PRIMARY MESENCHYMAL (NONANGIOMATOUS NONLYMPHOMATOUS) NEOPLASMS OCCURRING IN THE CANINE SPLEEN - ANATOMIC CLASSIFICATION, IMMUNOHISTOCHEMISTRY, AND MITOTIC-ACTIVITY CORRELATED WITH PATIENT SURVIVAL

Surgical submissions from canine splenectomy cases spanning a 3-year p eriod (1988-1990) were evaluated. Eighty seven neoplasms of the spleen considered to be of nonangiomatous and nonlymphomatous origin were se lected for morphologic classification, mitotic index determination, im munohistochemical analysis, and patient survival determination. In 76/ 87 cases, patient survival information was available, and the mitotic index was determined in 83/87 cases. Immunohistochemistry for selected antigens (vimentin, desmin, smooth muscle actin, myosin, and factor V III-related antigen) was performed in 58/87 of the cases. Morphologic classification of these lesions in standard HE preparations yielded th e following neoplastic groups: fibrosarcoma (19/87), undifferentiated sarcoma (19/87), leiomyosarcoma (14/87), osteosarcoma (8/87), mesenchy moma (7/ 87), myxosarcoma (6/87), histiocytic sarcoma (6/87), leiomyom a (3/87), lipoma-myelolipoma (2/87), liposarcoma (2/87), and malignant fibrous histiocytoma (1/87). A lack of distinct morphologic character istics among many of the neoplasms that were classified as either fibr osarcoma, leiomyosarcoma, or undifferentiated sarcoma contrasted these groups with the relatively unambiguous features that distinguished th e other sarcoma groups. Using immunohistochemical staining for muscle- specific antigens (desmin, smooth muscle actin, and myosin), specific staining often overlapped extensively within the neoplastic groups of fibrosarcomas, leiomyosarcomas, and undifferentiated sarcomas, suggest ing either ambiguous morphologic findings or the possibility of a comm on histogenesis from smooth muscle trabeculae or a distinct population of splenic myofibroblasts. The biological behavior of all tumors exam ined could be placed into three categories of patient survival: (1) be nign, noninvasive tumors (leiomyoma, lipoma) with prolonged survival i ntervals; (2) malignant tumors (fibrosarcoma, undifferentiated sarcoma , leiomyosarcoma, osteosarcoma, myxosarcoma, histiocytic sarcoma, and liposarcoma), showing severely truncated survival (median 4 months wit h 80-100% mortality after 12 months; and (3) intermediate survival per iods (median 12 months with 50% 1 year survival) attributed to a singl e group of neoplasm, the mesenchymomas. The biological behavior of pri mary splenic nonangiomatous, nonlymphomatous sarcomas was most closely correlated with observed mitotic index. Splenic neoplasms of this typ e with a mitotic index < 9 showed significantly (P < 0.0001) longer su rvival intervals than those with an index > 9. With the exception of o steosarcoma, all anatomically defined tumor groups contained one or mo re specimens with a mitotic index < 9. The clinical prognosis given fo r splenic sarcomas should be modified according to the mitotic index a s a predictive value for patient survival.