Identification of a spontaneous pleomorphic rhabdomyosarcoma in the thoracic and abdominal cavities of a female Wistar rat.

Abstract

Spontaneous rhabdomyosarcomas are uncommon in the rat, although they are readily induced experimentally. Rhabdomyosarcomas, especially of the adult pleomorphic type, are most often seen in the heart and the lower extremities; the neoplasms are deeply situated within muscle. Striated muscle tumors are also known to occur in the urinary bladder, kidney, uterus, and l~ng.~.~ Diagnosis of rhabdomyosarcomas is often difficult. Soft tissue tumors frequently contain fibroblasts, striated muscle, bone, cartilage, and collagen, illustrating the plasticity ofmesenchymal cells, and it is now more generally, although not universally, accepted that soft tissue neoplasms develop from multipotential stem cell^.^.^ The application of histochemical, immunocytochemical, and ultrastructural techniques has done much to assist in the diagnosis of these tumors. However, the use ofthese relatively sophisticated techniques still leaves several questions relating to the identification of poorly differentiated soft tissue sarcomas unanswered. Morphologic resemblance of a tumor to a particular tissue does not necessarily provide evidence of the tumor’s tissue of origin,5 especially when the cell of origin is a multipotential stem cell. Under these circumstances, phenotypic expression may only provide an indication of the degree of differentiation. In this report, we describe the steps taken to identify a spontaneous rhabdomyosarcoma found in the thoracic and abdominal cavities of a female Wistar strain Crl:(WI)BR rat at 67 weeks of age and discuss the limitations of the methods available in accurately diagnosing tumors of this type. A mass in the thoracic and abdominal cavities of a female Wistar strain Crl:(WI)BR rat, 67 weeks of age. was found at necropsy. The tumor was a large mass attached via a constriction through, but not obviously involving, the diaphragm. The thoracic component of the tumor consisted of a pale, firm, solid, lobular mass 40 x 30 x 30 mm. The cut surface revealed firm homogeneous regions and localized areas of mineralization. The abdominal component consisted of a dark, firm, lobular area approximately 35 x 35 x 35 mm surrounded by six fluid-filled cysts 10-25 mm in diameter. The cystic structures contained dark serosanguinous fluid. The cut surface resembled that of the thoracic mass where the two sites joined but was darker towards the caudal extremity of the tumor. The abdominal cavity contained excess serosanguinous fluid. The mass was fixed in 10% buffered formalin, processed by standard histologic techniques, embedded in paraffin wax, sectioned at 4 pm, and stained with hematoxylin and eosin (HE). Additional sections were taken for staining with phosphotungstic acid-hematoxylin (PTAH) and for immunocytochemical processing. The tumor contained many of the elements associated with a poorly differentiated pleomorphic adult rhabdomyosarcoma. The cell morphology within the mass showed considerable heterogeneity; neoplastic cells varied in appearance from elongated to plump and round, and straplike cells with abundant eosinophilic cytoplasm and numerous multinucleate giant cells were also present. There was no evidence of muscle striations in sections stained with HE, but a small number of cells did exhibit striations in sections stained with PTAH (Figs. I, 2). These striated cells formed a minor component in a connective tissue stroma in which areas of poorly differentiated cartilage and bone, frank areas of calcification, and areas of necrosis and hemorrhage were also present. There was little evidence of mitotic figures