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==Introduction==
Two types of thymic neoplasia occur, the '''thymoma''' and the '''thymic lymphosarcoma''' ''aka'' [[Lymphoma|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 [[Lymphoma|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|vomiting]], [[Colonic Impaction - Dog and Cat|constipation]], malaise, bradycardia, muscle tremors and [[Dehydration|dehydration]].
* [[Myasthenia Gravis]] - this can be induced by a thymoma
==Diagnosis==
[[File:Thymoma B1 (2).JPG|thumb|150px|right|'''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===
[[File:Primary mediastinal large B-cell lymphoma - very high mag.jpg|thumb|150px|right|'''Mediastinal Lymphoma''' (Wikimedia Commons)]]
'''Lymphomas''' have high numbers of '''malignant''' '''[[Lymphocytes|lymphocytes]] and lymphoblasts'''.
'''Thymomas''' usually contain both '''lymphocytes and [[Epithelial Histology resource|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 [[Lymphoma|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.
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==Pathology==
==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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[[Category:Lymphoreticular and Haemopoietic Diseases]]
[[Category:Neoplasia]]
[[Category:Expert Review]]