Peritoneal Metastases

Micronodular pattern: Earliest findings may be subtle peritoneal thickening and hyperenhancement

– Stranding and nodularity in mesentery may result in “pleated” or “stellate” appearance

Nodular pattern: More discrete nodules may be present measuring > 5 mm in size

Omental caking: Discrete omental masses coalesce into larger conglomerate omental masses

Thickening and nodularity along surface of bowel may reflect tumor implants on serosal surface

Ascites (often loculated) may be present

• MR: Sensitivity of MR is comparable to, or greater than, CT for implants > 1 cm, but limited for small implants

Diffusion-weighted imaging (DWI) may ↑ sensitivity

Tumor implants typically T1WI hypointense, intermediate signal on T2WI, and variable enhancement on T1WI C+ images depending on type of tumor

TOP DIFFERENTIAL DIAGNOSES

• Tuberculous peritonitis

• Abdominal mesothelioma

• Peritoneal lymphomatosis

• Primary peritoneal serous papillary carcinoma

• Pseudomyxoma peritonei

PATHOLOGY

• Usually due to peritoneal spread of surface epithelium tumors, although hematogenous spread also possible

• Most common: Ovarian and GI adenocarcinomas (gastric, colorectal, pancreas, appendix, gallbladder)

CLINICAL ISSUES

• Common complications: Bowel and ureteral obstruction

(Left) Axial anatomic rendering of peritoneal metastases. Note the anterior omental cake and serosal implants .

(Right) Axial CECT demonstrates extensive omental caking in the anterior pelvic omentum, compatible with carcinomatosis. Notice the presence of ascites , which is commonly associated with carcinomatosis.

(Left) Axial T1 C+ FS MR in the same patient demonstrates enhancing soft tissue in the omentum. Although debatable, some sources suggest that MR may have slightly increased sensitivity for carcinomatosis compared to CT.

(Right) Axial PET/CT image in the same patient demonstrates that the omental caking shows avid FDG uptake. No primary tumor was discovered in this case, and this was found to be peritoneal serous papillary carcinoma.

TERMINOLOGY

Synonyms

• Peritoneal carcinomatosis, peritoneal implants, omental caking

Definitions

• Metastatic disease to omentum, peritoneal surface, peritoneal ligaments, or mesentery

IMAGING

General Features

• Best diagnostic clue

Peritoneal stranding, nodularity, omental caking, or complex ascites in a patient with a known history of malignancy

• Location

Peritoneum, mesentery, peritoneal ligaments

• Size

Variable, ranging from tiny micronodules (< 5 mm) to large, confluent omental caking

• Morphology

Nodular, plaque-like, or large omental mass

Radiographic Findings

• Radiography

Plain film findings of significant ascites

– Medial displacement of cecum in 90% of patients

– Pelvic “dog’s ear” in 90% of patients

– Medial displacement of lateral liver edge (Hellmer sign) in 80% of patients

– Bulging of flanks, central displacement of bowel loops, indistinct psoas margin

Plain film findings of small bowel obstruction (SBO)

– Dilated small bowel > 3 cm

– Fluid-fluid levels in small bowel on upright film

– String-of-pearls sign

– Collapsed gasless colon

CT Findings

• CT has limited sensitivity for peritoneal metastases (25-90%), particularly implants measuring < 1 cm (7-50%)

Particularly difficult to identify tumor implants in certain anatomic locations (liver hilum, bladder dome, subdiaphragmatic positions, mesenteric root, lesser omentum, serosal surface of small bowel)

Utilizing positive oral contrast media may be helpful in better differentiating tumor implants from adjacent bowel loops

• 3 primary patterns of carcinomatosis on imaging

Micronodular pattern

– Earliest findings may be subtle peritoneal thickening and hyperenhancement ± nodularity

– Omentum often easiest site to appreciate carcinomatosis with nodularity, stranding, and infiltration

Other common sites include paracolic gutters, along surface of liver, along undersurface of diaphragms, and pelvic cul-de-sac

– Stranding and nodularity in mesentery may result in a “pleated” or “stellate” appearance

Nodular pattern

– More discrete peritoneal nodules measuring > 5 mm in size

Omental caking

– Omental nodules coalesce into larger conglomerate omental masses

• Thickening and nodularity along surface of bowel may reflect tumor implants along serosal surface of bowel

Thickened bowel loops may appear encapsulated as result of extensive serosal metastases

• Tumor implant density will vary based on histology of primary malignancy, with most hypovascular tumors appearing as solid, hypodense soft tissue nodules

Mucinous tumors may appear as low density or cystic tumor implants

Hypervascular tumors such as renal cell carcinoma may have hyperenhancing peritoneal metastases, which are more conspicuous on arterial phase imaging

Calcifications may be associated with certain mucinous neoplasms

• Ascites usually present with loculated ascites common in cases with advanced peritoneal carcinomatosis

• Tumor implants may cause bowel obstruction with thickening and nodularity at site of transition from dilated to nondilated bowel

MR Findings

• Sensitivity of MR is comparable to, and perhaps even greater than, CT for implants > 1 cm

Fat suppression increases conspicuity of tumor implants in omentum and mesentery

Diffusion-weighted imaging (DWI) offers increased sensitivity for tumor implants, which demonstrate restricted diffusion

Sensitivity of MR, even with inclusion of DWI, is still limited for small implants < 1 cm in size

• Caution needed when interpreting SSFSE/HASTE images, as bulk motion of fluid within ascites can lead to signal voids that might be confused for tumor implants

• Tumor implants typically are hypointense on T1WI, intermediate to high signal on T2WI, and demonstrate variable enhancement on T1WI C+ images depending on type of tumor