Identifying the primary site of malignancy in carcinomatous serous effusions using immunocytochemistry

Malignant effusions are commonly encountered in day-to-day cytology practice. Determining the primary site of malignancy in carcinomatous effusions is a Herculean task. Cytology coupled with immunocytochemistry (ICC) is often found to be helpful in this context.

This study was conducted to evaluate the diagnostic utility of ICC on sections from cell blocks (CBs) in the detection of the primary site of origin in cases of metastatic carcinomatous effusions. To determine the origin of the primary tumour, TTF1 (lung), PAX-8 (ovary), CDX2 (colorectal), GATA3 (breast), and CK19 (pancreaticobiliary) were employed, depending on the clinical and radiological findings, and serum tumour markers.

A total of 13,459 serous effusion samples were received for cytological evaluation from January 2017 to December 2021, of which 2708 (20.1%) were carcinomatous effusions. Out of these, 1044 (38.5%), 1611 (59.5%), and 53 (2.0%) were from pleural, peritoneal and pericardial cavities, respectively. Of these, the majority were adenocarcinoma. ICC was performed in 309 (11.4%) cases. The ovary was the most common primary site in 179 cases (57.9%), followed by the lung (75, 24.3%), pancreaticobiliary system (12, 3.9%), colon/rectum (8, 2.6%), breast (6, 1.9%), prostate (2, 0.6%) and kidney (1, 0.3). The lung was the most common primary site in pleural (67/113, 59.3%) and pericardial (6/8, 75%) effusions. The ovary (168/188, 89.4%) was the most common primary site for carcinomatous effusions in the peritoneal cavity. However, in 17 (5.5%) cases, the exact primary site could not be established.

Judicious and methodical use of ICC on CBs helps to identify the primary site of the tumour in most carcinomatous effusions. This is of immense help to the treating clinician in directing appropriate therapy.