Peritoneal disease: key imaging findings that help in the differential diagnosis.

BJR Received: 15 March 2021 © 2022 The Authors. Published by the British Institute of Radiology Revised: 12 October 2021 https://doi.org/10.1259/bjr.20210346 Accepted: 09 November 2021 Cite this article as: Campos NMF, Almeida V, Curvo Semedo L. Peritoneal disease: key imaging findings that help in the differential diagnosis. Br J Radiol 2022; 95: 20210346. REVIEW ARTICLE Peritoneal disease: key imaging findings that help in the differential diagnosis 1 NUNO M F CAMPOS, MD, 2VÂNIA ALMEIDA, MD and 1,3LUÍS CURVO SEMEDO, MD, PhD 1 Department of Medical Imaging, Coimbra Hospital and University Centre, Coimbra, Portugal Department of Pathology, Coimbra Hospital and University Centre, Coimbra, Portugal 3 Faculty of Medicine, University of Coimbra, Coimbra, Portugal 2 Address correspondence to: Luís Curvo Semedo E-mail: ABSTRACT The peritoneum is a unique serosal membrane, which can be the site of primary tumors and, more commonly, secondary pathologic processes. Peritoneal carcinomatosis is the most common malignant condition to affect the peritoneal cavity, and the radiologist plays an important role in making the diagnosis and assessing the extent of disease, especially in sites that may hinder surgery. In this review, we address the role of the radiologist in the setting of peritoneal pathology, focusing on peritoneal carcinomatosis as this is the predominant malignant process, followed by revising typical imaging findings that can guide the differential diagnosis. We review the most frequent primary and secondary peritoneal tumor and tumor-like lesions, proposing a systemic approach based on clinical history and morphological appearance, namely distinguishing predominantly cystic from solid lesions, both solitary and multiple. INTRODUCTION The peritoneum is a unique serosal membrane, which can be the site of primary tumors and, more commonly, secondary pathologic conditions. The peritoneum has a complex anatomy, which dictates the distribution and flow of fluid within the abdomen and pelvis and consequently the anatomic location and distribution of pathology. When evaluating peritoneal masses, metastatic disease should be accounted as the most common neoplastic process that involves the peritoneal cavity. However, the peritoneum is frequently the site of secondary benign processes and other tumor-like lesions, or more rarely of primary peritoneal tumors, and therefore the radiologist should be aware of their main imaging features, thus allowing a discussion of the most likely differential diagnoses. The purpose of this article is to review the spectrum of peritoneal pathologic conditions, focusing on imaging characteristics and possible differential diagnoses. PERITONEAL ANATOMY The peritoneum is a membrane which consists of a single layer of simple low-cuboidal epithelium called a mesothelium. The peritoneum that lines the abdominal wall is the parietal peritoneum, whereas the peritoneum that covers the organs is the visceral peritoneum. The peritoneal cavity is a potential space between the parietal peritoneum and the visceral peritoneum.1 The peritoneal ligaments and mesenteries are double folds of peritoneum that suspend and support the intraperitoneal organs and divide the peritoneal cavity into interconnected compartments. The peritoneal cavity normally contains only a small amount of sterile fluid similar to plasma, and the patterns of circulation and clearance of this fluid from the peritoneum is one of the major factors that dictate the location of disease.1 The peritoneal fluid usually circulates upward to the subdiaphragmatic spaces where the subphrenic submesothelial lymphatics provide most of the lymphatic clearance from the peritoneal cavity. The right subphrenic submesothelial lymphatics provide most of the lymphatic clearance from the peritoneal cavity, making this a common site of disease.2,3 The upward movement of peritoneal fluid to reach the undersurface of the hemidiaphragm is due to the fluctuations in intra-abdominal pressure during respiration and the intestinal peristalsis. In pathologic conditions that result in ascites, relative stasis of fluid at specific sites promotes seeding of malignant cells or development of infection at these locations: the right paracolic gutter; the ileocolic region; the root of the sigmoid mesocolon; and the peritoneal recesses of the pelvis2,3 (Figure 1). Campos et al BJR Figure 1. CT of the abdomen in a patient with voluminous ascitis. The phrenicocolic ligament (arrow in a and c) partially separates the left paracolic gutter from the left subdiaphragmatic space, limiting the spread of peritoneal disease in the left upper quadrant. The root of the small bowel mesentery (arrowhead in b) divides the inframesocolic compartment into the right and left infracolic spaces. The predominant sites of ascitic fluid accumulation (* in c) are prone to seeding of malignant cells or development of infection. IMAGING IN PERITONEAL DISEASE On CT, the normal peritoneum can occasionally be recognized as a very fine uniform linear structure, although usually it is not visible. Thus, visualization of peritoneal lining on CT scans should raise the suspicion of peritoneal thickening. Benign conditions such as infectious or inflammatory peritonitis commonly cause smooth peritoneal thickening, compared to more irregular, nodular and often discontinuous peritoneal thickening in malignancy. Although CT is still the imaging modality most used for evaluating peritoneal pathology, MRI and positron- emission tomography (PET)-CT have some advantages over the former technique. Therefore, at present, MRI could be regarded as the primary imaging tool in the evaluation of patients with peritoneal disease, especially suspected carcinomatosis, providing the highest sensitivity for the detection of peritoneal tumors. It is, however, less available, with many centers using it only for selected patients, particularly those with indeterminate or equivocal involvement of mesentery and small bowel on CT.4 The main advantage of PET-CT is the fact that it is a whole body imaging technique that can detect distant metastases elsewhere. Its major limitations include radiation exposure, higher cost, limited depiction of small tumors (current spatial resolution: 4 mm) and lower detection rate of mucinous carcinomas by 18 F-FDG-PET.5,6 The role of ultrasound imaging is limited in this setting. However, this imaging modality is often the first used when peritoneal disease is discovered incidentally, and it remains one of the diagnostic techniques for image-guided biopsy to obtain a histological diagnosis.7 PERITONEAL PATHOLOGY The peritoneum is the site of both neoplastic and non-neoplastic pathology. The most frequent involvement occurs secondarily, 2 of 12 birpublications.org/bjr either in an acute setting (surgery complications, bowel perforation) or more chronic conditions (carcinomatosis, ascites secondary to portal hypertension). Nonethel (...truncated)