We utilize here a diverse panel of syngeneic tumors to represent the heterogeneity found within patient tumors with respect to differences in vascularization, contributions to the EPR effect and response to PD-1 checkpoint inhibition, and found that maximum tumor-to-heart values of 89Zr-DFO-CD8a and to a larger degree 89Zr-DFO-CD4 were indicators of response to therapy. The gene discussed is CD8A; the disease is neoplasm.