Plasma Cell Tumors and Multiple Myeloma

By Malcolm Weir, DVM, MSc, MPH; Christopher Pinard, DVM

What is a plasma cell tumor?

Plasma cell tumors develop because of a disorganized, uncontrolled production of plasma cells. Plasma cells are a type of immune system cell that create antibodies used to combat infection and infectious diseases. Plasma cell tumors of the skin or mouth are usually benign (non-cancerous). They form small tumors and can grow rapidly. These types of tumors are very rare in cats.

Some plasma cell tumors are malignant, meaning they are cancerous (e.g., multiple myeloma), and affect one or multiple organs, bone and/or bone marrow, and the gastrointestinal tract. Multiple myelomas represent less than 1% of all malignant tumors in cats and dogs. Approximately 8% of dogs with tumors affecting the bone have this condition.

What causes these types of tumors?

The reason why a particular pet may develop this, or any 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 plasma cell tumors or multiple myeloma, no direct cause is known. Certain breeds seem to be more susceptible to developing plasma cell tumors, including Airedale Terriers, Cocker Spaniels, Kerry Blue Terriers, Scottish Terriers, and Standard Poodles. Multiple myeloma is seen more often in German Shepherds.


What are the clinical signs of these tumors?

Benign plasma cell tumors typically appear as raised nodules or masses that range from a few millimeters to several centimeters in diameter and are pink or red in color. They can be hairless, ulcerate, and bleed, and can develop anywhere on the skin. The most common locations for these tumors are the head, feet, lips, mouth, and ears. These tumors are generally not bothersome; however, this is dependent on location. They may bleed or ulcerate if irritated by your pet, so trauma to the region should be avoided.

Pets with multiple myeloma are typically ill and appear to feel unwell. Pets with bone lesions may exhibit lameness or reluctance to stand because the lesions are painful. Weight loss, loss of appetite, and abnormal increases in drinking and urination may also be observed. Other signs include changes in internal organs (changes seen in the spleen or liver), which can cause abdominal distention (swollen appearance) and enlarged lymph nodes.

"Pets with multiple myeloma are typically ill and appear to feel unwell."

How are these types of tumors diagnosed?

In some cases, a fine needle aspiration (FNA) may be performed. FNA involves taking a small needle with a syringe to suction a sample of cells directly from the tumor and placing them on a microscope slide. A veterinary pathologist then examines the slide under a microscope. If a diagnosis is not confirmed by this method, a biopsy may be needed. Either a piece or the whole tumor can be removed surgically and sent to a pathologist for microscopic examination. This is called histopathology.

Multiple myeloma, on the other hand, is usually diagnosed after observing elevated protein levels on routine blood work or in a dog that is feeling unwell. Abnormalities may be observed during physical examination, abdominal ultrasound, or radiographs (X-rays). Dogs with multiple myeloma can also have specific proteins present in their urine. A special test can differentiate these from other proteins and can help conclusively diagnose multiple myeloma. Bone marrow assessment may also be needed to confirm multiple myeloma.

How do these tumors typically progress?

Benign plasma cell tumors are generally solitary and do not spread. They may grow in a confined area and cause discomfort, especially if in the oral cavity. However, these tumors can be removed successfully when they are small.

Multiple myeloma is a very malignant cancer. Depending on the type of myeloma, it can involve multiple organs and bones, causing pain and discomfort. Elevated protein levels can cause kidney damage and elevated blood pressure. A hemorrhage in the retina (tissue in the back of the eye) is observed in some dogs. Lesions affecting bones can be painful and are susceptible to fracture, depending on the location. Some patients with myeloma have elevated blood calcium, which can cause kidney damage. Multiple myeloma is fatal without treatment.

What are the treatments for these types of tumors?

Benign plasma cell tumors are generally treated with surgery. Given that these tumors are benign, they are less likely to recur.

Because multiple myeloma is a systemic disease of plasma cells, it is generally treated with chemotherapy. Once chemotherapy is initiated, signs associated with the tumor tend to resolve. Multiple myeloma tends to recur, and your pet may exhibit signs while being treated or after chemotherapy is completed. If this does occur, other chemotherapy treatments may be pursued.

Both benign plasma cell tumors and multiple myeloma are treatable; however, multiple myeloma has a high recurrence rate. The goal of chemotherapy is to provide resolution of the clinical signs that your pet is experiencing.

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