After the diagnosis of primary splenic neoplasia, treatment options depend on tumor type and extent of disease. Diagnostic tests useful for determining the extent of disease include complete blood work, thoracic radiographs, abdominal ultrasound, peritoneal fluid analysis, and rectal palpation. Use of laparoscopy in diagnosis of both benign and malignant splenic tumors has been reported and may also be helpful in evaluating for intraabdominal spread. In horses with isolated solid tumors or even in rare cases of splenic lymphoma, the treatment of choice is splenectomy. Several approaches have been described for splenectomy, and most necessitate rib resections for adequate exposure. Transthoracic approaches, from the 16th rib cranially, also involve incision of the diaphragm, whereas a purely transabdominal approach can be accomplished by resection of the 17th rib. Laparoscopy-assisted abdominal approaches have also been used successfully for splenectomies, reducing both incision size and overall morbidity. For horses with lymphoid malignancies that are not candidates for primary splenectomy, chemotherapy may be attempted for disease palliation. Several chemotherapy regimens have been used in equine patients and include varying doses of prednisolone, vincristine, cyclophosphamide, and doxorubicin (Table 100-1). The effectiveness of these drugs either singly or in combination is unknown but is generally thought to be low.

Summary

Primary splenic tumors in the horse are rare, whereas metastatic tumors are relatively common, with melanoma being the most common metastatic lesion of the spleen. The biologic behavior of primary splenic tumors is unknown but is generally considered aggressive. For example, hemangiosarcoma and lymphoma of the spleen are commonly described in the literature as widespread diseases at the time of diagnosis. Nevertheless, in horses with solitary tumors, splenectomy may be considered. The prognosis for horses treated with splenectomy for primary tumors of the spleen depends on the histologic nature of the tumor and disease extent; however, long-term survival has been reported for both benign and malignant variants.

Histologic Types

Benign tumors of the soft tissues typically arise from either spindyloid-type cells (fibromas) or adipose tissue (lipomas). Fibromas most commonly develop in the bony tissues of the oral cavity or in the nasal or paranasal sinuses but are rarely reported in appendicular locations (Figure 100-2). In contrast, lipomas are generally found internally within the mesenteric adipose tissue but may occasionally be seen at external locations. Benign tumors grow by expansion, resulting in compression and local mass effects on surrounding tissues. Malignant tumors of the soft tissue are also generally composed of spindyloid-type cells and classified as fibrosarcomas or nerve sheath tumors. These so-called fibroblastic sarcomas are clinically considered more aggressive variants of sarcoids. Other histologic types commonly diagnosed include angiosarcoma (hemangiosarcoma, lymphangiosarcoma), undifferentiated sarcoma, histiocytic sarcoma, and leiomyosarcoma; extraskeletal osteosarcoma, giant cell tumor, and other subtypes are more rarely seen. Histologic grading systems have not been developed for equine soft tissue malignancies, but vascular invasion, increased mitotic index, and significant tumor necrosis are clinically associated with more aggressive behavior.

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