Peritoneal carcinomatosis (PC) is a well-known condition that indicates the presence of metastatic malignant tumor cells in the peritoneal cavity, which harbors a poor prognosis in patients with malignant neoplasms [1, 2]. Various malignancies, including those of the stomach, ovary, small bowel, appendix, pancreatico-biliary tracts, and breast, metastasize to the peritoneal surface [2, 3]. In addition to PC, a variety of tumorous or inflammatory conditions can occur in the peritoneal cavity [3, 4]. Patients with peritoneal spread of malignancy have only palliative treatment options, but patients with other conditions require either no treatment or different treatment strategies [2, 5, 6]. Therefore, for patient management, it is clinically important to discriminate PC from other conditions. Computed tomography (CT) is a relatively inexpensive and easily accessible modality commonly used to evaluate abdominal disorders. Furthermore, CT can reveal previously undetected PC, but radiologists must exercise caution because other disease entities can mimic PC on CT  Thus, the purpose of this essay is to assist in the differentiation of PC from its mimics through CT findings.
The typical CT findings of PC are multifocal discrete nodules to infiltrative masses in the peritoneal cavity, omental haziness, ascites, and peritoneal thickening, nodularity, and enhancement (Figure 1) . Intraperitoneal seeding depends on the flow of the peritoneal fluid and is determined by the ligaments and the mesentery attached to the peritoneum (Figure 2). The frequent locations where peritoneal seeding occurs are as follows: peritoneal reflexion of the pelvis (Figure 3A), lower small bowel mesentery (Figure 3B), sigmoid mesocolon (Figure 3C), right paracolic gutter (Figure 3D), and right subphrenic space (Figure 3E) . The omentum, which absorbs the peritoneal fluid, contains numerous macrophages and lymphocytes, thus malignant tumor cells frequently settle in the omentum (Figure 3F) [3, 7]. The ovary is another common seeding site in PC, and a metastatic lesion at the ovary is called a Krukenberg tumor. When solid, bilateral, ovarian masses containing intratumoral cystic component are detected on CT, Krukenberg tumor should be considered, even if no primary malignancy is observed (Figure 4) .
Pseudomyxoma peritonei (PP) is a clinical or radiological term, not a pathological term, and refers to a state in which a large amount of thick mucinous material is spread to the peritoneal cavity . PP frequently arises from appendiceal mucinous neoplasms in men and ovarian mucinous tumors in women, and the fluid may or may not contain malignant cells according to the primary lesion [3, 9]. On the CT image, mucin within the peritoneal cavity usually presents as low attenuated fluid with some soft tissue attenuation and omental haziness (Figure 5) . Scalloping of the visceral surfaces of intraperitoneal organs, which indicates extrinsic pressure from the mucinous implants, is a typical CT finding in PP (Figure 5B) .
Most peritoneal lymphomatoses (PL) are non-Hodgkin lymphomas. Although primary lymphomas can develop on the peritoneal surface as a primary process, almost all PLs are secondary to a pre-existing lymphoma . The CT findings observed in PL are similar to those of PC: diffusely thickened peritoneum, ascites, omental haziness, and multifocal nodules or masses in the peritoneal cavity [12, 13]. However, the presence of coexisting splenomegaly and extensive lymphadenopathy are imaging features that suggest PL over PC (Figure 6) .
Peritoneal malignant mesothelioma (PMM) is an uncommon neoplasm that originates from the mesothelial or mesenchymal cells of the pleura, pericardium, and peritoneum . The majority of malignant mesotheliomas arise from the pleura, but 6%–10% originate from the peritoneum . It is difficult to distinguish PC from PMM by abdominal imaging findings alone. However, a history of asbestos exposure or the presence of pleural plaques could be helpful for differentiating PMM from PC (Figure 7) [15, 16].
Leiomyomatosis peritonealis disseminata (LPD) is a rare benign disorder that is understood as nodules of smooth muscle throughout the peritoneal cavity, and it may be correlated with pregnancy or the use of oral contraceptives . It is usually found incidentally on an imaging study of fertile women . At CT images, LPD reveal well-circumscribed solid masses with smooth contour and delayed enhancement in the peritoneal cavity, without evidence of omental haziness and ascites (Figure 8) . The aforementioned imaging features, prior history of cesarean delivery or hysterectomy, and the presence of a uterine leiomyoma are the clues for diagnosing LPD . The leiomyomas in most patients with LPD regress either spontaneously or after the withdrawal of ovarian hormones .
Tuberculosis can involve the peritoneum and can spread to the peritoneal cavity in several ways, including systemic infection, direct extension from the hallow viscus to the peritoneum, or lymphatic spread . When this happens tuberculous peritonitis (TP) is diagnosed. The CT findings of TP, such as soft tissue masses or nodules in the peritoneal cavity, retroperitoneal lymphadenopathy with central low attenuation, ascites, and infiltration to the omentum, can mimic the CT findings of PC (Figure 9) . Although hepatic or splenic microabscesses, splenic or lymph node calcifications, and splenomegaly are CT features suggesting TP rather than PC, it is difficult to differentiate TP from PC in daily practice . Further evaluations, such as paracentesis or laparoscopic biopsy, may be needed for the correct diagnosis if there is no pronounced primary malignancy.
Although PP, PL, PMM, and LPD can mimic PC on CT imaging studies, they can be distinguished from PC by the following findings: scalloping of the visceral surfaces of the intraperitoneal organs, coexisting extensive lymphadenopathy, discovering history of asbestos exposure or the presence of pleural plaques, and history of cesarean delivery, hysterectomy, or uterine leiomyoma. However, the precise distinction between PC and TP on CT images remains a challenge. Differentiating between PC and other similarly-appearing diseases is paramount for patient management, therefore radiologists must be aware of their distinguishing imaging features.
CT = Computed tomography
LPD = Leiomyomatosis peritonealis disseminata
PC = Peritoneal carcinomatosis
PL = Peritoneal lymphomatosis
PMM = Peritoneal malignant mesothelioma
PP = Pseudomyxoma peritonei
TP = Tuberculous peritonitis
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. This article does not contain any studies with animals performed by any of the authors.
Statement of informed consent was not applicable because the manuscript does not contain any patient data.
This work was supported by the Soonchunhyang University Research Fund and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2018R1C1B5085419).
The authors have no competing interests to declare.
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