Thymoma in the Horse By Brian S. Burks, DVM, Dipl. ABVP, Board-Certified in Equine Practice About the lymph system The lymph system is made up of thin tubes that branch out to all parts of the body. The lymph system carries lymph, a colorless fluid containing a type of white blood cell called lymphocytes. Lymphocytes fight germs in the body. B-lymphocytes, or B cells, make antibodies to fight bacteria, and T-lymphocytes, or T cells, destroy viruses and foreign cells and trigger the B cells to make antibodies. The thymus is involved in the development of T-lymphocytes. As part of the lymph system, groups of tiny, bean-shaped organs called lymph nodes are located throughout the body at different sites. Lymph nodes are found in clusters in the abdomen, groin, pelvis, underarms, and neck. In addition to the thymus, other parts of the lymph system include the spleen, which makes lymphocytes and filters blood, and the tonsils, located in the throat. Thymoma Cancer begins when normal cells change and grow uncontrollably, forming a mass called a tumor. A tumor can be cancerous or benign. A cancerous tumor is malignant, meaning it can spread to other parts of the body. A benign tumor means the tumor will not spread. The thymus contains two main types of cells: epithelial cells and lymphocytes. Thymic epithelial cells are the cells that line the thymus, and this is where thymoma and thymic carcinoma start. The tumor is uncommon in people, and rare in the horse. It seems to occur with some frequency in goats. Signs and Symptoms in people: • Persistent cough • Shortness of breath • Pain or pressure in the chest • Muscle weakness • Drooping eyelids • Double vision • Arm or facial swelling • Difficulty swallowing • Anemia (low red blood cell count) • Frequent infections • Fatigue • Dizziness Signs and Symptoms in animals: • Exercise intolerance • Shortness of breath • Edema of the head, neck and forelimb Classification In addition to stage, thymoma can be classified into different categories developed by the World Health Organization (WHO), which are based on what the tumor cells look like under a microscope: Type A thymoma. This is also called spindle cell thymoma or medullary thymoma. Type AB thymoma. Also called mixed thymoma, type AB thymoma is similar to type A thymoma; however, type AB thymoma has lymphocytes in the tumor. Type B1 thymoma. This is also known as lymphocyte-rich thymoma, lymphocytic thymoma, predominantly cortical thymoma, and organoid thymoma. This type of thymoma has many lymphocytes in the tumor, but the cells of the thymus appear normal. Type B2 thymoma. Type B2 thymoma also has many lymphocytes, like type B1 thymoma; however, the thymus cells do not appear normal. Type B2 thymoma is also known as cortical thymoma and polygonal cell thymoma. Type B3 thymoma. Type B3 thymoma is also known as epithelial thymoma, atypical thymoma, squamoid thymoma, and well-differentiated thymic carcinoma. This type of thymoma has few lymphocytes, and the thymus cells look abnormal. Thymic carcinoma (Type C thymoma). Thymic carcinoma is more aggressive. Thymic carcinoma cells do not look like normal thymus cells but like cancers that start in other parts of the body. This type of tumor is often advanced when diagnosed. It is sometimes classified into two categories: low grade, which has a better chance of recovery, and high grade, which is more likely to grow and spread quickly. Low-grade thymic carcinoma includes basaloid, mucoepidermoid, and well-differentiated squamous cell types. High-grade thymic carcinoma includes anaplastic/undifferentiated, clear cell, poorly differentiated squamous cell, sarcomatoid, and small cell/neuroendocrine types. Equine thymoma is often malignant, spreading both locally and to more distant sites. Spread may be via lymphatic or hemolytic systems, or via direct extension into another organ such as the heart. Distant sites include the ribs, neck and head. Diagnosis Diagnosis of thymoma is based upon clinical signs, a tumor in the cranial thorax where the thymus would be found, and histopathology. Radiography is used in many species to locate cranial thoracic masses, but is less useful in the horse due to size and the inability to move the forelimb very far by extension or retraction. In small animals and people, MRI, PET scans or CT. Theses again have limited utility in the horse due to their size. Ultrasound can be used to locate a mass and guide fine needle aspiration or biopsy. Most thymomas are found during necropsy examination. Treatment Surgery is possible in many smaller species including dogs, cats, rabbits, and humans. The size of the horse makes thoracic surgery near impossible. To my knowledge this has not been attempted. There are chemotherapeutic medications that can be used, and radiation may be delivered by teletherapy (beam). Radiation can be difficult as general anesthesia is required and other important structures such as the heart are in close proximity. In the future, cancer vaccines may be developed to help treat this and other neoplastic tumors. Dr. Brian Burks, Dipl. ABVP is the owner/veterinarian at Fox Run Equine Center, a 24-hour medical-surgical center near Pittsburgh, Pennsylvania. He is Board Certified by the American Board of Veterinary Practitioners (Equine Practice). This certifies him as an expert in all categories of equine practice. He enjoys the diagnostic and treatment challenges of internal medicine, oncology, surgery, and ophthalmology. Find out more and contact us : foxrunrequine, [email protected] 724-727-3481 Fox Run Equine Center foxrunequine
Posted on: Sat, 22 Nov 2014 11:00:00 +0000
Trending Topics
Recently Viewed Topics
© 2015