Liver masses on CT is a concerning finding but has many different diagnostic possibilities from benign to deadly. The radiologist who reads your CT scan will provide possibilities based on the appearance. The radiologist May suggest additional testing based on the appearance.
The most concerning cause would be cancer which has spread to the liver. If you have a known cancer, then this becomes the most likely cause. Common cancers which spread to the liver in practice include gastrointestinal tumors, especially colon. Breast and lung cancer are some other cancers which tend to spread to the liver in advanced cases. Many other types of cancer can also spread to the liver. Additional testing or biopsy may be ordered by your doctor to confirm the suspicion of spread of cancer to your liver. PET scan may be a non invasive way of confirming this diagnosis.
Malignant and benign tumors of the liver can be multiple. Most commonly, the malignant tumors are seen in the setting of cirrhosis or hepatitis for hepatocellular carcinoma. Benign tumors of the liver such as hemangiomas and focal nodular hyperplasia can also be seen throughout the liver. The appearance on CT in many cases is characteristic. These are often asymptomatic and found incidentally on imaging performed for other reasons. Cysts can be multiple but are usually easily diagnosed and left alone. Abscesses or pockets of pus can also be multiple but are suspected based on your clinical presentation.
Often, your clinical doctor will increase the confidence of the diagnosis as he knows your history and clinical presentation best. Your clinical doctor may order blood tests, further imaging or biopsy based on the suspicion. The radiologist will guide your doctor in providing the most accurate diagnosis and may suggest further testing.