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Thymomas in Fischer 344/N Rats in the National Toxicology Program Database
Gabrielle A Willson1, Rebecca R. Moore1, Hiroaki Nagai2, Rodney A. Miller1, Jerry F. Hardisty1, Neil Allison1, David E. Malarkey2
1Experimental Pathology Laboratories, Inc., Research Triangle Park, NC, USA
2Cellular and Molecular Pathology Branch, Division of the National Toxicology Program, National Institute of Environmental Health Sciences, Research Triangle Park, NC, USA
Thymoma is an uncommon tumor in most strains of rats and mice.
Primary tumors of the thymus of Sprague-Dawley-derived rats
(strain Tif:RAI) were reported from 7 different long-term studies
(Naylor, et al., 1988). In that study, 192 primary thymic tumors were
identified (out of 4281 animals), of which 171 appeared benign and
21 appeared malignant. In the F344/N rat spontaneously occurring
thymomas are rare, and most of those have been reported to be
benign (Stefanski, Elwell, and Stromberg, 1990; Haseman, Hailey,
and Morris, 1998). The literature contains no reports of large studies
of thymoma in F344/N rats. The purpose of this study is to describe
the morphological features, incidence, and behavior of thymoma in
F344/N rats in NTP studies.
Two hundred seventy-seven thymomas recorded in chronic studies in
the NTP archives database were reviewed. No thymomas were
recorded in any of the subchronic studies. Thymomas in F344/N rats
in this survey were rare occurrences (<0.2% incidence). No
occurrences of this tumor were found to be associated with
treatment. Thymomas occurred more commonly in male rats
(156/277 [56.3%]) than in female rats (121/277 [43.7%]). Benign
thymomas comprised 84.8% (235/277) of the thymomas reviewed,
and the remaining 15.2% (42/277) were diagnosed as malignant
thymomas based upon metastasis, unequivocal tissue invasion, or
cytological malignant features (Table 1). In male rats 27/156 (17.3%)
thymomas were malignant, and in female rats 15/121 (12.4%) were
malignant. Metastases were observed in 14 of the 42 animals with
malignant thymomas, and in all 14 cases metastatic lesions were
observed in the lung. Metastases also were present in the lymph
node of 1 animal and in the liver of another animal. All of the
malignant thymomas that metastasized had features of the main
tumor mass and no malignant epithelial neoplasms were present in
other organs. Seventeen of the remaining 28 malignant thymomas
that exhibited no metastases were diagnosed due to their
unequivocal tissue invasion, and 11 cases were diagnosed based
upon cytological features, such as pleomorphism, cellular atypia,
karyomegaly, anisokaryosis, prominent nucleoli, and increased
mitoses. These cytologic features typically associated with
malignancy seemed to have little to no relationship to a propensity for
invasion or metastasis.
Thymomas in Fischer 344/N rats were morphologically
heterogeneous but could be divided into 6 general categories
according to morphologic features (Table 2). Most tumors displayed
a mixture of more than one morphologic pattern, and these were
grouped according to the predominant pattern. The most common
morphologic pattern consisted of epithelial cells arranged in cords
and tubules and was present in 155/277 (56.0%) of the tumors in this
review (Figure 1). This was followed by thymomas with a
predominantly spindloid pattern, which occurred with a frequency of
79/277 (28.5%) (Figures 2 and 3). A papillary pattern was observed
in 17/277 (6.1%) of the tumors and was frequently associated with
cystic features (Figure 4). A squamous epithelial pattern was present
in 12/277 (4.3%) of the tumors (Figure 5). A myoid pattern was
present in 11/277 (4.0%) of the tumors (Figure 6A). The myoid cell is
a striated muscle cell which exhibits expression of desmin (Figures
6B and 6C). While it is possible that myoid cells within the thymomas
in this series represented invasion of pre-existing muscle, myoid
differentiation of neoplastic cells could not be ruled out. A
neuroendocrine pattern was observed in only 3/277 (1.1%) of the
tumors (Figure 7). No particular pattern was unequivocally correlated
with a metastatic or invasive propensity.
Four benign thymomas (4/228; 1.8%), 11 malignant thymomas
without metastasis (11/28; 39.3%), and 11 malignant thymomas with
metastasis (11/14; 78.6 %) were considered to be the cause of death.
The survival days of animals with malignant thymomas were less
than those of animals with benign thymomas (Table 3).
Thymoma is defined as the neoplastic proliferation of thymic
epithelial cells. By virtue of the cellular composition of the thymus,
thymomas contain varying numbers of non-neoplastic lymphocytes.
Various classification schemes have been described in veterinary
medicine, and most are based upon histomorphological features.
One such scheme creates divisions based upon the relative
proportions of lymphocytes and epithelial cells, thus specifying tumor
types as either epithelial type, mixed epithelial and lymphoid cell
type, or “pure” lymphoid type (containing scattered epithelial cells to
distinguish it from well-differentiated lymphoma) (Greaves, 2012).
Others suggest that thymomas are divided into two types, consisting
of tumors with and without medullary differentiation (Kuper, Beems,
and Hollanders, 1986; Kuper and Beems, 1990). Tumors exhibiting
lobular, medullary differentiation have pale staining areas containing
fewer lymphocytes, fibrous trabeculae, and may contain epithelial
cells forming cords, tubules, and cysts. Thymomas without medullary
differentiation consist of a mixture of small lymphocytes and epithelial
cells without a lobular architecture. While epithelial cell morphology
can be quite variable, cellular atypia has been found to be rare
(Kuper and Beems, 1990). Epithelial cells of thymomas can form
several morphological patterns, which have been described by one
author as epidermoid (non-keratinizing squamous epithelium);
squamoid (with keratinization); papillary; ribbons, cords, or tubules;
spindloid; endocrine-like; neuroendocrine; and myoid (Pearse, 2006).
Thymoma is an uncommon tumor in most strains of rats and mice.
Primary tumors of the thymus of Sprague-Dawley-derived rats
(strain: Tif:RAI) were reported from 7 different long-term or lifespan
studies (Naylor, et al., 1988). In that study, 192 primary thymic
tumors were identified (out of 4281 thymuses), of which 171 were
classified as benign and 21 were called malignant. In the F344/N rat
spontaneously occurring thymomas are rare, and most of those have
been reported to be benign (Stefanski, Elwell, and Stromberg, 1990;
Haseman, Hailey, and Morris, 1998). The purpose of this study is to
describe the morphological features, incidences, and behavior of
thymoma in Fischer 344/N rats in NTP studies.
Thymomas are neoplasms of thymic epithelial cell origin which may
contain variable numbers of non-neoplastic lymphocytes. Thymomas
appear grossly as a firm, smooth mass in the anterior mediastinum,
and compression of the surrounding tissues may or may not be
present. These tumors are rare in humans and most lab animals. In
the Fischer 344/N rat, thymomas are rare and are said to occur with
no apparent sex difference (Stefanski, Elwell, and Stromberg, 1990),
although in this review more thymomas were observed in male rats.
There is no meaningful established classification system of
thymomas in veterinary medicine, whereas in human medicine there
is the widely used classification system established by the World
Health Organization (WHO) (Rosai and Sobin, 1999). Nevertheless,
the WHO system has been adapted for use in one of the largest
hamster thymoma papers to date (Brandes et al., 2004).
Diagnostic paradigms have been described for rat thymomas,
including those dividing them according to the proportions of
epithelial and lymphoid cells (Greaves, 2012), as well as divisions
based upon thymomas with and without medullary differentiation
(Kuper and Beems, 1990; Kuper, Beems, and Hollanders, 1986).
While understandable and useable, these paradigms do not seem to
add significantly to our ability to interpret the rat thymoma in relation
to toxicologic or biological significance.
The thymomas in this review were variable in microscopic
appearance and were separated into 6 categories based upon
morphological patterns. Although this could be done with some
consistency, there was no apparent difference in biological behavior
between the categories. Since separating thymomas into several
sub-classifications appears to have limited, if any, scientific relevance
in terms of biological behavior, it seems adequate to designate them
only as benign or malignant until new information is obtained.
Classifying thymomas as benign or malignant traditionally has been
based upon cytologic features and biological behavior such as
invasion and metastasis as discussed in two volumes of International
Classification of Rodent Tumors (Hailey et al., 1993; Frith et al.,
2001). In this review there was little evidence that cytologic features
typically associated with malignancy were necessarily correlated with
invasion or metastasis
The NTP studies evaluated for this review ended at 104 to 105
weeks for chronic studies and 13 weeks for subchronic studies.
Complete necropsies were performed on all animals. All gross
lesions and approximately 43 tissues were collected, fixed in 10%
neutral buffered formalin, stained with H&E, and examined. Thymic
tissue and/or thymic masses were collected from all rats on each
The NTP historical databases were reviewed for the diagnosis of
benign thymoma, malignant thymoma, and thymoma. The
databases contained data on ~190,000 Fischer 344/N rats (~175,000
rats from chronic studies and ~15,000 rats used in subchronic
studies). All sections of thymomas were evaluated for unique
morphologic features, patterns of growth, and the presence or
absence of invasion and/or metastasis that could be used to
categorize them as benign or malignant. Tabulation of data was
designed to determine thymoma occurrences in male and female
rats, classification of tumors as benign or malignant, and to
characterize general morphologic patterns that may relate to
The classification of the neoplasms as benign or malignant was
based upon tumor expansion and invasion of surrounding tissues, as
well as presence of metastatic lesions. Thymomas were classified as
benign when they were confined to the mediastinal space, discrete,
exhibited an expansile non-invasive growth pattern, and there was no
evidence of metastasis. Extension into the mediastinal fat was
considered a benign feature as long as there was connection to the
main tumor mass and as long as the other benign tumor criteria were
met. Malignant thymomas were characterized by implantation on
serosal surfaces, metastasis to other organs, invasion of adjacent
tissues (body wall, diaphragm, or thoracic organs), and/or infiltration
of mediastinal fat with nests of neoplastic cells but no apparent
connection with the main tumor mass.
Figure 1A. Benign thymoma from a male F344/N rat from a 2-year NTP
Figure 1B. Higher magnification of 1A, the neoplastic epithelial cells form
cords and tubules. (H&E)
Figure 2. Spindloid pattern in a malignant thymoma from a female F344/N
rat from a two-year NTP study. (H&E)
Figure 3. 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
Figure 4. Benign thymoma is from a male F344/N rat from a 2-year study.
Neoplastic epithelial cells form papillary projections, which protrude into
cystic spaces. There are small clusters of non-neoplastic lymphocytes
interspersed with the neoplastic cells. (H&E)
Figure 5. Malignant thymoma is from a male F344/N rat from a 2-year
study. The neoplastic epithelial cells form a squamous pattern. (H&E)
Figure 6A. Malignant thymoma is from a female F344/N rat from a 2-year
study. The neoplastic epithelial cells form a myoid pattern. (H&E)
Figure 6B. Immunohistochemical staining of neoplasm from 6A showing
expression of desmin in myoid cells. (Desmin)
Figure 6C. Immunohistochemical staining of neoplasm from 6A showing
expression of desmin with prominent cross striations in myoid cells.
Figure 7. Benign thymoma is from a female F344/N rat from a 2-year study.
Packets of elongated neoplastic cells form a neuroendocrine pattern.
without metastasis with metastasis
Number of animals 228 a 28 14
Mean survival days b 713 ± 4 609 ± 27 506 ± 42
Cause of death: thymoma 4 11 11
Natural death 16 8 8
Moribund sacrifice 48 9 6
Terminal sacrifice 164 11 0
a Seven animals were not included in this table because there were 2 accidental deaths
and 5 animals were sacrificed for interim necropsy.
b Study days on which animals were sacrificed. Value shown is group mean ± S.E.
Morphologic pattern Benign Malignant Total
Cords/Tubules 139 16 155
Spindloid 66 13 79
Papillary 17 0 17
Squamous 5 7 12
Myoid 6 5 11
Neuroendocrine 2 1 3
Table 2. The morphologic patterns of thymomas
Benign Malignant Total
Male 129 27 (10*) 156
Female 106 15 (4*) 121
Total 235 42 (14*) 277
Table 1. The incidence of thymomas in male and
female Fischer 344/N Rats
Table 3. Correlation of the malignancy of thymoma with
survival days, cause of death and sacrifice type
*The number within parentheses denotes the number of malignant thymomas with metastasis.
Grateful thanks for assistance with this poster to Karen Cimon, Kim
Pernicka, Maureen Puccini and Emily Singletary, of EPL and Lois
Wyrick of Image Associates.