Pseudomyxoma peritonei refers to syndrome of progressive intraperitoneal accumulation of mucinous ascites related to a mucin-producing neoplasm. It is most commonly caused by a mucinous tumor of the appendix .
Much less commonly, mucinous tumors of colon, rectum, stomach, pancreas, and urachus are implicated. There is some ongoing contention as to whether primary ovarian tumors are a frequent source in their own right, or whether in these cases the appendix is the primary site and the ovarian lesion is metastatic .
The terminology regarding pseudomyxoma peritonei can be a bit confusing, and continues to evolve over time. In 2016, a consensus group, the Peritoneal Surface Oncology Group International, published guidelines for classification for mucinous appendiceal tumors and associated peritoneal disease (see appendiceal mucinous neoplasm article).
An "appendiceal mucocele" is a descriptive term which refers to the appearance of a dilated mucin-filled appendix. Although this is the imaging manifestation of appendiceal mucinous tumors, appendiceal mucoceles may also represent non-neoplastic etiologies such as mucosal hyperplasia or retention cyst related to luminal obstruction .
Pseudomyxoma peritonei (of appendiceal origin) is due to invasion or rupture of the appendix from a mucin-secreting appendiceal tumor.
It is important to understand that pseudomyxoma peritonei represents a spectrum of disease. The characteristic mucinous ascites is composed of acellular mucin and a variable amount of neoplastic epithelial cells. Indeed, pseudomyxoma peritonei is not a histologic diagnosis - the label merely implies grossly mucinous intraperitoneal deposits .
The remarkable feature of pseudomyxoma peritonei is that this neoplastic, progressive process often arises from a seemingly benign or well-differentiated primary tumor . However once there is intraperitoneal spread, the mucinous deposits tend to follow routes of normal peritoneal fluid flow, spreading along the pelvis, paracolic gutters, liver capsule, and omentum, while often sparing mobile organs, e.g. small bowel .
Pseudomyxoma peritonei may be divided into two pathological subtypes which have etiological and prognostic significance :
- peritoneal adenomucinosis
- a peritoneal neoplasm composed largely of mucin associated with fibrosis with minimal cytological atypia and mitoses
- the primary tumor is generally an adenoma
- peritoneal mucinous carcinoma
- characterized by proliferative epithelium, glands, nests, or individual cells with marked cytologic atypia
- the primary is a mucinous adenocarcinoma
Not all cases fit neatly into these categories, and many patients have intermediate or discordant features.
However, a long term follow-up study of 109 patients found 5-year survival rates were markedly different: 75% for the adenomucinosis group and 14% for the mucinous carcinoma group. The intermediate/discordant group 5 year survival was 50%.
Pseudomyxoma peritonei is characterized by loculated collections of fluid which accumulate along peritoneal surfaces, classically resulting in a scalloped appearance of coated abdominal organs and omental caking. An appendiceal mucocele (representing a mucinous appendiceal tumor) may be visualized.
- since appendiceal tumors are most common etiology, early peritoneal disease may be limited to the right lower quadrant abdomen
- deposits tend to localize to site of physiologic lymphatic absorption of ascites (omentum, underneath right hemidiaphragm), and dependent areas (e.g. paracolic gutters, right retrohepatic space, lower pelvis)
- deposits tend to spare the more mobile small bowel until later in disease, when it eventually causes bowel obstruction
May show evidence of ascites with centrally displaced small bowel loops and scattered punctate or curvilinear calcifications.
- echogenic peritoneal masses or ascites with echogenic particles which (unlike other forms of particulate ascites such as hemoperitoneum or pus in the peritoneum) do not move
- small bowel loops displaced medially
- may show scalloping of the liver, spleen and at times other organs
- simple or loculated low attenuation fluid throughout intraperitoneal spaces, omentum, and mesentery
- scalloping of visceral surfaces, particularly the liver
- often with scattered (curvilinear or punctate) calcifications
- tends to remain localized to peritoneal cavity - thoracic and nodal lesions metastases uncommon
Reported signal characteristics of the collections include
- T1: typically low signal
- T2: typically high signal
- T1 C+ (Gd): may show enhancement
Treatment and prognosis
This is a progressive and often fatal disease. Recurrent bowel obstructions are commonly due to the fibrosis and adhesions in advanced disease .
Treatment is surgical debulking, followed by infusion of intraperitoneal chemotherapy. Aggressive surgical treatment has been considered based on the calculation of the peritoneal cancer index (PCI), which is a prognostic indicator based on the amount of tumor found at laparoscopy. This index has been performed using CT or MRI .
General imaging differential considerations include: