###Human: Is it possible to see lung cancer in CAT scan if pleural effusion is present? ###Assistant: Hi. Detection of tumor on CAT (computed tomography) scan has nearly 85 to 90 percent efficacy which further increases to 95 percent in post-contrast studies. The pleasurable effusion can be due to numerous causes and in your case due to mucous plugging causing lung lobe collapse-tumor need to be the cause always so do not be apprehensive about that. Any heterogenous abnormal enhancement can be a tumor seen in contrast scan which is not present in your scan.