Cytotoxic T-cells (CTLs) in the tumor microenvironment (TME) orchestrate anti-tumor immunity. Immunotherapy (IO) aims to prevent anergy and stimulate CTLs, thereby enhancing tumor cytotoxicity. However, CTL presence alone is a poor predictor of IO success due to patient-specific resistance mechanisms driven by the evolving TME. Studying the therapeutic effects on intratumoral CTLs within tumors, as opposed to artificial co-culture, provides deeper insights into heterogeneous patient responses. Farcast’s TruTumor platform preserves near-native tumors, stroma, and immune cells for up to 72 hours of culture. This study characterizes CTL abundance and functionality to define differential patient responses to IO across multiple solid tumor indications