Neuroendocrine Tumors: Medullary Thyroid Carcinomas and Pheochromocytoma

What are neuroendocrine tumors?

Neuroendocrine tumors develop due to disorganized, uncontrolled proliferation of neuroendocrine cells, which are special cells that make up the neuroendocrine system.

Neuroendocrine cells are like nerve cells (neurons), but they also make hormones like cells of the endocrine system (endocrine cells). They receive messages from the nervous system and respond by making and releasing hormones. These hormones enter the blood and travel all over the body to control many body functions.

Neuroendocrine tumors can be benign (non-cancerous and only grow locally) or malignant (cancerous and spread to other areas of the body).

• Functional tumors over-secrete their normal hormone products and cause signs of disease due to excessive hormone levels.
• Non-functional tumors cause signs of disease due to the presence of a physical mass and/or metastasis (spread to other areas of the body).

A growing tumor can damage nearby healthy tissues, even if it’s benign, and can sometimes secrete excess hormones. Generally, neuroendocrine tumors are rare in dogs and cats. Very rarely, more than one type of neuroendocrine tumor may occur at the same time.

What is a medullary thyroid carcinoma and what clinical signs does it cause?

A medullary thyroid carcinoma is a rare tumor of the neuroendocrine cells in the thyroid gland. This tumor may release more thyroid hormone, causing signs of hyperthyroidism, including weight loss, increased appetite, and hyperactivity. However, most tumors are non-functional, which means they don’t release hormones and instead cause clinical signs due to the growing tumor, including: a mass in the neck, difficulty breathing, coughing, a change in the sound of the bark or meow, facial edema (swelling), and reduced appetite with weight loss.

What is a pheochromocytoma and what clinical signs does it cause?

A pheochromocytoma is a rare tumor of the adrenal gland that secretes excess hormones like epinephrine (adrenaline) and norepinephrine. Clinical signs result from the increased hormones and from the presence of the mass itself. Increased hormones may cause signs such as hypertension (high blood pressure), weakness, collapse, vomiting, diarrhea, panting, increased respiratory rate, and arrhythmia (irregular heartbeat). The mass itself may cause signs such as abdominal enlargement or seizures from metastasis (spread).

How are these tumors diagnosed?

These tumors may be diagnosed in a variety of ways.

• Imaging may be needed to identify the presence of a tumor, including radiographs (X-rays), ultrasounds, computed tomography (CT scans), and magnetic resonance imaging (MRI). Imaging may demonstrate one or more masses or signs that a tumor has spread to other areas of the body, such as the lungs. Some neuroendocrine tumors are found while taking images for a completely different reason.
• Tissue biopsy is needed to definitively diagnose these tumors. A tissue biopsy involves taking a small sample of tissue from the body for examination under a microscope. The tissue may be obtained by surgically removing the entire tumor or a portion of the tumor. To identify the type of cancer, a veterinary pathologist examines the tumor or pieces of the tumor under the microscope, a process called histopathology.

For neuroendocrine tumors, special procedures are often necessary to make a diagnosis. Histopathology is not only helpful to make a diagnosis but can indicate how the tumor is likely to behave.

How do these tumors typically progress?

Both of these tumors have the tendency to metastasize (spread to other areas of the body). Common sites of metastasis include the liver, lungs, lymph nodes, and nearby blood vessels. 

Staging (searching for potential spread to other locations in the body) is highly recommended for these tumors. Staging may include bloodwork, urinalysis, X-rays of the lungs, abdominal ultrasound, or other imaging procedures. If any lymph nodes are enlarged or feel abnormal, further sampling may be required to determine if spread is present.

What are the treatments for these tumors?

Treatment depends on the type, size, and malignancy of the tumor, as well as the presence of metastasis. Treatment typically involves several options, including surgery, chemotherapy, radiation therapy, and medical and dietary approaches. Your veterinarian may refer your pet to a veterinary oncologist to develop a care plan. 

If surgery is not possible, then your veterinarian may recommend certain medications to manage your pet’s symptoms.

Is there anything else I should know?

Neuroendocrine tumors are rare, so if they are suspected or diagnosed, your veterinarian may refer you to a veterinary oncologist. Many of the signs of these tumors are symptomatic of many common illnesses, and the signs may come and go, so these tumors can be difficult to diagnose. Prognosis is variable, depending on the size of the mass, presence of metastasis, and whether the tumor has invaded nearby tissues.