Omentum and Peritoneum

The omental fat and peritoneal serosa may be involved by various inflammatory and neoplastic disorders.

Inflammation: acute due to appendicitis or a perforated viscus (GU, diverticulitis), or granulomatous, e.g., tuberculosis, fungal peritonitis (chronic ambulatory peritoneal dialysis (CAPD)) or after previous surgery. CAPD can also be associated with the rare condition of fibrous or scle-rosing peritonitis.

Infarction: spontaneous, idiopathic omental infarction in the right iliac fossa mimicking acute appendicitis (rare), or more commonly, infarction of an appendix epiploica (pericolonic fat tag) which may then undergo saponification and calcification. The latter are also seen as a consequence of acute pancreatitis or abdominal trauma. Omentum incarcerated within a hernial sac may also undergo ischaemia.

Keratin granulomas: an unusual finding most often related to treatment and follow-up of a previous gynaecological cancer, e.g., endometrioid adenocarcinoma of the uterus or vulval squa-mous carcinoma.

Peritoneal inclusion cysts: relatively common, solitary or multiple and should be distinguished from lymphangitic cysts (cytokeratin negative endothelial lining) and well-differentiated multicystic peritoneal mesothelioma. The latter is rare, occurring on the surfaces of the uterus, ovary, bladder, rectum and pouch of Douglas with potential for recurrence and invasion locally into retroperitoneum, bowel mesentery and wall. Some have a previous history of surgery, endometriosis or pelvic inflammatory disease.

Mesothelial proliferation: other mesothelial proliferations include:

• Mesothelial hyperplasia - commonly seen as a reactive phenomenon in omentum adherent to an inflammatory or neoplastic abdominopelvic lesion and within a hernia. Sometimes it is florid and distinction from mesothelioma can be difficult.

• Well-differentiated papillary peritoneal mesothelioma - rare, with most being an incidental finding at hysterectomy, usually localised and benign but occasionally diffuse.

• Diffuse malignant mesothelioma - epithelioid/sarcomatoid or mixed in pattern and a strong association with occupational asbestos exposure and spread from a primary pleural mesothelioma. Prognosis is poor with the majority of patients dying from their disease within months or 1-3 years. Of very limited suitability for resection.

Peritoneal serous epithelial proliferation: strongly associated with ovarian serous borderline tumours and either regarded as benign (endosalpingiosis), potentially progressive (invasive proliferating implants) or frankly malignant - primary peritoneal carcinoma. The former two conditions are microscopic findings while the latter is an ovarian serous-type adenocarcinoma with extensive peritoneal disease but minimal ovarian involvement.

Pseudomyxoma peritonei: characterised by filling of the peritoneal cavity with abundant mucin in which there is a component of either variably bland, atypical or frankly malignant epithelium. Prognosis is poor as it is refractory to treatment, slowly progressive and leads to bowel obstruction. There is a strong association with appendiceal and ovarian mucinous tumours and occasionally secondary colorectal or pancreaticobiliary neoplasms. Appendicectomy should be considered in the presence of bilateral cystic ovarian tumours associated with peritoneal disease. Secondary adenocarcinoma: staging and therapy are considered:

• Staging - diagnosed either by peritoneal aspiration cytology, laparoscopic biopsy, or open biopsy with frozen section at exploratory laparotomy as a prequel to consideration of suitability for operative resection of an abdominopelvic cancer. Postoperative pathological staging of ovarian carcinoma also partly relates to the size (< or >2 cm) of the peritoneal deposits - it requires removal of the primary ovarian lesion, biopsy of the contralateral ovary, omentum and peritoneum, and peritoneal washings for cytology if ascitic fluid is not present. • Therapy - tumour debulking or cytoreductive surgery of extensive omental disease is an important initial step prior to adjuvant chemotherapy in ovarian and other abdominopelvic cancers.

Other cancers: these are rare, e.g., intra-abdominal desmoplastic small round cell tumour -divergent cellular differentiation, aggressive, pelvis and abdomen of young people.

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