Inflammatory Pleural Effusion versus Malignant Pleural Effusion

Pleural effusion (fluid in the pleural cavity) is a finding that is often reported on imaging examinations in patients with tumors or other related diseases.

Pleural effusion is divided into two types: inflammatory pleural effusion and malignant pleural effusion. The former is mostly caused by pneumonia, pleurisy, viral infection, pulmonary tuberculosis, hemothorax, chylothorax, chest trauma and other conditions. It is a secondary exudative effusion, and the underlying disease should be treated; once the primary disease is cured, the pleural effusion can usually be absorbed. The latter is a common complication in patients with malignant tumors, and cancer cells can be detected in the pleural fluid. The appearance of a malignant pleural effusion means the tumor has reached an advanced stage—once a cancer patient develops a malignant pleural effusion, it is considered that distant metastasis has occurred, regardless of whether metastatic lesions are visible elsewhere.

In cancer patients, pleural effusion found on examination may be either inflammatory or malignant, because cancer patients often have other diseases that can cause inflammatory effusion. For example, patients who have received radiotherapy to the lungs are very likely to develop radiation pneumonitis, which can also cause pleural effusion.

It should be noted here that radiation pneumonitis mostly appears within one to three months after radiotherapy, but a few patients develop it years later, because the damage caused by radiation to the body is a continuously accumulating process. For instance, Marie Curie, who studied radioactive elements, later suffered deeply from nuclear radiation, developing intractable anemia and other sequelae.

So how can inflammatory and malignant pleural effusion be distinguished? The ideal method is, of course, pathological examination of the pleural fluid. If cancer cells are present in the pleural fluid, it is malignant; if cancer cells are absent, it is inflammatory. But this method requires drawing off the effusion and is relatively complicated to perform. We can also use CT to differentiate between them.

Generally speaking, unilateral, large-volume effusion accompanied by irregular pleural thickening and thickening of the involved mediastinal pleura is mostly malignant. Inflammatory effusion usually shows no pleural thickening, or only diffuse linear thickening. A loculated pleural effusion is always inflammatory, because so far not a single case of loculated effusion has been reported to be malignant; therefore, whether the effusion is loculated has important differential-diagnostic value.

The great majority of malignant pleural effusions are caused by primary or secondary tumors within the lung. There are also some tumors—for example, pleural mesothelioma—whose typical feature is pleural effusion accompanied by a rise in the tumor marker CA-125. Malignant effusions are mostly bloody, and only a few are straw-colored serous fluid. The fluid accumulates rapidly, and after it is drawn off it quickly returns to its original volume.

Whether malignant or inflammatory, the root of treatment lies in treating the primary disease. If it is a malignant effusion, once anticancer therapy is effective, the volume of pleural fluid will be greatly reduced. If the effusion is caused by another disease, treatment of that disease will also allow the fluid to be absorbed. If the effusion is large in volume and seriously impairs the patient's quality of life or even endangers life, then both the root and the symptoms must be treated. Diuretic drugs can be used to promote drainage of the fluid, or some fluid can be drawn off by physical means to relieve the patient's suffering.