Gastrointestinal Stromal TumorsProto-Oncogene Proteins c-kitGastrointestinal NeoplasmsBenzamidesPiperazinesPyrimidinesReceptor, Platelet-Derived Growth Factor alphaStromal CellsAntineoplastic AgentsMesenchymomaNeoplasms, Connective TissueEpithelioid CellsMitotic IndexPyrrolesDuodenal NeoplasmsLeiomyosarcomaImmunohistochemistryIleal NeoplasmsJejunal NeoplasmsNeoplasms, Multiple Primary