Liver cysts on CT are one of the most common findings encountered in the liver. They can range from barely visible and under a centimeter to tens of centimeters in size. Cysts are filled with fluid and usually have an imperceptible wall. On CT, they are smooth, homogenous and water filled. Rarely some complexity inside can be appreciated. Cysts are rarely symptomatic. They can push on normal structures, rarely rupture and have internal bleeding or infection, but these are all rare.
Multiple cysts can be associated with polycystic diseases of the liver and kidney. Multiple cysts can occur in patients without these diseases as well. Cysts in the liver can be small in size, under a centimeter. In these cases, it is sometimes difficult to tell they are cysts with confidence. This becomes more important when a patient with cancer is being staged initially to see if there is any cancer spread. In these cases, an abdominal MR or short term follow up can help.
Cysts usually grow slowly, slower then tumors or metastasis (spread of cancer). The radiologist has to be careful before calling a cyst and consider many mimickers. For example, a necrotic metastasis to the liver can look like a cyst. This means the tissue inside the metastasis is dead or no longer receiving blood supply. In these cases, the cyst will have a thickened wall and more complex inside. Often a cancer history is present and there are multiple lesions.
Some cysts can be infectious. Hydatid infection occurs due to a parasite which is present in Australia and North America. The cysts are more complex in appearance then the simple cyst described above. The feared complication is rupture of the cyst. An abscess is another mimicker. An abscess or pus pocket is usually more complex and occurs in the setting of a sick patient. The appearance is is not that of a simple cyst on CT. A bile leak can also look like a cyst in the liver.
Some cancers of the liver can appear cystic. The most common to mimic a cyst is called a biliary cystadenoma or cystadenocarcinoma. These usually appear more complex with septations, nodules, and calcifications. Imaging is not very good at distinguishing the benign variant from the malignant cystadenocarcinoma. Even benign tumors are usually excised since they can degenerate into the cancer variant. Other tumors of the liver may have cystic components but are usually easily distinguished by more solid areas.
Hepatic cysts are therefore almost always benign. In rare cases, they can cause symptoms from pressing on normal structures, becoming infected, bleeding or rupturing. There are mimickers of simple liver cysts that are more dangerous to the patient. Tumors, infections, and bile leaks can look like cysts in the liver. Confusing imaging cases need to be correlated with the symptoms and possible additional imaging such as MRI of the abdomen. Close follow up of some cysts is appropriate as well.