosed only once peritoneal disease and ascites are established. Importantly, ovarian cancer with peritoneal spread often responds well to a combination of surgery and chemotherapy, and prolonged remissions are achievable—an outcome quite different from peritoneal spread of many other tumours.
Colorectal cancer produces peritoneal spread in a minority of patients, more often with right-sided tumours, mucinous histology, tumours that have perforated, and locally advanced disease. It may be present at diagnosis or emerge as an isolated site of recurrence after apparently curative surgery.
Gastric cancer spreads to the peritoneum frequently, particularly the diffuse type, and peritoneal involvement is a major determinant of prognosis. Deposits in the ovaries from gastric or other gastrointestinal primaries are termed Krukenberg tumours and can be mistaken for a primary ovarian cancer.
Appendiceal mucinous neoplasms produce pseudomyxoma peritonei, in which mucin-producing cells disseminate through the abdomen and generate large volumes of gelatinous material. This condition behaves quite differently from typical carcinomatosis—it is often low grade, spreads by redistribution rather than invasion, and can be controlled for many years with appropriate specialist treatment.
Pancreatic, biliary, small bowel, uterine, and cervical cancers all account for a proportion of cases, and breast cancer, particularly lobular carcinoma, can spread to the peritoneum.
Primary peritoneal malignancies arise from the peritoneum itself. Peritoneal mesothelioma, linked to asbestos exposure, is rare but important because it is treatable with specialist approaches. Primary peritoneal carcinoma is closely related to ovarian cancer, shares its biology and treatment, and is diagnosed when disease is distributed across the peritoneum without a dominant ovarian mass.
The imaging appearances form a recognisable set. Ascites—fluid accumulating in the abdominal cavity—is present in most cases and often the first abnormality noticed. Peritoneal thickening and enhancement of the lining membrane may be subtle. Discrete nodules appear along peritoneal surfaces, in the paracolic gutters, and beneath the diaphragm. Omental caking, a thickened sheet of tumour replacing normal omental fat and lying between the abdominal wall and bowel, is a hallmark finding. Mesenteric involvement produces stranding, nodularity, and a stellate or pleated appearance. Bowel wall thickening and tethering can produce obstruction. Mucinous disease produces low-density collections that scallop the surface of the liver and spleen, a distinctive appearance in pseudomyxoma peritonei.
Symptoms are frequently vague and are a substantial reason for delayed diagnosis. The most common is progressive abdominal distension caused by ascites, often accompanied by bloating, early satiety, reduced appetite, and a sense of fullness or heaviness. The combination of persistent bloating, feeling full quickly, abdominal or pelvic pain, and urinary urgency or frequency lasting more than a few weeks is a recognised presentation of ovarian cancer and warrants evaluation, particularly in women over 50. Weight loss occurring while the abdomen enlarges is a specific and important pattern.
As disease progresses, bowel obstruction becomes a common and clinically dominant problem, arising from tumour encasing and tethering bowel loops rather than from a single blocking mass. It may develop gradually, with intermittent crampy pain and vomiting before becoming complete. Breathlessness arises when a distended abdomen limits diaphragm movement or when pleural fluid accumulates. Leg swelling can result from venous or lymphatic compression. Fatigue, poor nutrition, and progressive weakness accompany advanced disease.
Diagnosis combines imaging, fluid or tissue sampling, and assessment of the extent of disease.
CT of the abdomen and pelvis with intravenous contrast is the primary test and is used for initial assessment, identification of the likely primary tumour, and evaluation of disease elsewhere. A recognised limitation is that CT substantially underestimates small-volume peritoneal disease, and deposits under about 5 mm are frequently invisible. This matters when surgery is being considered, since the disease found at operation can exceed what imaging predicted.
MRI with diffusion-weighted sequences is more sensitive for small peritoneal deposits and is used in specialist centres for mapping disease before cytoreductive surgery.
PET-CT contributes to staging and to detecting disease outside the abdomen. Its major limitation in this context is that mucinous and low-grade tumours, including pseudomyxoma peritonei and some ovarian and colorectal deposits, are often not metabolically avid and can be underestimated.
Paracentesis—drainage of ascitic fluid—serves both diagnostic and therapeutic purposes. Cytology can confirm malignant cells, though sensitivity is moderate and a negative result does not exclude the diagnosis. The volume of material obtained is often insufficient for the molecular and genomic testing that increasingly determines treatment.
Image-guided core needle biopsy of an accessible deposit provides more tissue and is frequently the preferred diagnostic step. Omental deposits are often the easiest and safest target because of their superficial position.
Diagnostic laparoscopy allows direct visualisation of the peritoneal surfaces, targeted biopsy, and accurate assessment of disease extent. Extent is commonly quantified using the Peritoneal Cancer Index, which scores tumour size across thirteen abdominal regions to produce a total that informs whether complete surgical removal is realistic. This assessment is central to selecting patients for cytoreductive surgery.
Tumour markers including CA-125, CEA, and CA 19-9 assist with assessment and are useful for monitoring response, though none is diagnostic alone. CA-125 in particular is elevated in many benign conditions that irritate the peritoneum, including endometriosis, liver disease, heart failure, and infection.
Important to Know
The most important development in this field is that peritoneal carcinomatosis is no longer treated as a single prognosis. Outcomes differ substantially depending on the primary tumour, the volume and distribution of disease, whether it can be completely removed, and how it responds to systemic therapy. Care is coordinated by specialist multidisciplinary teams including medical oncology, gynaecologic oncology, surgical oncology, radiology, pathology, and palliative care.
Systemic therapy forms the foundation of treatment for most patients. The specific regimen depends on the primary tumour and increasingly on molecular characteristics identified through genomic testing, which may open access to targeted agents or immunotherapy. Response to systemic treatment is also used as a guide to whether more aggressive local approaches are appropriate.
For ovarian and primary peritoneal cancer, cytoreductive surgery combined with platinum-based chemotherapy is the standard of care. The objective is removal of all visible disease, since the completeness of cytoreduction is one of the strongest predictors of outcome. This typically includes omentectomy, removal of the ovaries and uterus, and resection of peritoneal deposits, sometimes with bowel resection or diaphragmatic stripping. Whether surgery is performed first or after several cycles of neoadjuvant chemotherapy depends on disease extent and patient fitness. Genetic and molecular testing, including BRCA mutation and homologous recombination deficiency status, now directs maintenance therapy with PARP inhibitors, which has meaningfully improved outcomes.
For appendiceal, colorectal, and mesothelial primaries, selected patients may benefit from cytoreductive surgery combined with heated intraperitoneal chemotherapy, in which warmed chemotherapy is circulated within the abdomen at the time of operation. Patient selection is careful and depends on tumour type, the Peritoneal Cancer Index, the absence of significant disease outside the abdomen, and fitness for a long and demanding operation. The evidence supporting this approach is strongest for pseudomyxoma peritonei from appendiceal mucinous neoplasms and for peritoneal mesothelioma, where prolonged survival is achievable and the approach has changed the outlook considerably. For colorectal peritoneal metastases, complete cytoreduction clearly benefits selected patients, while the additional contribution of the heated chemotherapy component has been questioned by trial evidence and remains an area of active investigation and differing practice. For gastric cancer, the role remains investigational in most settings.
These operations are performed at specialised centres and are substantial undertakings, often lasting many hours with a recovery measured in months. Referral for assessment is worthwhile when the situation might qualify, and a specialist opinion is reasonable to seek even when a general recommendation has been that no surgical option exists, since practice varies and the field continues to change.
Symptom-directed treatment is central rather than secondary. Ascites causing distension and breathlessness can be drained by paracentesis, which often provides substantial and immediate relief. For recurrent ascites, an indwelling tunnelled drain allows drainage at home and avoids repeated hospital visits.
Malignant bowel obstruction requires careful, individualised decision-making. Options range from surgery and endoscopic stenting to a venting gastrostomy tube, and medical management with antiemetics, corticosteroids, and antisecretory agents such as octreotide. Because obstruction in this setting is often at multiple levels, surgery is not always feasible or beneficial, and the least invasive option that achieves comfort is frequently the right one.
Nutritional support matters throughout. Reduced intake from early satiety, obstruction, and treatment effects contributes to weakness and to reduced tolerance of therapy. Dietitian input, and in selected cases parenteral nutrition, forms part of care.
Palliative care involvement improves quality of life and, in several cancer types, is associated with outcomes at least as good as delayed involvement. It is appropriately introduced alongside active treatment rather than only when treatment ends, and encompasses symptom control, decision support, and planning.
Follow-up follows protocol-based oncologic monitoring with imaging and, where informative, tumour markers, adapted to the tumour type and treatment given.
Care is coordinated by specialist multidisciplinary teams, and second opinions at peritoneal surface malignancy centres are reasonable for patients whose disease might be amenable to specialist surgical approaches. Imaging, laboratory, and clinical findings are interpreted alongside the patient’s symptoms, examination, tumour biology, and overall goals rather than in isolation.
Patient education plays an important role. Understanding that peritoneal involvement does not carry a single fixed prognosis, that some forms respond very well to treatment, why the extent of disease is assessed so carefully before surgery is offered, why drainage of ascites can be repeated safely, what a referral to a specialist peritoneal centre involves, and which symptoms warrant prompt contact with the treating team all contribute to appropriate care and informed decision-making.
Red flag symptoms include progressive abdominal distension or rapid abdominal swelling; persistent bloating with early satiety, particularly when accompanied by pelvic or abdominal pain or urinary urgency lasting more than a few weeks; unintentional weight loss with an enlarging abdomen; crampy abdominal pain with vomiting, distension, and inability to pass gas or stool; new or worsening breathlessness; fever with abdominal pain, which may indicate infection of ascitic fluid; jaundice; new leg swelling; and severe or uncontrolled pain. These warrant prompt medical evaluation, as they may indicate bowel obstruction, accumulating ascites, infection, or disease progression requiring a change in treatment.