Three-year-old, male, RCS rat (Rattus norvegicus).The rat was kept as a non-treated animal in a long-term rat study. No clinical signs except coarse hair were found until the scheduled sacrifice at 152 weeks old.

Gross Description:  

A mass measured 40 x 30 x 15 mm, was observed in the anterior mediastinum, and adhered to the pleura of the lung. Small masses had found around main mass, and adhered to the lung and esophagus. The mass was soft, and the cut surface was milky white to yellow and grey-white.

Histopathologic Description:

Affecting approximately 80% of this section of thymus is an unencapsulated, poorly circumscribed, moderately cellular neoplasm composed of round to spindle-shaped cells arranged in sheets on a pre-existent fibrovascular stroma. Neoplastic cells have variably distinct cell borders, an abundant amount of lightly eosinophilic to amphophilic cytoplasm that is often obscured by fine eosinophilic granules. The nuclei of the tumor cells are generally centrally placed, round to elongate, with finely stippled chromatin, and 1-2 variably distinct nucleoli. Mitotic count averages about 1 per HPF. Almost all tumor cells have metachromatic granules when stained with toluidine blue stain. Immuno-histochemically, tumor cells are positive for c-kit, strongly positive for rat mast cell protease, and mostly negative for histamine as mast cell tumor markers. All tumor cells are negative for cytokeratin AE1/AE3, and positive for vimentin. Ultrastructurally, the various sized granules contained homogeneous electron-dense material consistent with mast cell granules. The tumor metastasized and disseminated to the pleura of the lung and esophagus, and adventitia of the left ventricles and aorta.

The basophilic area at the periphery of the mass consists of a large number of lymphocytes and thymic epithelial cells similar to normal thymic tissue. Lympho-cytes in high cell density areas of epithelial cells are mostly a small cell type with coarse chromatin and fewer large lymphocytes. On the other hand, lymphocytes in low cell density areas of epithelial cells are nearly uniform medium-sized cell with fine stippled chromatin. Immunohistochemically, the hymic epithelial cells in high-cell-density areas exhibit positive cytokeratin 8 staining, which is expressed in thymic cortical epithelial cells. Thymic epithelial cells in low-cell-density areas are positive for cytokeratin 14 which is expressed in thymic medullary epithelial cells.

Morphologic Diagnosis:  

Thymus: Malignant mast cell tumor with thymic epithelial hyperplasia

Lab Results:  



Mast cell tumor

Contributor Comment:  

This malignant tumor of thymic origin was located in the anterior mediastinum, invaded the adjacent thymus, and metastasized to the thoracic cavity. The tumor is characterized by a dense round cell proliferation, and has the features of a malignant round cell tumor. Neoplastic cells have intracytoplasmic metachromatic granules with toluidine blue stain and are strongly immunopositive for mast cell markers. Therefore, the tumor is diagnosed as a malignant mast cell tumor. Mast cell tumor is a very common neoplasm of the skin in the dog and cat5,6, and is composed of round cells with basophilic granules which are metachromatic when stained with toluidine blue. With regard to rodents, chemical and radiation-induced and spontaneous mast cell tumor have been reported in mice.3,8 However, to our knowledge, mast cell tumors are extremely rare in rats. Rat mast cell tumors have only been reported in two case reports; and there are 12 cases/entries in the National Toxicology Program (NTP) pathology database.2,3 Histopathologically, the mast cell tumors described in the two case reports originated in the thymus and in the eyelid, and they were characterized by a sheet-like proliferation of round cells with fine cytoplasmic eosinophilic granules. However, infiltration of eosinophils and an increase in collagen fibers are not observed, unlike in cases of these tumors in dogs and cats.1,2,3,4 Because our case has similar morphologic features to previously reported cases, mast cell tumor in the rat may be characterized by eosinophilic granules in the cytoplasm. However, since this case has variable cell morphology and evidence of metastasis, the mast cell tumor in this case may have more malignant potential compared with previous reports. This case is characterized by sheet-like proliferation of spindle to round cells with eosinophilic granules of various sizes. Differential diagnoses for the present tumor included thymoma, granular cell tumor, and globule leukocyte tumor. Thymoma, an epithelial tumor, is easily distinguishable from a round cell tumor; however, the patterns of cellular proliferation observed in the present case resemble that seen in tumors of epithelial origin, making the rat tumor difficult to differentiate from thymoma. Negative immunohistochemical staining for cytokeratin AE1/AE3 helps to rule out a thymoma in this rat. Granular cell tumors and globule leukocyte tumors are characterized as round cell tumors with eosinophilic granules, similar to those observed in the current case. Accordingly, metachromasia, using toluidine blue, was required to confirm the diagnosis.2,7,9,12

The thymic region of the tumor described here is composed of two areas with different densities, as well as different epithelial cell and lymphocyte morphologies, suggesting that cortical and medullary thymic components may have been maintained in the tumor. In humans, cortical and medullary thymic epithelial cells yield different expression patterns of cytokeratin. Medullary thymic epithelial cells express cytokeratin 5 and cytokeratin 14, whereas cortical thymic epithelial cells express cytokeratin 8 and cytokeratin 18.5,11 Accordingly, immunohistochemical staining for different cytokeratins distinguishes between the cortex and the medulla of the thymus.

In this study, the expression patterns of cytokeratins 8 and 14 were analyzed in thymic epithelial cells of a normal RCS rat and shown to be similar to those observed in humans. Cytokeratins 8 and 14 were thus considered suitable markers for distinguishing between the cortex and medulla in RCS rats. In the RCS rat described here, the distribution of cytokeratins 8 and 14 corresponded to the areas with high and low epithelial cell densities, respectively. It was clear that the thymic area possessed both cortical and medullary components but that the epithelial cell density differed from that of the normal thymus. However, growth of solid tubules and epithelial cords, which represent a characteristic feature of benign thymoma, is not observed in our case. The area exhibiting high epithelial cell density represented the cortical component of the tumor with thymic epithelial hyperplasia; accordingly, this region was diagnosed as thymic epithelial hyperplasia. In our laboratory, we previously detected only one benign thymoma in about 20 RCS rats of over 120 weeks of age; however, almost all thymus showed severe involution. Therefore, this strain may not be prone to the development of thymic epithelial hyperplasia and thymoma.

JPC Diagnosis:  

Thymus: Mast cell tumor, RCS rat, Rattus norvegicus.

Conference Comment:  

Conference participants had great difficulty with the diagnosis and tissue identification in this case. While most attendees agreed that this case represented a malignant round cell neoplasm, none had mast cell tumor as a differential or thymus as the affected tissue. The neoplasm effaces the majority of the tissue; however, normal thymic parenchyma is present at the periphery of all examined tissue sections. As a result, the conference moderator led a discussion of the anatomical features of the rodent thymus.

The thymus is located in the mediastinum, cranial and ventral to the base of the heart and aortic arch with extension into the cervical region in the rat. It consists of two bilateral lobes joined by a connective tissue isthmus. Within the lobe, a thin capsule surrounds each lobule and gives rise to septae; however, septation is not apparent in this section.10,11

The thymus is unique among the lymphoid organs because it is supported by an epithelial framework, highlighted by the contributor’s cytokeratin immuno-histochemical stains. It is divided into a cortex and medulla separated by a vascular corticomedullary zone. Histologically, the darkly staining cortex contains densely packed, small, immature T-lymphocytes, which obscure the epithelial cell population.10,11 The medulla is less densely cellular than the cortex, and contains more mature T-cells, prominent epithelial cells, macrophages, dendritic cells, and B lymphocytes. Hassall’s corpuscles are rare in rodents when compared with many other species, contributing to the difficulty participants had in tissue identification. In addition, given the age of this rat, the tissue is likely in an advanced stage of involution, further obscuring the normal architecture.10,11

Neoplastic cells in this section have numerous prominent pale eosinophilic cytoplasmic granules which are inconsistent with the deeply basophilic granules seen in well-granulated mast cell tumors in dogs and cats, as well as in normal rat mast cells. Conference participants considered other differentials including undifferentiated malignant round cell neoplasm, granular cell tumor, oncocytoma, balloon cell melanoma, and large granular lymphoma. In addition to the histochemical and immunohistochemical stains mentioned by the contributor that support the diagnosis of mast cell tumor, the provided transmission electron microscopy (TEM) image nicely demonstrates numerous intracytoplasmic homogenous electron dense granules consistent with mast cell granules.


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Click the slide to view.

3-1. Anterior mediastinum, rat.

3-2. Thymus, rat.

3-3. Thymus, rat.

3-4. Thymus, rat.

3-5. Thymus, rat.

3-6. Thymus, rat.

3-7. Thymus, rat.

3-8. Thymus, rat.

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