Thymus Neoplasia

Introduction

Two types of thymic neoplasia occur, the thymoma and the thymic lymphosarcoma aka mediastinal lymphoma. With both tumour types the animal may present with very similar clinical signs, and histopathology may be necessary to confirm diagnosis. It is important to definitively diagnose the tumour as the two tumour types have very different treatments and prognoses.

Pathology

Thymoma

Thymomas are benign and localised. They are most commonly seen in cats. Their cell type can be either predominantly epithelial, or predominantly lymphocytic.

Thymic lymphosarcoma

Thymic lymphsarcoma is a malignant tumour that often metastasises. Involvement of the lung is unusual. It is also known as mediastinal lymphoma.

Normally the mass is poorly encapsulated, and there may be infiltration of the atria and heart base infiltration and the sternal pleura.

Clinical signs

Common presenting signs are associated with compression of structures in the cranial thorax:

• Dyspnoea
• Coughing
• Tachypnoea
• Weightloss
• Regurgitation
• Loss of compressibility of the cranial thorax in cats
• Displacement of heart sounds caudally
• Decreased lung sounds ventrally

Other clinical sign that may be present:

• Swelling of the head and neck due to compression of the cranial vena cava
• Horners syndrome due to compression of the sympathetic nerves within the mediastinum
• Hypercalcaemia - normally only present in lymphosarcoma in the dog. Signs include polydypsia/polyuria (PU/PD), vomiting, constipation, malaise, bradycardia, muscle tremors and dehydration.
• Myasthenia Gravis - this can be induced by a thymoma

Diagnosis

Thymoma (Wikimedia Commons)

Diagnosis of a cranial mediastinal mass may be achieved using imaging modalities such as radiography and ultrasound.

Radiography will reveal a soft tissue opacity in the cranial thorax, normally with displacement of the heart caudally and the trachea dorsally. The cranial lung fields are normally obscured by the mass. There may be evidence of pleural fluid, which can be confirmed by ultrasound.

As the thymoma and mediastinal lymphoma often look the same clinically it is necessary to differentiate them by performing an ultrasound guided fine needle aspiration of either the mass or pleural fluid, and if this proved inconclusive - a surgical biopsy.

Cytology

Mediastinal Lymphoma (Wikimedia Commons)

Lymphomas have high numbers of malignant lymphocytes and lymphoblasts. Thymomas usually contain both lymphocytes and epithelial cells. However because lymphocyte numbers can be high in lymphocytic thymomas, unless the epithelial cells in the sample are atypical or suggestive of neoplasia, it can be hard to differentiate thymomas from lymphomas. In this instance a surgical biopsy and histopathology are required.

Treatment

Treatment of the two types of neoplasia is very different:

• The treatment of choice for a thymoma is complete surgical excision.
• The treatment of choice for mediastinal lymphoma is chemotherapy.

Prognosis

Prognosis following successful surgical excision of a thymoma is good. Prognosis of mediastinal lymphoma is very poor, with a survival time of weeks if left untreated.

Thymus Neoplasia Learning Resources
Flashcards Test your knowledge using flashcard type questions	Feline Medicine Q&A 12 Small Animal Emergency and Critical Care Medicine Q&A 04

References

Elwood, C (2006) Diagnosis and management of canine oesophageal disease and regurgitation In Practice 2006 28: 14-2

Hayes, A (2006) Feline lymphoma 2. Specific Disease Presentations In Practice 2006 28: 578-585

Sparks, AH & Caney, SMA (2005) Self-Assessment Colour Review Feline Medicine Manson

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