Lymphosarcoma (LSA) is defined as a proliferation of malignant lymphoid cells. It is the most common hematopoietic tumor affecting cats and dogs. A unique feature of feline LSA is that cats tend to show symptoms of illness earlier and more severely than dogs, which provides an opportunity for an earlier diagnosis. Lymphoid tissues are found throughout the body, and there are multiple forms of LSA, most of which tend to be responsive to treatment. Without early initiation of treatment, the average survival time of cats that have been diagnosed with LSA is measured in weeks. Fortunately, 70% of feline cases respond to treatment, and survival time is often measured in months. In some cases, cats have been reported to go into remission for up to two years.
Another unique feature of feline LSA is in regard to predisposing factors. Historically, a strong link has been shown between feline leukemia (FeLV) positive cats and at least one form of LSA. Twenty years ago, when FeLV had a higher incidence, the average age of cats diagnosed with LSA was 4-6 years. Currently, with a much lower incidence of FeLV in the cat population, the average age of diagnosis has shifted to 9-10 years. LSA can affect any cat, but those with FeLV are at an increased risk.
The three main types of LSA are multicentric, mediastinal, and alimentary. The mediastinal form is the type that tends to affect younger cats and is strongly associated with FeLV positive status (80%). Mediastinal LSA is found in the chest cavity and affects the thymus and associated lymph nodes. The most common presentation that owners detect is respiratory distress. The veterinarian may detect muffled heart sounds and pitting edema of the head, neck, and forelimbs secondary to compression of the largest blood vessel in the chest cavity. This is referred to as precaval syndrome, because the vessel is called the cranial vena cava. Hypercalcemia is commonly present with this type of LSA, which can cause cats to urinate and drink excessively (polyuria polydypsia, or PU/PD).
Alimentary LSA is currently the most common form, and tends to affect older cats. It is not considered to be strongly linked to FeLV status, although FeLV positive cats that also get alimentary LSA have a more guarded prognosis. This form affects the digestive tract and surrounding lymph nodes. Symptoms tend to be very nonspecific gastrointestinal signs such as diarrhea, vomiting, and lack of appetite.
Multicentric LSA, as the name implies, involves multiple lymph nodes and often multiple organs (ie liver and spleen). About 30% of cats diagnosed with this form are FeLV positive, and those that are tend to be younger. This suggests that there are multiple predisposing factors, and FeLV may cause the disease to develop sooner than it would have otherwise. The presenting signs of this form are extremely variable since multiple organ systems can be involved. Often times, these patients are systemically ill and lethargic. Hypercalcemia can occur, as with mediastinal LSA, so PU/PD can be an early presenting sign. PU/PD may also develop if the kidney is infiltrated. In some cases, especially early, the palpable lymph nodes may be enlarged with no other signs. Anemia and generalized cytopenias are often present in late stages because of bone marrow involvement, which prevents new blood cells, platelets, and white blood cells from being formed normally.
Cats have some benign lymph node syndromes that are unique to the species. When working up a feline patient with enlarged lymph nodes, it is important that the veterinarian rule out idiopathic peripheral lymphadenopathy, plexiform vascularization of lymph nodes, and peripheral lymph node hyperplasia of young cats. All of these syndromes can cause the lymph nodes to harmlessly enlarge. Therefore, a feline patient with enlarged lymph nodes and no other signs of illness is not necessarily afflicted with LSA, as is usually the case in a dog with the same presentation.
There is no single test to diagnose LSA in the feline patient. Diagnosis is based on multiple observations and tests and a complete clinical picture. Physical exam may or may not reveal enlarged peripheral lymph nodes. Abdominal palpation may reveal enlarged organs or thickened intestines, but these signs are more easily appreciated with radiographs and ultrasound. In addition, most lymph nodes in the body are not palpable and can only be visualized through radiography or ultrasound. In some species, a fine needle aspirate of any enlarged lymph nodes is often very informative, but in the cat it is better to biopsy a suspect lymph node to prevent confusion with the above mentioned, benign syndromes that are unique to cats.
Bloodwork is often helpful in the diagnostic process, especially in multicentric forms. Most cases of this form will have abnormalities depending on which organ systems are affected. Increased calcium is a common finding in mediastinal and multicentric forms, but is not specific to LSA. Anemia is not an uncommon finding, especially in FeLV positive cats. When multicentric LSA is determined, a bone marrow aspirate should be performed to stage the disease and determine the prognosis. If the bone marrow is involved, the prognosis is significantly reduced. All cats that are diagnosed with any type of LSA should be FeLV/FIV tested because the prognosis is greatly reduced in these patients.
The treatment of choice for LSA is chemotherapy. In cases where a distinct tumor is present, surgical removal and radiation treatments may also be used, but these treatments should also be combined with chemotherapy. As mentioned previously, 70% of feline patients respond to treatment. This statement, however, does not reflect a cure rate. Chemotherapy is expected to slow the progression of the disease and improve the patient?s quality of life, and will sometimes result in remission for up to two years depending on the type and stage of the disease as well as other health factors.
The decision to undergo chemotherapy treatment is a very personal decision, and must be carefully considered for each patient and guardian. It is worth mentioning that the approach to cancer treatment in animals is not the same as it is for humans. Most veterinarians will agree that if the patient's quality of life is reduced due to treatment, then the treatment should not be administered. In human medicine, the patient understands that aggressive treatment may result in many years of good health, and can make a choice to go through very difficult times in hopes of a long term cure. Our feline patients do not have that ability, of course, so most veterinarians and veterinary oncologists will not put them through treatments that make them worse. Each and every case is carefully considered, and the guardians that know the cat best are always heavily involved in every step of the decision making process to insure that the best plan is implemented and adjusted as necessary.