CT mimics of peritoneal carcinomatosis

Abstract Peritoneal carcinomatosis is a term used to describe widespread metastases of cancerous tumors in the peritoneal cavity. It is most common in carcinomas of the gastrointestinal tract (GIT) and ovaries, and must be considered to be the main diagnosis even when the primary is not known. A wide variety of disease processes mimic peritoneal carcinomatosis. Precise diagnosis based on imaging alone is often difficult and very often the final diagnosis is only obtained after appropriate histopathology or microbiology.


Introduction
Peritoneal carcinomatosis is a metastatic manifestation of many organ-based malignancies, particularly carcinomas of the gastrointestinal tract (GIT) and ovaries, and must be considered as the fi rst possibility even in the absence of a known primary. There are several neoplastic and non-neoplastic conditions that may mimic peritoneal carcinomatosis on CT scan. These include lymphomas, gastrointestinal stromal tumors (GIST), granulomatous infections like tuberculosis, and primary peritoneal malignancies such as mesotheliomas.

Peritoneal carcinomatosis
Peritoneal carcinomatosis without distant metastases represents locoregional disease and calls for aggressive locoregional treatment. Most CT scan fi ndings are however nonspecifi c as both neoplastic and non-neoplastic pathologies of the peritoneum present as soft -tissue masses, with or without ascites. [1] In addition, there may also be a cystic component, necrosis, calcifi cation, or signifi cant contrast enhancement. Sometimes, peritoneal nodules can simulate unopacified bowel loops and hence adequate bowel opacification is important for accurate diagnosis. [2] The CT appearance of neoplastic infi ltration of the greater omentum can range from increased density of fat anterior to the colon or small bowel, to large masses, called omental cakes, separating the colon and small bowel from the anterior abdominal wall, [ Figure 1].
Very oft en though, the diagnosis is relatively easy when associated ovarian [ Figure 2a and 2b] or gastric neoplastic disease is seen. In the absence of a primary neoplasm and sometimes even in the presence of ovarian or gastric and bowel masses, other disease entities such as GIT lymphomas, GIST of the omentum and mesentery, peritoneal tuberculosis, and primary neoplasms of the peritoneum like primary peritoneal mesothelioma, can all mimic peritoneal carcinomatosis.
This pictorial essay is based on our experience with patients with CT features that mimicked peritoneal carcinomatosis; in all cases, the diagnosis was confi rmed on histopathology.

Lymphoma
Peritoneal lymphomatosis due to GIT lymphoma may be seen on CT as omental caking or masses, with diff use peritoneal thickening [ Figure 3a] or ascites. [3] Associated fi ndings that may help in distinguishing lymphoma from peritoneal carcinomatosis include aneurysmal dilatation of a bowel segment, with a thickened wall [ Figure 3b and c] and splenic enlargement [4] [ Figure 4]. The classic appearance on CT is of confluent masses causing encasement of the superior mesenteric artery and vein, producing a 'sandwich sign'; [5] these masses are bulky, soft , non-obstructing neoplasms [ Figure 5a and b], with a tendency to be less vascular than carcinomas. There is usually homogenous att enuation, without signifi cant necrosis with marked bowel wall thickening. The diff erential diagnosis of mesenteric lymphadenopathy also includes metastases and reactive lymphadenopathy due to granulomatous infections, Crohn disease, etc. [2] Splenic involvement and large non-necrotic masses and lymph nodes help make this diagnosis.

Primary peritoneal mesothelioma
Malignant primary peritoneal mesothelioma, though rare, can be seen as a large confl uent mass [ Figure 6a], which may be nodular or diff use, with or without ascites [ Figure 6b]. Calcifi cation is uncommon. [1] Approximately 30% arise primarily from the peritoneum, with the rest arising from the pleural surface. It can cause scalloping of,

Gastrointestinal stromal tumors
GIST refers to tumors arising from the mesenchymal tissue of the GIT; they commonly possess spindle cells and show c-kit protein positivity. [6] Although c-kit expression may be seen in other malignant tumors, it has a high specifi city for GIST. GIST is oft en solitary and arises most commonly from the stomach (60-70%), followed by small bowel (20-25%) and, rarely, the rectum (5%), esophagus, colon, and appendix. [3] GIST is rarely seen arising from the mesentery, omentum, and retroperitoneum and is usually large in size at the time of presentation. It may sometimes be an incidental fi nding owing to the submucosal origin of the tumor and exophytic nature of the tumor growth. [7] GIST of the omentum and mesentery may present with diffuse peritoneal seeding, mimicking mesenteric carcinomatosis. In a study by Kim et al., primary GIST in the or a mass eff ect on, adjacent abdominal organs. A history of exposure to asbestos is found in a few cases. Unlike in pleural mesothelioma, associated calcifi ed peritoneal plaques are uncommon. High-power microscopy may Figure 7 (a,b): GIST. Axial, contrast-enhanced CT scan of the abdomen in a 65-year-old male with diffuse abdominal pain shows a large, heterogenously enhancing intraperitoneal mass having central necrosis (arrow in a) with multiple peritoneal deposits (arrow in b). This was confi rmed on histopathology omentum and mesentery were seen as well-circumscribed, large masses containing areas of hemorrhage, necrosis [ Figure 7a], or cystic degeneration. [8] Peritoneal deposits may also be seen [ Figure 7b]. One diff erentiating feature of GIST is their hypervascularity because of which, even if central necrosis or cystic degeneration is present, there may be peripheral enhancement with surrounding dilated vessels. [9] Peritoneal tuberculosis Peritoneal tuberculosis, in particular, can be a diffi cult and elusive diagnosis to make and may mimic metastases from ovarian cancer and other nontuberculous granulomatous diseases because of the vague symptoms and nonspecifi c radiographic, pathologic, and laboratory findings. Tuberculous peritonitis may be of wet, fi xed fi brotic, and dry plastic types. [10] The wet type presents as free or loculated ascites with septae. The fi xed fi brotic type may present as an omental and mesenteric mass, with matt ed bowel loops, and the dry plastic type can show thickened peritoneum and necrotic lymph nodes, though there is oft en an overlap between these two types. [2] A high index of suspicion for peritoneal tuberculosis is important if unnecessary elaborate surgery and delay in treatment are to be avoided. [11] The CT scan fi ndings include omental cake-like masses [ Figure 8], nodules, [10] and a smudge patt ern. The peritoneal thickening is usually smooth as compared to the nodularity seen in peritoneal carcinomatosis. [12] Peritoneal tuberculosis can mimic peritoneal carcinomatosis [ Figure 9]. A few cases of abdominal tuberculosis may even show elevation of CA 125. The presence of necrotic mesenteric and retroperitoneal lymph nodes, especially in younger patients helps clinch this diagnosis.
Other lesions such as papillary serous carcinoma, desmoplastic small round-cell tumor, and mesenchymal tumors, including both benign and malignant tumors may occur but are diffi cult to diagnosis on imaging fi ndings alone.
CT scan plays an important role in the detection of peritoneal carcinomatosis and its mimics. However, the exact diagnosis and characterization of lesions may be diffi cult due to the overlap of imaging fi ndings. CT scan can also play an important role in guiding biopsy for tissue diagnosis and can provide the surgeon with a 'road map' prior to cytoreductive surgery. Since a precise diagnosis based on imaging fi ndings alone is oft en not possible, histopathology is mandatory to confi rm the diagnosis.