A large peritoneal inclusion cyst in a postmenopausal female without risk factors.

A 55-year-old female presented with a 5-month history of persistent abdominal fullness, distension, bloating, and dull, diffuse abdominal pain. She reported an unexplainable weight gain of approximately 5 pounds despite a decreased appetite. She also noted a progressive increase in abdominal girth, causing her clothes to feel tighter. Her medical history was unremarkable, with no significant prior surgical history. She had 2 previous vaginal deliveries. On physical examination, a soft, palpable fullness or mass was noted in the mid and left abdomen, with tenderness upon palpation. A contrast-enhanced computed tomography (CT) scan of the abdomen ( Fig. 1 ) demonstrated a large, fluid-filled mass with lobulated margins in the left peritoneal cavity, extending into the pelvis. The mass measured approximately 9.0 × 14.0 × 19.5 cm, most consistent with a lymphangioma. It abutted the transverse and descending colon and caused superior and rightward displacement of the small bowel. Peripheral displacement of vessels was also observed. Additionally, a 1.7 cm soft tissue mass-like lesion was identified in the right hemiabdomen with differential diagnoses including mesenteric panniculitis, a lymph node, or a carcinoid tumor. Fig. 1 A 55-year-old female presented with complaints of dull abdominal pain, fullness, bloating, weight gain and increased abdominal girth. A CT scan in the venous phase including axial (A), coronal (B), and sagittal (C) views show a large fluid density mass with lobulated margins in the left peritoneal cavity extending into the pelvis and also displacing the small bowel. No evidence of bowel obstruction was seen. Additionally, an ill-defined elongated mesenteric soft tissue mass was seen in close association with the small bowel in the right hemiabdomen (arrows). Fig 1: A 55-year-old female presented with complaints of dull abdominal pain, fullness, bloating, weight gain and increased abdominal girth. A CT scan in the venous phase including axial (A), coronal (B), and sagittal (C) views show a large fluid density mass with lobulated margins in the left peritoneal cavity extending into the pelvis and also displacing the small bowel. No evidence of bowel obstruction was seen. Additionally, an ill-defined elongated mesenteric soft tissue mass was seen in close association with the small bowel in the right hemiabdomen (arrows). Magnetic resonance imaging (MRI) was performed for further characterization ( Fig. 2 ), which redemonstrated the lobulated fluid-filled mass with enhancing septa within the small bowel mesentery. The soft tissue mass in the right paramedial small bowel mesentery measured approximately 1.5 cm and demonstrated T2 intermediate signal intensity with mild heterogeneous enhancement. Fig. 2 A 55-year-old female presented with complaints of dull abdominal pain, fullness, bloating, weight gain and increased abdominal girth. MRI images with contrast were obtained to further characterize the abdominal mass. Axial T2-weighted fat-suppressed (A) and coronal T2-weighted MR images (B) showed a large lobulated mass with T2 hyperintense fluid and numerous thin internal septations. Postcontrast T1-weighted fat-suppressed images in axial (C) and coronal (D) views showed the mild enhancement of the thin septations. A soft tissue mass along the right paramedial small bowel with T2 intermediate signal intensity and mild heterogeneous enhancement can also be appreciated (arrows). Fig 2: A 55-year-old female presented with complaints of dull abdominal pain, fullness, bloating, weight gain and increased abdominal girth. MRI images with contrast were obtained to further characterize the abdominal mass. Axial T2-weighted fat-suppressed (A) and coronal T2-weighted MR images (B) showed a large lobulated mass with T2 hyperintense fluid and numerous thin internal septations. Postcontrast T1-weighted fat-suppressed images in axial (C) and coronal (D) views showed the mild enhancement of the thin septations. A soft tissue mass along the right paramedial small bowel with T2 intermediate signal intensity and mild heterogeneous enhancement can also be appreciated (arrows). All laboratory investigations, including a complete blood count and comprehensive metabolic panel were within normal limits. The patient was monitored for 2 months, and a follow-up CT scan done with IV contrast and 3D cinematic rendering showed no significant change in the findings ( Fig. 3 ). Fig. 3 A 55-year-old female presented with complaints of dull abdominal pain, fullness, bloating, weight gain and increased abdominal girth. A CT scan with 3D cinematic rendering including (A) axial, (B) coronal, and (C) sagittal views redemonstrated a large fluid density with lobulated margins in the left peritoneal cavity extending into the pelvis displacing the small bowel. Internal septa (arrows) can be better appreciated on the 3D rendering compared to 2D images shown in Fig. 1 . Fig 3: A 55-year-old female presented with complaints of dull abdominal pain, fullness, bloating, weight gain and increased abdominal girth. A CT scan with 3D cinematic rendering including (A) axial, (B) coronal, and (C) sagittal views redemonstrated a large fluid density with lobulated margins in the left peritoneal cavity extending into the pelvis displacing the small bowel. Internal septa (arrows) can be better appreciated on the 3D rendering compared to 2D images shown in Fig. 1 . Given the presence of a possible solid lesion and the obstructive symptoms, surgical resection was performed. During an exploratory laparotomy, the cystic mass was found to have replaced most of the jejunal mesentery, with terminal mesenteric vessels running within and around it. Even with gentle manipulation, the smaller, thin-walled cysts tended to rupture, releasing thin, clear, serous fluid. The mass involved the jejunum, and intraoperative frozen section analysis of the mesenteric fibrotic lesion confirmed a benign process with adjacent benign-appearing lymph nodes. Final pathology revealed that the cystic mass was a multilocular peritoneal inclusion cyst ( Fig. 4 ). The resected small intestine with the mesenteric mass consisted of a segment of the small intestine with mesenteric fibrosis and 5 benign lymph nodes. Fig. 4 Surgical histopathology of a large, lobulated, fluid filled mass in a 55-year-old woman who presented dull abdominal pain, fullness and increased abdominal girth. H&E stain (A) shows a fibrous cyst wall lined by bland mesothelial cells without atypia. It is cytokeratin (B) and calretinin (C) positive, and ERG (vascular marker) negative (D) which helps establish it as a mesothelial cyst. Fig 4: Surgical histopathology of a large, lobulated, fluid filled mass in a 55-year-old woman who presented dull abdominal pain, fullness and increased abdominal girth. H&E stain (A) shows a fibrous cyst wall lined by bland mesothelial cells without atypia. It is cytokeratin (B) and calretinin (C) positive, and ERG (vascular marker) negative (D) which helps establish it as a mesothelial cyst.

The patient reported in the manuscript signed the informed consent/authorization for participation in research, which includes the permission to use data collected in future research projects such as the presented case details and images used in this manuscript.

We present a rare case of a postmenopausal female with an exceptionally large intra-abdominal peritoneal inclusion cyst (PIC) causing obstructive symptoms by displacing the bowel, despite the absence of common risk factors. This case highlights the characteristic imaging features and patient profiles of PICs while also emphasizing that these cysts can occur in atypical locations, age groups, and without prior peritoneal insults. Accurate preoperative diagnosis is crucial in preventing unnecessary or excessively aggressive interventions. We emphasize the importance of clinicians carefully evaluating the advantages of surgical excision versus aspiration or other nonsurgical treatments to determine the most suitable management strategy.

Peritoneal inclusion cysts (PICs) are uncommon mesothelial proliferations most commonly originating from the pelvis [ 2 , 3 ]. They can be uniloculated or multiloculated cysts lined by a single layer of mesothelial cells, lacking cytologic atypia, stratification, or infiltration into the underlying stroma and filled with serous fluid [ 1 , 2 ]. Due to their cystic appearance and reactive nature, many medical terms are used to describe them such as “multilocular peritoneal inclusion cyst,” “multicystic mesothelioma,” “benign cystic mesothelioma,” “benign cystic peritoneal mesothelioma,” “peritoneal cystic mesothelioma” and “inflammatory peritoneal cyst” [ 2 ]. While widely considered to be benign, a case report documented the transformation of a PIC into malignant mesothelioma [ 4 ]. PICs mostly occur in females of reproductive age, with infrequent cases reported in males and the postmenopausal age group [ 5 ]. The exact cause remains unclear and is widely debated; however, it is commonly associated with a history of abdominal or pelvic surgery, endometriosis, or pelvic inflammatory disease [ 6 ]. A study spanning a 19 year period with 288 PIC cases found over 70% had a history of a peritoneal insult [ 2 ]. Common clinical symptoms include abdominal pain, a sensation of fullness, and, in some, a palpable mass, as demonstrated in our case [ 7 ]. However, it can often be an incidental finding in many patients, especially if it is small and without obstructive symptoms. PICs can be identified on ultrasound, CT, or MRI. Classic ultrasound findings include a large anechoic cyst with or without septa [ 8 , 9 ]. When involving the ovaries, a classic “spider web pattern” can be present as adhesions surrounding the ovary [ 10 ]. CT is helpful to determine the full extent of these cysts and findings include a cystic mass with regular or irregular borders, containing material with the attenuation properties of fluid and/or hemorrhage and possible presence of septa [ 11 ]. There are no calcifications on CT. On MRI, PICs have low signal intensity on T1-weighted images and high signal intensity on T2-weighted spin-echo images, suggesting that the fluid is serous in nature, and usually have enhancing septa [ 10 , 12 , 13 ]. These findings are reflected in our case, accompanied by mesenteric fibrosis, consistent with the histologic nature of PICs. Recognizing these imaging features can be pivotal as the differential diagnosis of intra-abdominal cystic lesions includes both benign and malignant entities. Benign differential diagnoses include PICs, lymphangiomas, mesenteric or duplication cysts, endometriosis-related cysts, Müllerian cysts, epidermoid cysts, and cystic teratomas. Malignant differential diagnoses include pseudomyxoma peritonei, mucinous cystadenomas, cystic mesotheliomas, and cystic changes in solid neoplasms which require careful evaluation due to their potential for malignancy or aggressive behavior [ 14 ]. Lymphangiomas are among the most common differential diagnosis of a PIC and imaging can help differentiate them. Lymphangiomas predominantly arise in the pediatric population and exhibit a characteristic elongated configuration. The presence of fat can result in varying attenuation, ranging from fluid to fat, reflecting their lymphatic nature. This feature serves as a key distinction from PICs [ 14 ]. While multiple treatment options exist for managing PICs, recurrence remains a significant concern. Treatment strategies include observation, hormonal therapy such as oral contraceptives, image-guided aspiration using CT or ultrasound, image-guided sclerotherapy, laser ablation, and surgical removal [ 15 , 16 ]. While these have demonstrated benefits, surgical methods have more available data on long-term outcomes and adverse effects compared to other approaches. Among surgical options, laparoscopic excision is preferred compared to an exploratory laparotomy due to its advantages of minimal blood loss and a shorter hospital stay [ 16 ]. The two surgical options have similar recurrence rates which can be as high as 50% [ 14 ]. Both surgery and image-guided aspiration provide the benefit of obtaining a biopsy for definitive diagnosis; although, they also carry the risk of procedural complications.

Peritoneal inclusion cysts (PICs) are fluid-filled cystic lesions frequently occurring in the abdominopelvic region, primarily in females with a history of peritoneal injury due to surgery or inflammation [ 1 ]. These benign mesothelial proliferations originate from the peritoneal lining and can either present with compressive symptoms or as an incidental finding [ 2 ]. This report aims to highlight a rare cause of an abdominal mass in a female patient who does not align with the typical demographic for PICs. Additionally, we seek to examine key imaging features that facilitate an accurate diagnosis.