What is the diagnostic approach and treatment for a patient with suspected abdominal mesothelioma, likely due to asbestos exposure, presenting with abdominal pain, weight loss, and ascites?

Diagnostic Approach for Abdominal Mesothelioma

For a patient with suspected abdominal mesothelioma presenting with abdominal pain, weight loss, and ascites, obtain a detailed asbestos exposure history, perform contrast-enhanced CT of the abdomen and pelvis, and proceed directly to laparoscopy with tissue biopsy for definitive histological diagnosis, as cytology of ascitic fluid alone is insufficient for diagnosis and treatment planning. 1, 2

Initial Diagnostic Workup

Essential History and Imaging

• Document occupational asbestos exposure history in detail, including construction work, shipyards, mining, or asbestos product manufacturing, as 70-80% of cases have documented occupational exposure 3
• Obtain contrast-enhanced CT of the abdomen and pelvis as the initial imaging modality, which typically reveals peritoneal thickening in an "omental cake" pattern, mesenteric or parietal peritoneal nodules, visceral peritoneal thickening, ascites, or omental masses 2, 3
• Extend CT imaging to include the chest to evaluate for pleural involvement or pulmonary metastases, as mesothelioma can present with both thoracic and abdominal disease 2, 4
• Perform basic laboratory tests including complete blood count (anemia, leukocytosis, thrombocytosis are common), liver enzymes, and renal function 2

Tissue Diagnosis - The Critical Step

Laparoscopy with biopsy is the preferred diagnostic approach for peritoneal mesothelioma, as it allows direct visualization of peritoneal surfaces and sampling of multiple sites 1, 2

• Avoid relying on ascitic fluid cytology alone for definitive diagnosis, as it is insufficient for histological subtyping and treatment planning 3
• Sample at least three distant peritoneal sites when feasible during laparoscopy to ensure robust histological subtyping and grading 3
• If laparoscopy is not feasible, perform CT-guided core needle biopsy of accessible peritoneal implants or masses 2, 4

Pathological Confirmation Requirements

Histological diagnosis with immunohistochemistry is the gold standard and must include 3:

• For epithelioid subtype: At least two mesothelial markers (calretinin, CK5/6, GATA3) and two adenocarcinoma markers to exclude metastatic adenocarcinoma 3, 4
• BAP1 and MTAP immunohistochemistry to aid diagnosis, as loss of these markers supports mesothelioma over reactive mesothelial hyperplasia 3
• Classification into histological subtypes (epithelioid, sarcomatoid, or biphasic), as epithelioid has better prognosis while sarcomatoid has worse survival 5, 1

Staging After Diagnosis

Once histological diagnosis is confirmed:

• Complete staging with FDG PET/CT if not already performed, to assess extent of disease and identify distant metastases 3
• Assess performance status (ECOG or Karnofsky), as this is the most important prognostic factor for treatment planning 3, 5
• Document prognostic factors including age, sex, histological subtype, presence of chest pain, and weight loss 3, 5

Treatment Approach Based on Disease Extent

For Localized Disease (Rare at Presentation)

Cytoreductive surgery with hyperthermic intraperitoneal chemotherapy (HIPEC) offers the only curative option, with 5-year survival rates of 29-63% 1

• This approach is only feasible in highly selected patients with limited peritoneal disease, good performance status, and no extra-abdominal metastases 1
• Surgery removes bulky tumor, leaving microscopic residual disease susceptible to intraperitoneal chemotherapy 1

For Advanced/Diffuse Disease (Most Common Presentation)

Systemic chemotherapy with pemetrexed plus cisplatin is the standard palliative treatment 5, 6, 7:

• Pemetrexed 500 mg/m² IV plus cisplatin 75 mg/m² IV on Day 1 of each 21-day cycle 6
• Mandatory supplementation with folic acid and vitamin B12 to reduce hematologic toxicity 6
• Median survival of 12-14 months with this regimen 1
• This combination has demonstrated improved survival, pulmonary function, and symptom control compared to cisplatin alone 5

For Patients Not Candidates for Systemic Therapy

Transition to comfort-focused care is appropriate for patients with poor performance status or rapid clinical deterioration 2

Critical Pitfalls to Avoid

• Do not rely on tumor markers (mesothelin, osteopontin, fibulin-3) for diagnosis, as they lack adequate specificity and sensitivity 3
• Do not delay tissue diagnosis by attempting multiple imaging studies, as peritoneal mesothelioma progresses rapidly and most patients present with advanced disease 2, 8
• Do not assume absence of asbestos exposure excludes the diagnosis, as some cases occur without documented exposure 4
• Do not confuse peritoneal mesothelioma with metastatic adenocarcinoma, which is far more common; immunohistochemistry is essential for differentiation 3

Prognosis

Median survival is 8-14 months from diagnosis, with death typically from local disease extension and respiratory failure 5, 1, 2. The epithelioid subtype has better prognosis than sarcomatoid or biphasic subtypes 5, 1. Most patients present with diffuse peritoneal involvement at diagnosis, which carries significantly worse prognosis than localized disease 2.