1. Myoid Thymoma in F-344 Rats
Neil Allison, DVM
Diplomate ACVP
Experimental Pathology Laboratories, Inc
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2. Signalment Rat, F-344 Female 1.5 year-old
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3. History National Toxicology Program (NTP) chronic carcinogenesis study
Thin, ataxic and breathing abnormally on day 526 of the study
Euthanasia
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4. Necropsy Findings Cranial mediastinal mass Discrete, oval, mottled
40 x 32 x 25 mm
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15. Desmin
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16. Cytokeratin
Note cytoplasmic weak cytoplasmic staining of mesenchymal cell
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17. Diagnosis Myoid thymoma
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18. Discussion Reviewed all thymomas in NTP Data from 1972-2003
Approximately 189,500 F344 rats
4/244 thymomas with myoid component
One myoid thymoma after 2003
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19. Discussion Benign or malignant? Gross = Huge and discrete mass
No invasion of viscera, diaphragm or body wall
No Metastasis
Cytology = Malignant
Behavior = Benign
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20. Discussion Other Names Carcinosarcoma Sarcomatoid carcinoma
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21. Acknowledgement Julie Foley Natasha Clayton Maureen Puccini
Emily Singletary
David Sabio
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22. References
1. Snover MS , Levine GD, Rosai J: Thymic carcinoma. Five distinct histological variants. Am j Surg Pathol 6: , 1982
2. Rosai J, Sobin LH: Histological typing of tumours of the thymus. In: World Health Organisation, International Histological Classification of Tumours, 2nd ed., pp Springer, Berlin, Germany, 1999
3. Henry K: An unusual tumour with a striated muscle (myoid) component (with a brief review of the literature on myoid cells). Brit J Dis Chest 66: , 1972
4. Murakami S, Shamoto M, Miura K, Takeuchi J: A thymic tumor with massive proliferation of myoid cells. Acta Pathol Jpn 34: , 1984
5. Pearse G: Histopathology of the thymus. Toxicol Pathol 34: , 2006
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