IMMUNOHISTOCHEMICAL STAINING CHARACTERISTICS OF CANINE GASTROINTESTINAL STROMAL TUMORS

Sections from 35 formalin-fixed, paraffin-embedded, canine gastrointes tinal stromal tumors consisting of 14 leiomyomas (five stomach, three small intestine, two colon, four rectum), 18 leiomyosarcomas (one stom ach, five small intestine, nine cecum, three rectum), two undifferenti ated sarcomas (two stomach), and one neurofibrosarcoma (small intestin e) were examined for the expression of vimentin, S-100 protein, alpha- smooth muscle actin, and desmin via immunoperoxidase methodology using an avidin-biotin complex technique. The leiomyomas were 4/14 (29%) vi mentin-positive, 3/14 (21%) S-100 protein-positive, 10/14 (71%) alpha- smooth muscle actin-positive and 13/14 (93%) desmin-positive. Leiomyos arcomas were 18/18 (100%) vimentin-positive, 11/18 (61%) S-100 protein -positive, 9/18 (50%) alpha-smooth muscle actin-positive, and 15/18 (8 3%) desmin-positive. The undifferentiated sarcomas were 2/2 (100%) vim entin-positive, 2/2 (100%) S-100 protein-positive, 1/2 (50%) alpha-smo oth muscle actin-positive, and 0/2 (0%) desmin-positive. The neurofibr osarcoma was vimentin and S-100 protein-positive and alpha-smooth musc le actin- and desmin-negative. Thirty-one of thirty-five (89%) of all neoplasms demonstrated reactivity for either desmin and/or alpha-smoot h muscle actin. S-100 protein reactivity occurred in 17/35 (49%) of al l specimens. Lack of desmin and alpha-smooth muscle actin reactivity o ccurred in 4/35 (11%) of all specimens, all of which were vimentin-pos itive. The immunohistochemical results indicate that the majority of c anine gastrointestinal stromal tumors (GIST) with light microscopic fe atures of smooth muscle cells have immunohistochemical staining patter ns supporting smooth muscle differentiation. Vimentin reactivity corre lated with a light microscopic diagnosis of malignancy. The lack of sm ooth muscle cell markers in some tumors and the high percentage of cas es positive for S-100 protein may suggest a more complex histogenesis or differentiation for subgroups of these tumors.