Pleural Effusion — Fluid build-up around the lungs

The pleura consists of a double-layered membrane: the visceral pleura covers the lungs, while the parietal pleura lines the inner chest cavity. A minute volume of lubricating fluid normally exists between these layers to facilitate smooth lung expansion during breathing. A pleural effusion develops when fluid production exceeds the rate of lymphatic absorption, causing liquid to accumulate and compress adjacent lung tissue.

When associated with cancer, the condition is termed a malignant pleural effusion (MPE). MPE arises when cancer cells metastasise to the pleural surfaces, triggering inflammation, increasing vascular permeability, and disrupting normal fluid drainage. Effusions can also stem from non-malignant factors in cancer patients, including congestive heart failure, severe hypoalbuminaemia, pulmonary embolism, or post-radiation complications, necessitating thorough diagnostic differentiation to guide appropriate therapeutic intervention.

A pleural effusion significantly impacts daily comfort and functional independence by causing distressing breathlessness and fatigue. For clinicians, the presence of malignant cells in pleural fluid confirms advanced or metastatic disease, which fundamentally alters staging, prognostic outlook, and therapeutic strategy. Timely intervention not only relieves debilitating respiratory symptoms but also prevents recurrent hospitalisations. Managing an effusion allows patients to regain exercise tolerance, maintain their quality of life, and stay sufficiently well to continue essential systemic cancer treatments without interruption.

Diagnostic evaluation typically begins with chest radiography, ultrasound, or computed tomography (CT) to visualise fluid volume and lung architecture. A thoracentesis—inserting a fine needle into the pleural space under local anaesthesia—is performed to sample fluid for laboratory analysis. Cytological examination checks the fluid for malignant cells. Biochemical markers, including protein and lactate dehydrogenase (LDH), categorise fluid as an exudate or transudate. If cytology is inconclusive, a video-assisted thoracoscopic surgery (VATS) biopsy of the pleural lining may be performed.

Pleural effusions are most commonly encountered in primary lung cancers (both non-small cell and small cell lung cancer) and metastatic breast cancer, which together account for over half of all malignant effusions. They also frequently occur in pleural mesothelioma, ovarian cancer, gastric cancer, and various lymphomas. Occasionally, a malignant pleural effusion is the initial presenting manifestation of an unknown primary cancer that has disseminated to the thoracic cavity.

Treatment prioritises symptom relief and preventing fluid recurrence. Therapeutic thoracentesis provides immediate drainage. For recurrent malignant effusions, chemical pleurodesis (using talc or bleomycin to fuse the pleural layers) or placement of an indwelling tunneled pleural catheter allows home drainage. Concurrently, effective systemic therapy—such as chemotherapy, targeted agents, or immunotherapy tailored to the primary tumour—can treat the underlying malignancy, ultimately reducing or eliminating fluid production.