Abstract
An adult, captive Taiwanese monkey (Macaca cyclopis) presented clinically with a large, rapidly growing mass located in the crus of the left hind leg. The overlying skin was severely ulcerated and necrotic. Radiographs suggested an invasive neoplasm in soft tissue with no bone involvement. The animal's clinical condition progressively worsened over the next 3 months until it died. Necropsy revealed that the mass infiltrated the surrounding skeletal muscle with no evidence of distant metastasis. Microscopically, the mass was highly cellular and composed of round to spindle cells with frequent rhabdoid cells characterized by abundant eosinophilic glassy cytoplasmic inclusions and large, bizarre nuclei. Phosphotungstic acid hematoxylin staining failed to reveal distinct cross-striations within the neoplastic cells. Neoplastic cells were strongly positive for smooth muscle actin and vimentin but were negative for sarcomeric actin, myoglobin, desmin, cytokeratin, S100, and lysozyme. The gross, microscopic, and immunohistochemical findings supported the diagnosis of pleomorphic leiomyosarcoma.
Among human soft tissue sarcomas, prominent pleomorphic cells are observed in storiform-pleomorphic malignant fibrous histiocytomas (MFHs), malignant melanomas, pleomorphic rhabdomyosarcomas, and pleomorphic leiomyosarcomas. Pleomorphic leiomyosarcoma was first described as an important differential diagnosis of pleomorphic MFH. 3 In humans, pleomorphic leiomyosarcomas are predominantly located in the extremities, and less frequently in the abdominal cavity or body wall. 7 An adult nonhuman primate that presented with a pleomorphic leiomyosarcoma in the left hind leg is described in the current report.
A 10-year-old, male, captive Taiwanese macaque (Macaca cyclopis; also known as Formosan rock macaque) developed a large mass in the crus of the left hind leg. The mass was first noticed by the attending veterinarian 3 months before presentation, and the mass gradually increased in size.
At presentation, the overlying skin was severely ulcerated and necrotic. Radiography revealed a soft tissue mass that invaded the surrounding skeletal musculature without involvement of the tibia and fibula. Because of inflammation and necrosis in the overlying skin, the monkey was hospitalized at the Animal Hospital of Seoul Grand Park and treated with antibiotics and fluids. The monkey was anesthetized, and biopsy specimens were taken from the mass and submitted for microscopic evaluation. While waiting for the histopathology results, the monkey experienced severe weight loss and depression and died in the animal hospital.
A necropsy was performed at the Animal Hospital of Seoul Grand Park. Gross lesions included a left crus mass measuring 6 cm × 12 cm × 10 cm that infiltrated the surrounding skeletal muscles, including the gastrocnemius and superficial flexor digitorum, but did not directly involve the adjacent tibia and fibula. On cut surface, the mass was tan with extensive areas of hemorrhage and necrosis (Fig. 1). There was a well-demarcated, tan, 1.5-cm mass within the cortex of the right kidney. No other findings were apparent on gross examination. Specimens of the tumor and major parenchymal organs were submitted to the Department of Veterinary Pathology, College of Veterinary Medicine, Seoul National University for microscopic examination.
The tissues were fixed in 10% neutral phosphate buffered formalin, routinely processed, embedded in paraffin wax, stained with hematoxylin and eosin, coverslipped, and examined microscopically. Additional serial sections of the tumor from the left hind leg were prepared for phosphotungstic acid hematoxylin (PTAH) staining and immuno-histochemistry (IHC). The standard avidin-biotin-peroxidase technique 6 and commercially available antibodies (Table 1) were used to detect vimentin, desmin, cytokeratin, lysozyme, sarcomeric actin, myoglobin, S-100, and smooth muscle actin (SMA).
Microscopically, the neoplastic mass was covered by an ulcerated epidermis. The mass was contained within the skeletal muscle and infiltrated the subcutis. Normal skeletal muscle was largely effaced and replaced by the neoplastic cells. The neoplastic tissue was composed of variably sized, round to spindle to polygonal cells that separated and entrapped the preexisting skeletal muscle fibers (Fig. 2a). Neoplastic cells had spindle to pleomorphic nuclei with prominent nucleoli, and abundant eosinophilic cytoplasm (Fig. 2b). There were occasional rhabdoid cells characterized by abundant eosinophilic glassy cytoplasmic inclusions and eccentric bizarre nuclei (Fig. 2b). Neoplastic spindle cells were infrequently arranged in interwoven bundles intermixed with pleomorphic cells. There were 0–2 mitoses per 40 × field of view. Extensive areas of necrosis and hemorrhage were also present. The PTAH staining failed to reveal distinct cross-striations within neoplastic cells (data not shown). Histologically, the renal mass was a well-demarcated, expansile neoplasm that compressed the surrounding renal cortical parenchyma. The mass was composed of well-differentiated tubules lined by a single layer of cuboidal epithelium, consistent with a renal adenoma.
Pleomorphic leiomyosarcoma, left hind leg, Taiwanese macaque (Macaca cyclopis). Neoplastic cells replace preexisting skeletal muscle. Notice the extensive areas of necrosis and hemorrhage.
Results of immunohistochemical staining of a pleomorphic leiomyosarcoma in the hind leg of a Taiwanese macaque (Macaca cyclopis).*
- = negative; + = positive.
Normal skeletal muscle fibers adjacent to the neoplastic tissue.
BioGenex Laboratories Inc., San Ramon, CA; Dako North America Inc., Carpinteria, CA; Cell Marque Corp., Rocklin, CA.
The results of IHC staining on the left crus mass are summarized in Table 1. Throughout the neoplastic tissue, the cytoplasm of the neoplastic cells was variably positive for SMA (Fig. 3) and vimentin (Fig. 4) but negative for sarcomeric actin (Fig. 5a), desmin (Fig. 5b), myoglobin (Fig. 5c), cytokeratin, lysozyme, and S-100. Normal skeletal muscle fibers adjacent to the neoplastic tissue were positive for sarcomeric actin, myoglobin, and desmin but negative for SMA and vimentin. Proliferating satellite cells adjacent to the preexisting skeletal muscle fibers were also positive for SMA.
The microscopic and IHC findings suggest that the tumor was of muscle origin with smooth muscle differentiation. To date, only 1 rhabdomyosarcoma of spontaneous origin has been reported in the musculature of a male, 11-year-old, rhesus monkey (Macaca mulatta). 1 This monkey was euthanized without treatment, and the prior clinical course of disease was not reported. The neoplasm developed in the left pelvic region and extended from the anterior ilial wing to the ischium and proximal aspect of the left hind leg with lysis of the ilium and destruction of the pelvic musculature. 1 Microscopic features of the previously reported neoplasm were similar to the findings in the monkey in the present report; however, the neoplastic cells were reported to be positive for vimentin and myoglobin, suggestive of striated muscle differentiation. 1 In the rhesus monkey study, SMA staining of these neoplastic cells was not performed.
Given that the histologic features of the neoplastic cells in the present case were strongly suggestive of rhabdomyosarcoma, the IHC profile of the tumor was unexpected. In human and animal neoplasms, SMA has generally been used as a definitive marker to distinguish smooth muscle differentiation from skeletal muscle origin. Although rare, some cases of human pleomorphic rhabdomyosarcoma are positive for SMA, 4 and the medical literature has suggested that SMA is a marker of early myoid differentiation. 4,8 In support of this speculation, proliferating satellite cells surrounding preexisting skeletal muscle fibers in the current case were strongly positive for SMA. A previous study 4 has indicated that the diagnosis of human pleomorphic rhabdomyosarcoma is dependent on the expression of at least one skeletal muscle-specific marker (e.g., myoglobin, sarcomeric actin, nuclear MyoD1, nuclear myf4, or fast myosin) in addition to nonspecific myoid markers (e.g., desmin, myogenin, or muscle-specific actin). In the present case, neoplastic cells were negative for desmin, myoglobin, and sarcomeric actin, excluding the differential diagnosis of pleomorphic rhabdomyosarcoma. 2
The neoplasm in the monkey in the current report was diagnosed as a pleomorphic leiomyosarcoma based on the microscopic, histochemical, and IHC findings. Histologically, human pleomorphic leiomyosarcomas are characterized by the presence of foci showing typical features of smooth muscle neoplasms including spindle cells with abundant eosinophilic cytoplasm, cigar-shaped cells, fascicular architecture, and aggregates of pleomorphic cells. 9 Although pleomorphic cells similar to those seen in the present case may also be observed in rhabdomyosarcoma, they are reportedly seen in other types of malignant mesenchymal 9 and epithelial 5 neoplasms as well. Moreover, malignant melanomas and peripheral nerve sheath tumors, there are several reports of leiomyosarcoma that contain as well as lysozyme, which is a marker for MFH. rhabdoid cells. 9 The neoplastic cells in the current case were In humans, pleomorphic leiomyosarcoma has recently also negative for S100 protein, which is a marker for malignant melanomas and peripheral nerve sheath tumors, as well as lysozyme, which is a marker for MFH.
In humans, pleomorphic leiomyosarcoma has recently been described as a morphologic variant of leiomyosarcoma. 7 Pleomorphic leiomyosarcoma predominantly occurs in adults with an age range from 31 to 89 years. 7
Pleomorphic leiomyosarcoma, left hind leg, Taiwanese macaque (Macaca cyclopis). a, neoplastic tissue is highly cellular with pleomorphic cells that infiltrate adjacent skeletal muscle. Hematoxylin and eosin (HE). Bar = 800 μm. b, individual neoplastic cells range from small and round to large and polygonal with eosinophilic cytoplasm. Fewer spindle cells are present. Notice the rhabdoid cells (arrows) with abundant eosinophilic glassy cytoplasmic inclusions and large, eccentric nuclei. HE. Bar = 100 μm.
Pleomorphic leiomyosarcoma, left hind leg, Taiwanese macaque (Macaca cyclopis). The neoplastic cells are strongly positive for smooth muscle actin. Notice the positive staining of adjacent normal blood vessels (arrow) and proliferating satellite cells. The adjacent normal skeletal muscle cells fail to stain. Avidin-biotin complex method with hematoxylin counter-stain. Bar = 150 μm.
Pleomorphic leiomyosarcoma, left hind leg, Taiwanese macaque (Macaca cyclopis). Neoplastic cells are positive for vimentin. The adjacent normal skeletal muscle (arrows) fails to stain. Avidin-biotin complex method with hematoxylin counterstain. Bar = 120 μm.
Pleomorphic leiomyosarcoma, left hind leg, Taiwanese macaque (Macaca cyclopis). Neoplastic cells are negative for sarcomeric actin (a; bar = 100 μm), desmin (b; bar = 400 μm), and myoglobin (c; bar = 300 μm). Notice the positive staining of the adjacent normal skeletal muscle (arrows). Avidin-biotin complex method with hematoxylin counterstain.
Although follow-up data were available in only a small number of human cases, the overall prognosis for pleomorphic leiomyosarcoma is poor with frequent metastasis to local lymph nodes and lung. 7 The monkey in the current case was at least 10 years old. Although rapid growth of the neoplasm occurred in the left hind leg, there was no microscopic evidence of metastasis. The only other mass observed grossly involved the kidney and was diagnosed microscopically as a renal adenoma. Pleomorphic leiomyosarcoma may exhibit an aggressive biological behavior. Therefore, it should be differentiated from ordinary leiomyosarcoma to provide a more defined clinical prognosis.
Acknowledgements. This work was financially supported by KRF (2006-005-J502901). The authors thank Dr. Mark Hoenerhoff (National Institute of Environmental Health Sciences) for his discussion and constructive comments and Sandra Horton and Monica Mattmuller (Veterinary Teaching Hospital, North Carolina State University) for performing the special staining techniques.