Guideline for Management of Atypical Cells in Pleural Fluid

Diagnostic Imperative

• When atypical cells are identified in pleural fluid, proceed directly to pleural tissue biopsy—preferably via thoracoscopy—because cytology cannot reliably distinguish reactive mesothelial cells from malignancy and a definitive tissue diagnosis is essential for staging, prognosis, and treatment planning. 1
• Atypical cells are diagnostically indeterminate due to morphologic overlap between reactive benign mesothelial cells and malignant mesothelioma cells; therefore cytologic diagnosis alone is unreliable. 2
• Recognition of tissue invasion is mandatory for a definitive diagnosis of malignant pleural mesothelioma, which cannot be assessed on fluid cytology. 3

Biopsy Strategy

Thoracoscopic Pleural Biopsy (Gold Standard)

• Thoracoscopic (medical or surgical) pleural biopsy achieves >92 %–95 % diagnostic sensitivity and consistently outperforms cytologic examination. 4
• The procedure provides direct visualization, allows multiple biopsies from different pleural sites, and enables simultaneous therapeutic pleurodesis when indicated. 1
• Tissue obtained is sufficient for immunohistochemistry, molecular profiling, and receptor‑status testing required for targeted therapies. 4

Alternative Image‑Guided Core Needle Biopsy

• When thoracoscopy is contraindicated or unavailable, ultrasound‑ or CT‑guided core needle biopsy is an acceptable alternative. 5
• Blind (non‑image‑guided) pleural biopsies should never be performed. 5

Specimen Handling

• Place each tissue specimen in both saline (for microbiological culture) and formalin (for histopathology). 6

Pathology and Molecular Testing

Mesothelioma‑Specific Markers

• Loss of BAP1 expression and homozygous p16 deletion (detected by FISH) are highly specific for malignant mesothelioma, although negative results do not exclude the disease. 2
• Standard mesothelial immunostains (calretinin, CK5/6, D2‑40, WT‑1) confirm mesothelial origin. 1
• Documentation of histologic subtype (epithelioid, biphasic, sarcomatoid) is required because it carries prognostic significance. 3

Metastatic Adenocarcinoma Work‑up

• A panel of adenocarcinoma markers (CEA, B27.3, BerEP4, MOC‑31) helps differentiate malignant pleural involvement from primary pleural disease. 1
• Organ‑specific markers guide identification of the primary tumor:
  
  • TTF‑1 / Napsin A → lung
  • Mammaglobin, ER, PR, GATA3 → breast
  • CDX‑2 / CK20 → gastrointestinal tract
  • PAX‑8 → gynecologic or renal origin. 1

Molecular Profiling

• Adequate tissue permits gene‑expression profiling and receptor‑status testing to assess eligibility for molecular‑targeted therapies. 1
• Cell blocks derived from pleural fluid rarely provide sufficient material for comprehensive molecular testing. 4

Optimizing Pleural Fluid Studies (While Awaiting Biopsy)

• Collect a minimum of 25 mL (optimal 25–50 mL) of pleural fluid to maximize cytologic yield. 5
• Process fluid with both direct smear and cell‑block techniques to enable subsequent immunohistochemistry. 1
• Flow cytometry is a useful adjunct when lymphoma is suspected, offering 50 %–94 % sensitivity for non‑lymphoma malignancies. 2
• For microbiological exclusion of infection, send 5–10 mL in aerobic and anaerobic blood‑culture bottles plus sterile containers for Gram stain, AFB stain, and culture. 5

Critical Pitfalls and Recommendations

• The International Mesothelioma Interest Group recommends that mesothelioma diagnosis must always be based on tissue biopsy, never on cytology alone. 2
• Do not delay biopsy for repeat cytology; if the initial fluid shows atypical cells and clinical suspicion is high (especially for mesothelioma), proceed directly to tissue sampling. 1
• Diagnostic bronchoscopy is not indicated for undiagnosed pleural effusion work‑up unless the patient has hemoptysis or clear bronchial obstruction. 6
• Up to 15 % of patients whose initial pleural biopsy shows nonspecific pleuritis are later diagnosed with pleural malignancy (most often mesothelioma). 1

Management of Persistent Diagnostic Uncertainty

• If the first biopsy yields only nonspecific pleuritis but suspicion remains high, consider a repeat biopsy using an alternative approach (e.g., thoracoscopy if the first was image‑guided, or vice‑versa). 2
• Implement long‑term radiological monitoring to ensure that an occult malignancy is not missed. 4
• Systematically exclude alternative diagnoses such as tuberculosis and pulmonary embolism, which can present with atypical cells in pleural fluid. 6

Diagnostic Utility of Mesothelial Cells in Pleural Fluid

Limitations in Malignancy Detection

• The American Society of Clinical Oncology guidelines provide strong evidence-based recommendations that pleural fluid cytology can serve only as an initial screening test for mesothelioma, not a diagnostic confirmation 7, 8, 9
• The major limitation is the extreme difficulty in differentiating malignant mesothelioma cells from reactive benign mesothelial cells 7, 8, 9
• When definitive diagnosis is needed, tissue biopsy via thoracoscopy (>95% diagnostic yield) or CT-guided core biopsy must be performed 7, 8, 10, 11

Sensitivity for Malignancy

• The British Thoracic Society guidelines demonstrate that pleural fluid cytology has approximately 60% sensitivity for detecting malignant effusions overall 12, 13
• First specimen detects 65% of malignancies when positive 13, 12
• Second specimen adds another 27% 13
• Third specimen contributes only 5% additional yield 13
• The diagnostic yield is particularly poor for mesothelioma compared to adenocarcinoma 12, 13, 14, 10

Immunohistochemical Limitations

• Standard immunohistochemical studies have limited value in differentiating benign reactive mesothelial cells from malignant mesothelioma cells 7, 8, 9
• Newer markers (BAP1 loss, p16 deletion) show promise for distinguishing mesothelioma from reactive mesothelial cells 7, 8, 9
• However, immunocytochemistry on pleural fluid is useful for distinguishing mesothelioma from adenocarcinoma using markers like Claudin-4, CEA, calretinin, and cytokeratin 5/6 12, 13, 11

Clinical Algorithm for Mesothelial Cell Interpretation

• If atypical or suspicious mesothelial cells are present, do not diagnose mesothelioma; proceed directly to tissue biopsy 7, 8
• If malignancy is suspected, perform thoracoscopy with multiple pleural biopsies from different locations (diagnostic yield >95%) 7, 8, 10, 11
• If thoracoscopy is contraindicated, CT-guided core needle biopsy of pleural nodules can be performed 7, 8, 10
• Ensure biopsies are of sufficient depth to assess for invasion, which distinguishes malignant mesothelioma from benign mesothelial proliferation 14, 10

Rare Mesothelioma Types

• Sarcomatoid and biphasic mesothelioma are rarely detected in pleural fluid specimens, even when malignant mesothelial cells are present 14, 10, 11