The immune system of the cancer patient's body and the viral lytic cycle play important roles in cancer virotherapy. Most mathematical models for virotherapy do not include these two agents simultaneously. In this paper, based on clinical observations we propose a mathematical model including the time of the viral lytic cycle, the viral burst size, and the immune system response. The proposed model is a nonlinear system of delay differential equations in which the period of the viral lytic cycle is modeled as a delay parameter and is used as the bifurcation parameter. We analyze the stability of equilibrium points and the existence of Hopf bifurcation and obtain some conditions for the stability of equilibrium points in terms of the burst size and delay parameter. Finally, we confirm the results with a numerical example and describe them from a biological point of view.