Unveiling the Hidden: Malignant Peritoneal Mesothelioma Detected on ¹⁸F FDG PET-CT Scan

INTRODUCTION

Peritoneal mesothelioma is quite rare, with an annual incidence of 1.02 cases per million.^[1] It occurs most commonly in males and is aggressive in nature, with poor prognosis. Even though there is a well-established link between pleural mesothelioma and asbestos exposure, peritoneal mesothelioma does not show a very strong association, with 20%–40% of cases occurring spontaneously, especially in females.^[2,3] Since the presenting symptoms are very vague and nonspecific, average time to diagnose can take up to 4–6 months, resulting in extensive tumor burden at the time of diagnosis.^[4] Our case shows a 67-year-old male detected with peritoneal carcinomatosis and abdominal lymphadenopathy proven to be malignant primary peritoneal mesothelioma on histopathology.

CASE REPORT

A 67-year-old male presented with complaints of abdominal heaviness, bloating, loss of appetite, and constipation for 2 months. An abdominal ultrasound revealed moderate ascites with irregular omental and peritoneal thickening. Fine-needle aspiration cytology from the omental thickening was positive for malignancy, favoring metastasis. Colonoscopy and upper gastrointestinal (GI) endos-copy were unremarkable. The patient was referred to our department for a ¹⁸F Fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/ CT) scan to identify the primary disease and assess subsequent staging. The scan was performed 60 min aft er the intravenous injection of ¹⁸F FDG. It revealed diffuse FDG-avid omental and peritoneal thickening with multiple serosal soft tissue deposits and lymphadenopathy in the abdomen and pelvis, along with gross ascites and pleural effusion. The GI tract, pancreas, gallbladder, and lung appeared unremarkable, raising a high suspicion for primary peritoneal carcinomatosis [Figure 1]. A subsequent biopsy and immunohistochemistry (IHC) from the peritoneal and omental thickening confirmed the diagnosis of mesothelioma, deciduoid type, with IHC markers positive for Calretinin, WT 1, CK 5/6, and PAN CK [Figures 2 and 3].

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Figure 1:

(a) The MIP image of ¹⁸F fluorodeoxyglucose positron emission tomography-computed tomography (PET-CT) scan showing diffuse peritoneal carcinomatosis. (b,c,d) shows axial CT and fused PET-CT images of diffuse peritoneal and omental thickening, serosal soft tissue deposits, lymphadenopathy, and ascites. (b and c) Gallbladder and pancreas appear unremarkable

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Figure 2:

Histopathology images (stained with Hematoxylin & Eosin) of peritoneal mesothelioma (a) 10x (marked with arrow), (b) 40x. Sections studied show tumor tissue arranged in tubular, papillary and solid pattern, at places deciduoid cells are also seen ( marked with arrow). The cells show vesicular and prominent nuclei with abundant eosinophilic cytoplasm.

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Figure 3:

Immunohistochemistry is positive for (a) calretinin and (b) WT1

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DISCUSSION

Peritoneal carcinomatosis most commonly occurs as metastatic presentation from primary tumors of GI tract, ovaries, pancreas, appendix, gallbladder, breasts, uterus, and lungs.^[5] Primary peritoneal mesothelioma is a rare malignancy with a challenging diagnosis due to its nonspecific clinical presentation. The carcinogenic fibers due to asbestos exposure, when inhaled, can reach the peritoneal cavity via the lymphatic system or through direct ingestion, triggering chronic inflammation and DNA damage, ultimately leading to malignant transformation. However, asbestos is not the only cause. Other potential risk factors include radiation exposure, genetic predisposition (BRCA 1 mutations), chronic peritoneal inflammation, and, in rare cases, erionite exposure.^[6-8]

In this case, FDG PET/CT played a crucial role in diagnosis, staging, prognosis, and treatment selection. While cytology identified the presence of malignant cells, PET/CT revealed the full extent of disease by showing diffuse peritoneal and omental thickening, serosal soft tissue deposits, lymphadenopathy, ascites, and pleural effusion. In addition, it also helped exclude other primary sites. The absence of significant metabolic activity or any lesion in the GI tract, pancreas, gallbladder, and lungs increased suspicion for primary peritoneal carcinomatosis rather than metastasis from another malignancy. This was further corroborated with no abnormality detected in colonoscopy and upper GI endoscopy and confirmed by biopsy and IHC markers consistent with mesothelioma. The scan also demonstrated a maximum standardized uptake value of 6.6, suggesting more aggressive disease with a poorer prognosis.^[6,9,10]

Given the aggressive nature of the disease, intensive management strategies are warranted. The patient may benefit from cytoreductive surgery combined with hyperthermic intraperitoneal chemotherapy, which has been shown to improve overall survival in selected cases with resectable disease. For unresectable or extensive disease, systemic chemotherapy (e.g., pemetrexed + cisplatin) or palliative care may be considered.^[11] In our case, a peritoneal carcinomatosis index of more than 30 categorized the disease as extensive and systemic chemo-therapy was chosen as the choice of treatment.^[5]

CONCLUSION

Thus, not only did FDG PET/CT play a significant role in reaching the rare diagnosis of primary peritoneal mesothelioma but it also helped in comprehensive staging, prognostic evaluation, and treatment guidance, highlighting its valuable role in shaping clinical decisions.