Thymoma in Dogs and Cats

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Article Written by Dr. Daniel A. Degner, Board-certified Veterinary Surgeon (DACVS)

Animal Surgical Center of Michigan

5045 Miller Road

Flint, MI 48507

Phone: 810-671-0088

Key Points

Thymoma frequently has nonspecific clinical signs

Video Assisted Thoroscopic surgery is the best treatment option

Thymoma, if noninvasive carries a very good prognosis

What is the thymus?

The thymus is a an organ of the lymphatic system located within the chest cavity (just in front of the heart) that produces a subset of lymphocytes (white blood cells) called T-cells. These white blood cells are important to kill foreign cells, activate other immune cells to fight infection, and stop the immune response after a foreign cell or microbe has been eliminated. T-cells migrate away from the thymus to other parts of the body such as the lymph nodes, spleen, and the bloodstream. As the pet matures, the thymus has served its function, therefore it will wither away.



Tumors of the thymus gland

The thymus gland can develop a variety of cancers which most commonly include lymphoma and thymoma. Lymphoma is a cancer of the lymphocytes, whereas thymoma is a cancer that originates from the epithelial cells of the thymus. Thymoma can be either an invasive or noninvasive type of tumor. In fact, approximately 50% of the dogs have the non-invasive type of tumor which does not extend into the surrounding tissues and can be easily removed with surgery. The invasive thymoma is deeply rooted in surrounding structures such as the great vessels of the heart and the heart sac itself. Thymic carcinoma is a very rare, malignant tumor that spreads to other parts of the body.



Clinical signs

Thymoma most commonly affects medium to large breed dogs. Labradors and German shepherds are more commonly affected. Most pets with this condition are older, with the median age reported to be 11 years. Although likely coincidental, dogs having thymoma frequently have another type of cancer in their body. Only about 5% of the dogs with thymoma have hypercalcemia (high calcium level in the blood) due to the tumor. Hypercalcemia causes increased thirst, increased urination, decreased appetite, weakness, lethargy, and other nonspecific clinical signs. Most signs directly related to thymoma are nonspecific and may include decreased exercise tolerance, breathing difficulties, coughing, difficulty swallowing, and weight loss. The respiratory signs are due to enlargement of the tumor and compression of the windpipe within the chest cavity. Swallowing difficulties and regurgitation of fluid or food can be secondary to megaesophagus or compression of the esophagus within the chest. Paralysis of the voice box (laryngeal paralysis) can be secondary to nerve damage from the tumor. Invasive thymomas can cause cranial vena cava syndrome which manifests itself as swelling of the lower jaw, neck, forelimbs, and can result in fluid buildup in the chest (pleural effusion).

Para neoplastic syndromes secondary to thymoma include myasthenia gravis, hypercalcemia, aplastic anemia, immune-mediated diseases (immune mediated anemia, polymyositis dermatitis in cats), abnormal heartbeats due to inflammation of the heart muscle, and hypogammaglobulinemia.




A presumptive diagnosis of thymoma is based on finding a mass on an x-ray in front of the heart. Other tumors in this area may also include lymphoma of the thymus, carcinoma of the thymus, branchial cyst, displaced thyroid or parathyroid tumors, aortic body tumors, tumors that have spread from elsewhere into this region, and sarcomas that extend from the chest wall. Definitive diagnosis requires a biopsy of the mass, which is commonly done after the tumor has been surgically removed. Fine needle biopsy prior to surgery can be performed and may be useful to diagnose tumors other than thymoma. One of the challenges, however, is that thymoma tumors also have lymphocytes in them, which may confuse the diagnosis with lymphoma. Other tests that are commonly performed in dogs that have a suspected thymoma include 3 view chest x-rays, advanced imaging such as CT scan or MRI, chest ultrasound, and bloodwork including a complete blood count and biochemistry profile, and an urinalysis.



Surgery is the primary treatment for thymoma. Currently, in most surgical animal hospitals, tumors of the thymus gland are removed within an open surgical approach, which involves spreading the ribs or more commonly cutting the breast bone with a saw. I recommend to remove these tumors with video assisted thoroscopic surgery (VATS). During this procedure, a camera on a thin tube (thoroscope) is inserted into the chest via a small incision; this allows the surgeon to see the inside of the chest on a video monitor (see video right).


Two other small incisions are made to insert instruments into the chest cavity. With the use of a special cautery instrument called the Ligasure, the tumor is liberated from the surrounding tissues. The tumor is then placed in a surgical plastic bag and removed via one of the ports. A chest tube is placed after the surgery, regardless of whether it is done with the traditional open techniques or with VATS. The hospital stay following VATS is about 48 hours, versus 5 days with traditional surgery.



















Radiation can be a very effective treatment for thymoma in dogs and cats. Approximately 75% of the patients will respond to this therapy. Radiation side effects may include inflammation of the lung and heart sac (pericardium). Radiation treatments are administered daily, 5 days a week, for a period of 18 to 21 treatments. As a result, radiation is only recommended in patients that have a tumor that cannot be surgically removed. Chemotherapy is largely ineffective for thymoma.



Most dogs that have a noninvasive thymoma removed are cured of their disease. In fact most of them died of unrelated causes. The overall one-year survival rate following thymectomy is 83%. If a patient has other complicating factors such as pneumonia or megaesophagus the prognosis may not be as good. The median survival time of dogs receiving radiation therapy is 248 days. Overall, the prognosis for dogs that have an invasive thymoma is guarded.


Frequently Asked Questions After Surgery

When should my dog have the first bowel movement after surgery?

  • Many dogs will not have a bowel movement for the first 4 to 5 days after surgery
  • Reasons that a dog will not have regular bowel movements after surgery include:
    • The dog has been fasted prior to surgery
    • Dogs do not eat well during the hospital stay
    • They frequently do not eat well when they go home
    • They are fed highly digestible food that produces little stool
    • Pain medication that contain narcotics (such as morphine, fentanyl patches, and tramadol) can be constipating
  • If a pet does not have a bowel movement on the 5th day of being home, a stool softener such as metamucil can be fed
    • Dose of metamucil is 1 tsp per 25 Kg mixed in with each meal (canned dog food); feed immediately after mixing, as the metamucil will gel the food and may make it less palatable

My pet had surgery and will not eat.  What can be done?

  • Dogs
    • Most pets will not eat their regular dog food after surgery, especially if it is kibble.
    • Offer a cooked diet having a 1:1 ratio of a protein source and carbohydrate source.  The protein source can be any meat (example: chicken breast, turkey breast, lean hamburger) that is low in fat and should be cooked (drain off all fat after the meat has been cooked).   The carbohydrate can be pasta, potato or white rice.
    • Try canned dog food; to enhance the flavor sprinkle a very small amount of garlic powder or chicken or beef broth (Chicken-in-a- MugTM or Beef-in-a-MugTM products)
    • Try Gerber strained meats for babies such as the chicken, beef, turkey, or veal
    • Try Hill's A/D diet available at most veterinary hospitals
    • Hand feeding: place a small amount of food in the mouth so that your dog gets the flavor
    • Warm the food slightly in a microwave, as the food will be more aromatic; stir the food before feeding and test the temperature on the bottom side of your wrist; it should only be luke warm.
    • Remember that most pets will not eat the first day or two after they get home from surgery
  • Cats
    • Offer smelly foods that contain fish such as tuna or smelly cat foods
    • Try Gerber strained meats for babies such as the chicken, beef, turkey or veal
    • Hand feeding:  with your finger place a small amount of food on the roof of your cat's mouth; use a syringe to get soft food into the mouth
    • Warm the food slightly in a microwave as the food will be more aromatic; remember to stir the food before feeding and test the temperature; it should be only luke-warm
    • Some cats will only eat dry food, try kibble if your cat normally has been fed that food
    • Petting and stroking your cat frequently will help to stimulate appetite
    • Remember that most pets will not eat the first day or two after they get home from surgery
    • Appetite stimulants such as cyproheptadine may be helpful
    • If your cat refuses to eat anything for 7 days a stomach tube or nasogastric tube should be placed to provide nutrition so that a serious liver problem (hepatic lipidosis) does not develop

My pet is vomiting.  What can be done?

  • The first thing for you to discern is whether your pet is vomiting or regurgitating.  Both will result in fluid or food being brought up.  Vomiting always will have heaving or retching of the abdomen prior to expulsion of the vomitus.  Regurgitation is not associated with heaving and the pet usually just opens the mouth and fluid or food will be expelled.  Usually the regurgited material will be clear or brown colored fluid. 
  • Next is to identify the cause of the vomiting or regurgitation.
  • Causes and treatment of vomiting after surgery
    • When some pets return home after a stay in the hospital they may drink excessive amounts of water at one time and then vomit; if this appears to be the case, the water should be limited to frequent smaller amounts.
    • Medications such as antibiotics, narcotics or nonsteroidal anti-inflammatory medication commonly cause vomiting after surgery.  In order to see which medication is causing the problem, the administration of each drug should be separated 2 hours apart.  Usually the pet will vomit or appear nauseated (drooling and sick look) within 1 hour of administration of the medication that they are sensitive to.  The antibiotic in some cases may be changed to a different one, or may be discontinued. 
    • Stomach upset from anesthesia is a potential cause of vomiting and will pass within a couple of days. 
    • An uncommon cause of vomiting after surgery is internal organ failure.  Blood testing will confirm this problem. For this reason vomiting should not be ignored if it persists for more than 24 hours.
    • If your pet had surgery of the bowels or stomach, vomiting is always a concern, as it may indicate that infection of the abdominal cavity, called peritonitis, is present.  Do not ignore this sign.
    • Symptomatic treatment of vomiting involves withholding food for 12 to 24 hours, then introducing small amounts of bland food such as rice and lean cooked hamburger, if your pet does not vomit after that then gradually wean him/her back onto the regular diet after 3 days.  In order to decrease the acidity of the stomach, Pepcid AC 0.5 mg/kg can be given by mouth twice daily for 5 days.  Metoclopramide and Cerenia are good anti-vomiting medications for dogs and cats.  You should always consult a veterinary healthcare professional before administering medication.
  • Causes and treatment of regurgitation after surgery
    • The most common cause of regurgitation is reflux of acid from the stomach into the esophagus while your pet is under anesthesia.  Acidic fluid from the stomach can cause a chemical burn of the esophagus and result in a bad case of heart burn, called esophagitis.  This results in poor motility of the esophagus, therefore water and food will accumulate in this structure.  In most cases, esphagitis is self-eliminating and will resolve within two or three days. 
    • If the esophagitis is severe the esophagus may develop one or more strictures.  A stricture is a narrowing or stenosis of the esophagus and does not allow passage of food down the esophagus, in regurgitation that lasts longer than one week.  This problem should be brought to the attention of your pet's doctor within the first two weeks so that it can be treated by ballooning the stricture (minimally invasive procedure, as it is done with the aide of an endoscope).  If an esophageal stricture is chronic surgery is needed.
    • Symptomatic treatment of regurgitation caused by esophagitis includes feeding bland food, and administering a coating agent (sucralfate) and an acid blocker (omeprazole or other).  Consult a veterinary health care professional if the regurgitation continues for more than a couple of days.

How do I know that my dog is in pain following surgery?

  • Signs of pain include
    • crying
    • biting if you get near the surgical site
    • grimacing (lips are pulled back and the the dog looks anxious)
    • tragic facial expression
    • panting
    • restlessness and unable to sleep; pacing
    • if abdominal surgery was done the pet will not lie down on the incision, or will continually sit up in spite of appearing very tired
    • the worst pain will be for the first 2 to 3 days after surgery

What can I do to control my dog's pain?

  • Narcotic medications that control pain: tramadol, butorphanol, Duragesic (fentanyl patch)
  • Anti-inflammatories used to control pain: Deramaxx, Rimadyl, Previcox, or Etogesic
  • If an orthopedic surgery has been done cold packing the surgical site may be helpful
    • A cold pack may be a pack of frozen peas, crushed ice in a Ziploc bag, or a cold gel pack; place a thin barrier between the skin and the cold pack.  An alternative to a cold pack is to freeze water in a styrofoam cup; after frozen cut the bottom of the styrofoam cup out. Cool the surgical site around the incision by rubbing the exposed ice directly on the skin in a circular pattern.  Cooling the surgical site helps to numb the area.

How do I know that my cat is in pain following surgery?

  • Pain is more difficult to assess in cats versus dogs, as signs can be more subtle and they usually do not vocalize when in pain
  • Signs of pain in a cat include the following:
    • biting if you get near the surgical site
    • growling or deep cry
    • not wanting to eat
    • hiding and not wanting to be near owner (remember that this could also be caused by the cat just being upset about leaving home and coming back)

What can be done for pain at home for my cat?

  • Pain medication such as buprenorphine or a Duragesic (fentanyl) patch
  • Tylenol will kill a cat as they lack abundant glutathione enzyme in the liver
  • Anti-inflammatories can be used, but the dose is much less than dogs

Is it okay for my pet to lick the incision?

  • If a dog licks the incision, the healing process may be delayed.
  • Licking can remove stitches and cause the incision to open
  • Licking can become a severe habit that is difficult to break
  • Licking can cause infection as the mouth has many bacteria
  • Dogs will frequently lick the incision when the owner is not watching such as at night time; if the skin looks red or excoriated the most common cause is from licking.
  • To stop your pet from licking the following can be tried:
    • Elizabethan collar can be placed on the neck; this will not help stop your pet from scratching at the region
    • Cervical collar (bite not collar) is a less awkward device and can be effective at stopping a pet from licking the surgical site
    • A tee shirt can be used to cover an incision on the chest or front part of the abdomen; gather the waist of the shirt up over the dog's back and wrap an elastic band around this part of the shirt.
    • A bandage or sock can be used to cover an incision on a limb; fasten the top of the sock to the dog's limb with tape.
    • Bitter apple can be applied around the incision; many dogs will continue to lick  after application of this topical
    • Bitter Apple and Liquid HeetTM (obtain this from a is used for sore muscles) mixed in a 2:1 ratio can be applied around the skin incision
    • Antipsychotic medication in some cases is needed

Board-certification by the American College of Veterinary Surgeons

What does it mean?

  • Four years of advanced training in surgery beyond the Doctor of Veterinary Medicine Degree

  • Experience in the development of new surgical treatments

  • Rigorous examination by the American College of Veterinary Surgeons to ensure competency in advanced surgical techniques

  • Assurance that a veterinarian is a surgical specialist

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