This is a radiograph of a cat that has ascites (fluid in the abdomen). There is a mass (tumor) at the lower tip of the heart in the chest. The cat has a large tumor on the right side of the abdomen which the veterinarian could feel. The mass is not visible on the radiograph due to all of the accumulated fluid.
As much fluid as possible was drained via and abdominocentesis to make the kitty more comfortable.
When humans have abdominal cancer, fluid will also develop and fill up in the abdomen. Those people usually have the fluid drained weekly so they will be more comfortable also.