The Diagnostic Value of Cell Block Immunohistochemistry Combined with Imaging For Difficult Pleural Effusion

Introduction/ Background: Advanced pulmonary carcinoma frequently metastasises to the chest cavity, resulting in malignant pleural effusion (PE). Consequently, when PE is of pulmonary origin, diagnosis is generally straightforward. However, when PE originates from non-pulmonary sites, diagnosis becomes more challenging. This study aims to evaluate the diagnostic utility of combining cell block immunohistochemistry (IHC) with imaging modalities in diagnostically challenging cases of PE.

We collected PE data from difficult-to-diagnose cases admitted to Nantong Tumor Hospital from January to December 2024. Malignant PE of nonpulmonary origin and benign PE were defined as difficult cases. For each patient, pleural fluid underwent liquid-based cytology, cell block IHC, and imaging to differentiate between benign and malignant etiologies.

Among the 23 patients included, 9 presented with benign PE and 14 with malignant PE. 1. Benign pleural fluid cases: IHC analysis of cell blocks revealed proliferating mesothelial cells. CT identified six cases of PE, including five with concomitant pneumonia. PET-CT was performed in three patients; results indicated pneumonia in two cases and no significant abnormal uptake in the remaining case. 2. Malignant PE cases: The primary origins were the digestive system (eight cases), breast (five cases), and gynecological system (one case). 3. Cases of gastrointestinal origin: One patient was diagnosed with a gallbladder tumor via PET-CT; one with a pancreatic tail tumor via CT; two with colon malignancy via CT; and one with gastric cancer via CT. The remaining three cases had their primary lesions surgically removed, and CT revealed abdominal metastases. 4. Cases of breast origin (five cases): CT imaging showed lung metastasis in three cases, liver metastasis in two cases, and bone metastasis in two cases. 5. Case of gynecological origin: CT showed PE and lung metastasis.

IHC can help determine the origin of PE. However, IHC sometimes provides only a general direction, such as suggesting that malignant cells arise in the digestive tract. The digestive tract includes a wide range of organs (e.g., gastrointestinal tract, liver, gallbladder, pancreas). Therefore, to accurately localize the source of PE to specific organs or tissues, imaging examination is indispensable.

Cell block IHC is effective for determining the origin of PE, while imaging is useful for localization and identification of metastatic lesions. Combining these modalities helps differentiate between benign and malignant etiologies in difficult-to-diagnose PE cases.