Abstract
Pseudochylothorax is a rare disease entity. It is characterized by a milky white pleural fluid with a cholesterol/triglyceride ratio >1. According to published reports, most patients had associated diseases, including rheumatoid arthritis and tuberculosis, with only one patient having oropharynx carcinoma associated with lung metastases. Herein, we describe the development of pseudochylothorax in a patient with rheumatoid arthritis and lung squamous cell carcinoma. Cancer cells were confirmed in the pleural fluid, but there was no thoracic duct damage caused by cancer invasion or tuberculosis-related lesions in the lung field or intrathoracic lymph nodes. Anticancer treatment included immune checkpoint inhibitors, and the appearance of a cavity within the mass and pneumothorax were associated with lung cancer treatment. Although the mechanism of pseudochylothorax onset was unknown, we do believe that this clinical course would provide some suggestive information on treatment for patients with a similar course in the future.