Liver Tumors

What is a liver tumor?

A liver tumor is growth in the liver, due to abnormal and unregulated replication of cells within the liver. The liver is a large organ in the abdomen that is made of several parts, called lobes. It cleanses the blood and aids in digestion by secreting bile. The liver is composed of liver cells (hepatocytes) and other kinds of cells, including bile duct cells, neuroendocrine (hormone-secreting) cells, and connective tissue cells. As such, there are four types of primary tumors of the liver: hepatocellular tumors, bile duct tumors, neuroendocrine tumors, and sarcomas.

Liver tumors may be either benign (non-cancerous) or malignant (cancerous, invasive, and prone to spread). Benign tumors include hepatocellular adenomas, hepatomas, bile duct adenomas, hemangiomas, and leiomyomas. Malignant tumors include hepatocellular carcinomas (HCCs), bile duct carcinomas, neuroendocrine tumors, and various sarcomas, including hemangiosarcoma, fibrosarcoma, and leiomyosarcoma, among others.

Malignant tumors tend to metastasize (spread to other areas of the body). Most liver tumors in dogs are malignant, while in cats, most are benign. Malignant tumors can present in three different ways. The tumor can be massive, meaning the cancer is a single, large tumor; nodular, meaning there are several masses spread throughout the liver; or diffuse, meaning it involves the entire liver evenly.

• Hepatocellular carcinomas (HCC) are the most common liver tumors in dogs and the second-most common in cats. Most HCCs are massive in form.
• Bile duct adenomas are the most common liver tumors in cats and are uncommon in dogs. Bile duct adenomas are sometimes called biliary cystadenomas, as they are often cystic (fluid-filled).
• Bile duct carcinomas are the second-most common liver tumors in dogs. Cats can develop these tumors as well.
• Neuroendocrine tumors (also called carcinoids) are less common in dogs and cats, as are the various sarcomas.

Overall, primary liver tumors (i.e., tumors that originate in the liver) are rare in dogs and cats. More often, liver tumors are a result of metastatic cancer: a cancer that originated somewhere else in the body and then spread to the liver.

What causes these tumors?

The reason why a particular pet may develop this, or any other tumor or cancer, is not straightforward. Very few tumors and cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary. In the case of primary liver tumors, there are no known causes. They tend to arise in older pets (i.e., more than 9 years old).

What are the signs of liver tumors?

The signs of liver tumors vary, ranging from no signs at all (asymptomatic) to a wide range of symptoms. You may notice inappetence, fever, lethargy, and weight loss or gain. Some pets may have nausea, vomiting and diarrhea; increased drinking and urination; and yellowing of the skin and eyes (jaundice). Occasionally, neurological signs can be seen, such as seizures, disorientation, stumbling, and weakness.

With some pets, the first indication of a tumor is the sudden onset of weakness, lethargy, and collapse — the signs of tumor rupture with bleeding into the abdomen. When this happens, the gums may become pale and your pet may have difficulty breathing as their blood pressure drops. This is a life-threatening emergency that requires immediate veterinary attention.

How is this cancer diagnosed?

Your veterinarian may suspect a liver tumor based on your pet’s history, clinical signs, and the findings of a full physical examination. With an exam and abdominal palpation, your veterinarian may find that the liver is enlarged, or they may find a mass or fluid in the abdomen, sometimes with abdominal pain. Sometimes, the first indication of a liver tumor is an unexplainable rise in blood liver enzymes in blood tests repeated over time, and the pet otherwise appears normal.

Blood tests and urinalysis may be abnormal and show that your pet has signs of liver damage or bile duct obstruction (cholestasis). In the case of tumor rupture with bleeding, a low red blood cell count (anemia) will be found.

Your veterinarian may want to take abdominal radiographs (X-rays) or do an abdominal ultrasound. Ultrasound is more sensitive for detecting masses on the liver. Ultrasound can determine if the tumor is massive, nodular, or diffuse, and whether it is in more than one lobe of the liver. It can also detect the presence of bile duct or gall bladder obstructions. Occasionally, more advanced imaging is recommended, such as CT (computed tomography) scan or MRI (magnetic resonance imaging). Some liver tumors are found incidentally (by chance) while taking images for a different reason.

If one or more masses are found, it may be possible to obtain a sample of the tumor cells with an ultrasound-guided, fine-needle aspiration (FNA). This technique uses an ultrasound probe to guide a long fine needle through the skin and into the tumor to retrieve a sample of cells. The cells are placed on a microscope slide and examined by a veterinary pathologist in a process called cytology.

Although cytology can be helpful, the most accurate diagnosis of a liver tumor is made with an ultrasound-guided, core-needle biopsy. In this procedure, a piece of tumor tissue is obtained by inserting a long hollow needle through the skin and into the tumor. To identify the type of cancer, the piece of tumor is examined by a veterinary pathologist under the microscope, a process called histopathology. Histopathology is not only helpful in making a diagnosis, but also can indicate how the tumor is likely to behave.

How does this cancer typically progress?

Benign liver tumors, such as bile duct adenomas, do not spread. They generally do not cause signs of illness unless they grow to the point that they physically affect other organs or structures (e.g., blood vessels), or rupture and bleed.

Liver tumors overall are fragile and may rupture as they grow, potentially causing life-threatening internal bleeding.

Malignant liver tumors, such as HCCs, tend to metastasize, most commonly to the nearby lymph nodes, the lungs, and the peritoneum (inner lining of the abdomen). 

HCCs of the massive form are locally invasive, meaning they invade the surrounding liver tissue and can become quite large. As they grow, they can compress the main bile duct (the duct that drains bile from the liver into the intestine), the internal organs, or the large blood vessels in the abdomen, causing a variety of symptoms. Though they tend to metastasize, they have a lower rate of metastasis compared to the nodular or diffuse forms. Without surgical removal, dogs with HCCs are 15 times more likely to die of tumor-related complications.

Bile duct carcinomas, neuroendocrine tumors, and the various sarcomas also tend to spread, often quickly, and they are often spreading by the time they are diagnosed. Bile duct carcinomas can cause bile duct obstruction as they grow, which causes severe damage to the liver and gall bladder, at times risking gall bladder rupture.

What are the treatments for this type of tumor?

Surgery is the treatment of choice for most primary liver tumors. Surgery can involve removal of the tumor only or removal of the affected lobe of liver (called a lobectomy). Due to the liver’s regenerative capacity, a large portion of the liver can be safely removed. Surgery offers the benefit of preventing future cancer-related complications, as well as alleviating the clinical signs.

Surgical removal of benign tumors is usually recommended, because these non-cancerous tumors can transform into malignant forms and there is a risk the tumor will grow into a major blood vessel, with life-threatening consequences. Surgical removal of benign tumors may be curative, or at the least provide several cancer-free years.

The simplest malignant tumor type to remove is the massive form, as it is typically confined to one area of the liver. Surgical removal of massive HCCs may be curative, even with large tumors (as they are slow to metastasize). Nodular and diffuse forms, including neuroendocrine tumors, involve multiple liver lobes, and therefore surgery is not possible. The surgical removal of bile duct carcinomas and sarcomas is also possible, but only when they present as a single large mass. The prognosis with these cancers is poor because of the high likelihood of metastasis.

The role and effectiveness of systemic chemotherapy has not been well defined with primary liver tumors. In some cases, it may delay tumor progression, and is more commonly used with HCCs and sarcomas. Neuroendocrine tumors tend to be resistant to chemotherapy.

A newer form of therapy for localized, inoperable liver tumors is chemoembolization. In this process, a catheter is fed directly to the affected liver lobe. Chemotherapy drugs and a type of glue are injected into blood vessels that feed the tumor to block the blood supply and help shrink the tumor. The benefit of this treatment is not yet known. Although not curative, it may increase your pet’s survival time.

What is the prognosis for pets with liver tumors?

The prognosis for pets with liver tumors depends on many factors, including the type of tumor, the form (massive, nodular, or diffuse), and the presence of metastasis. Surgical removal and chemotherapy remain the primary therapeutic options, depending on the tumor type. Benign tumors and massive HCCs have a good to excellent prognosis. Bile duct carcinomas, neuroendocrine tumors, and the various sarcomas have a guarded to poor prognosis.