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INTRODUCTION
Malignant mesothelioma is a rare malignant tumor originating from the mesothelium and accounts for approximately 0.2% of all malignant neoplasms. The pleural membrane is the most common primary site, although it rarely arises in the peritoneum and other locations.1,2 Macroscopically, mesothelioma is classified into two types: localized and diffuse, with the diffuse form being more prevalent.3,4 We report a case of primary localized peritoneal malignant mesothelioma (PMM) involving both the stomach and transverse colon, which was successfully treated with laparoscopic surgery.
CASE
This study was approved by the Institutional Review Board (IRB) of Wonkwang University Hospital (IRB No: WKUH-2025-04-010). The requirement for informed consent was waived by the IRB.
A 55-year-old male presented to our outpatient department with right upper quadrant pain and a palpable mass in the upper abdomen. He had a medical history of hypertension and hyperlipidemia, with no prior history of abdominal surgery. Vital signs were within normal limits. Laboratory evaluation revealed a hemoglobin level of 14.1 g/dL, white blood cell counts of 9.65×10³/mm³, and a CRP level of 0.87 mg/L. Other laboratory findings, including CEA, were within normal ranges.
The patient underwent esophagogastroduodenoscopy (EGD), colonoscopy, abdomen-pelvis CT, and PET-CT. EGD revealed a large subepithelial lesion (SEL) extending from the lower to mid-body of the stomach along the greater curvature. Colonoscopy revealed an ulcerative lesion in the transverse colon, approximately 65 cm from the anal verge, suspected to be secondary to invasion by gastric SEL (Fig. 1). CT imaging demonstrated a 13 cm subepithelial gastric mass with heterogeneous enhancement invading the transverse colon (Fig. 2). MR imaging demonstrated a 13 cm lobulated mass between the gastric antrum and transverse colon, which showed low T1 and heterogeneously high T2 signal with focal nodular focus that exhibited opposed-phase signal drop, findings consistent with fat content. The lesion abutted the transverse colon (suggesting a colonic mesenteric origin) and externally compressed the gastric antrum with probable invasion, without definite intraluminal extension into the stomach. On diffusion- weighted imaging, it showed heterogeneous hyperintensity with low apparent diffusion coefficient values, suggestive of malignancy. These findings are more suggestive of a liposarcoma, although a gastrointestinal stromal tumor (GIST) remains in the differential diagnosis. PET-CT revealed a large mass involving the stomach and transverse colon, with suspected early metastasis to the regional lymph nodes.
Preoperative biopsy was planned using EGD, colonoscopy, and ultrasound-guided percutaneous biopsy. However, because the lesion was suspected to be in the mesentery, the ultrasound- guided biopsy was not performed owing to the anticipated risk of hemorrhage. Endoscopic biopsies obtained during EGD and colonoscopy revealed only nonspecific inflammation. As the preoperative biopsy failed to reveal a definitive tumor pathology, the patient underwent laparoscopic subtotal gastrectomy with D1+β lymphadenectomy and extended right hemicolectomy under the presumptive diagnosis of a liposarcoma or GIST. Intraoperatively, a 14×11 cm extraluminal mass was identified on the serosal surface of the transverse colon, invading the stomach. Histopathological examination confirmed epithelioid- type malignant mesothelioma with no lymph node metastasis. Immunohistochemical staining was positive for calretinin, Wilms tumor 1 (WT1), and cytokeratin (CK) 5/6, and negative for c-kit and CEA (Fig. 3).
Following the diagnosis of mesothelioma, a thorough review of the patient's occupational and environmental history revealed no evidence of asbestos exposure. The patient was discharged on postoperative day 25 after delayed recovery due to postoperative ileus. At the 9-month follow-up, he remained recurrence-free under regular surveillance.
DISCUSSION
Malignant mesotheliomas arise from the mesothelial cells of the serosal membranes, including the pleura, pericardium, and peritoneum, with the majority originating in the pleura. Primary PMM is much less common, accounting for approximately 10–30% of all cases. Mesothelial tumors arising within the peritoneal cavity include well-differentiated papillary mesothelioma (WDPM), multicystic mesothelioma, adenomatoid tumor, and PMM. Among these, multicystic mesothelioma and adenomatoid tumors are generally benign, whereas WDPM is regarded as a low-grade neoplasm with uncertain malignant potential. These tumors should be distinguished from PMM based on clinicopathological features.5-9
Macroscopically, PMM is classified into diffuse and localized types. Although there are no histological or immunohistochemical differences between the two types, the localized type does not exhibit diffuse peritoneal dissemination but tends to invade adjacent organs. This often leads to misdiagnosis of the primary tumor site.5-8,10 Histologically, malignant mesothelioma is classified into epithelioid, sarcomatoid, and biphasic (mixed) types. The epithelioid subtype is the most common, accounting for approximately 75% of cases, whereas the sarcomatoid type is the rarest and is associated with the poorest prognosis.1,3
Asbestos exposure is a definitive risk factor for malignant mesothelioma; however, this association appears to be weaker in peritoneal mesothelioma than in pleural mesothelioma. Other proposed etiologies include chronic serositis and infection with simian virus 40. However, a definitive causal relationship has not been established.1,7,11
Most patients with localized mesothelioma are asymptomatic or present with nonspecific symptoms. However, depending on the site of invasion, patients may experience abdominal pain, a palpable mass, or weight loss, which often contribute to delayed diagnosis. Although establishing a definitive histological diagnosis preoperatively can be challenging, preoperative biopsy, when feasible, aids diagnosis through immunohistochemical analysis. Characteristically, the tumor stains positive for calretinin, WT1, and CK 5/6.6,7,12
Complete surgical resection remains the primary treatment modality for PMM. However, most PMM cases present as the diffuse type, making complete resection challenging. Traditionally, treatment options for unresectable tumors have included pemetrexed-based systemic chemotherapy, palliative surgery, and rarely, total abdominal radiation. Recently, cytoreductive surgery (CRS) combined with hyperthermic intraperitoneal chemotherapy (HIPEC) has emerged as an effective therapeutic option for diffuse PMM, with previous studies demonstrating improved survival outcomes. In contrast, for localized PMM, definitive guidelines are lacking, and the evidence is limited to a few case reports and series with heterogeneous management approaches. Therefore, the decision to use HIPEC should be individualized based on patient- and tumor-specific factors, and further prospective studies are needed.1,5,8,10,13,14 In our case, a definitive diagnosis could not be established preoperatively through biopsy, and surgery was performed under the presumptive diagnosis of a liposarcoma or GIST. Therefore, HIPEC was not administered. The patient is currently undergoing short-term postoperative follow-up; in the event of recurrence, CRS with HIPEC or systemic chemotherapy should be considered.
The prognosis largely depends on the feasibility of complete surgical resection. The prognosis of diffuse-type mesothelioma is generally poor, with a median survival time of approximately 12 months. However, successful CRS combined with HIPEC significantly improved survival. In previous studies, the survival outcomes ranged from 27 to 97 months. Localized and epithelioid tumors are associated with favorable survival outcomes.6,8,10,13
In conclusion, we report a case of localized PMM arising from the mesentery of the T-colon with stomach invasion. Although mesothelioma is a rare malignancy with a generally poor prognosis, appropriate treatments such as CRS combined with HIPEC may lead to improved survival. In particular, localized tumors tend to have a more favorable prognosis when complete resection is achieved, underscoring the importance of accurate diagnosis and timely intervention.
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