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Comparative Study
. 2019 Oct;47(7):833-841.
doi: 10.1177/0192623319863119. Epub 2019 Aug 8.

Comparative Incidences and Biological Outcomes for Thymoma in Various Rat Strains in National Toxicology Program Studies

Rebecca R Moore  1 , Hiroaki Nagai  2 , Rodney A Miller  3 , Jerry F Hardisty  3 , Neil Allison  3 , Keith R Shockley  4 , David E Malarkey  2
Affiliations
Comparative Study

Comparative Incidences and Biological Outcomes for Thymoma in Various Rat Strains in National Toxicology Program Studies

Rebecca R Moore et al. Toxicol Pathol. 2019 Oct.

Abstract

Thymomas from 277 Fischer 344/N (F344/N), 10 Sprague Dawley (HSD:Sprague Dawley SD) (SD), 129 Wistar Han [Crl:WI(Han)] (WH), and 4 Wistar Outbred (WO) rats were reviewed from long-term studies in the National Toxicology Program (NTP) database. The incidence of thymomas in F344/N rats was slightly higher in males than in females, while the incidences in SD and WH rats were higher in females than in males. Only male WO rats were used in NTP studies. Of the 277 thymomas in F344/N rats, 235 (84.8%) were benign and 42 (15.2%) malignant, 14 of which exhibited metastasis. Of the 10 thymomas in SD rats, 5 (50%) were benign and 5 (50%) were malignant, one of which exhibited metastasis. Of the 129 thymomas in WH rats, 126 (98%) were benign and 3 (2%) were malignant, 1 with metastasis. Of the 4 thymomas in WO rats, 3 (75%) were benign and 1 (25%) was malignant, with no metastases. Malignant thymomas in F344/N and WH rats showed a propensity to be the cause of death and to result in early mortality, whereas the benign thymomas were associated less often with decreased survival. No occurrences of this neoplasm were reported to be related to exposure to any test articles.

Keywords: F344/N; National Toxicology Program; Sprague Dawley; Wistar Han; rat; thymoma; thymus.

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Conflict of interest statement

Declaration of Conflicting Interests

The author(s) declared no potential, real, or perceived conflicts of interest with respect to the research, authorship, and/or publication of this article.

Figures

Figure 1.
Figure 1.
This benign thymoma measuring 3.7 × 2.8 cm was removed from the cranial mediastinum of a female F344/N rat from a 2-year NTP study.
Figure 2.
Figure 2.
This is a well-circumscribed, nodular, benign thymoma from a male F344/N rat from a 2-year NTP study. Hematoxylin and eosin.
Figure 3.
Figure 3.
Higher magnification of Figure 2 showing the neoplastic epithelial cells forming cords and tubules. Hematoxylin and eosin.
Figure 4.
Figure 4.
Lung metastasis from a malignant thymoma in a male F344/N rat from a 2-year study. Sheets of elongated neoplastic cells form a spindloid pattern. Hematoxylin and eosin.
Figure 5.
Figure 5.
This benign thymoma is from a male F344/N rat from a long-term study. The neoplastic epithelial cells form a papillary pattern. The papillary projections (arrows) protrude into a cystic space (asterisk). Hematoxylin and eosin.
Figure 6.
Figure 6.
This malignant thymoma is from a male F344/N rat from a long-term study. The neoplastic epithelial cells form a squamous pattern. Hematoxylin and eosin.
Figure 7.
Figure 7.
This malignant thymoma is from a female F344/N rat from a long-term study. The neoplastic cells form a predominantly myoid pattern. Hematoxylin and eosin.
Figure 8.
Figure 8.
(A) Immunohistochemical staining of neoplasm from Figure 7 showing expression of desmin. (B) Higher magnification showing desmin-positive myoid cells with prominent cross striations (arrow).
Figure 9.
Figure 9.
This benign thymoma is from a female F344/N rat from a long-term study. Packets of elongated neoplastic cells form a neuroendocrine pattern. Hematoxylin and eosin.
Figure 10.
Figure 10.
Benign thymoma from a female Wistar Han rat from a long-term study. There is a predominance of small lymphocytes which partially obscure the underlying neoplastic epithelial cells. Hematoxylin and eosin.
Figure 11.
Figure 11.
Benign thymoma from a female Wistar Han rat from a long-term study. This lymphocyte-rich thymoma has a medullary pattern, consisting of numerous lymphocytes separated into lobules by fibrous trabeculae and surrounding central foci of large pale-staining cells. Hematoxylin and eosin.
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References

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