Clinical studies in radiotherapy with conventional dose fractionation have shown a reduction in the TCP with increasing initial total tumor volume as measured using computed tomography (CT) or magnetic resonance imaging (MRI). A plausible mechanism for the volume effects in the TCP would be useful for analysis of treatment outcomes and design of dose escalation protocols. Chapter 3 describes a derivation of equations for tumor control probability (TCP) equation for tumors with hypoxic regions. It is shown that the dose-volume effects in the TCP are dependent on tumor radiosensitivity and defined primarily by the hypoxic regions. A comparison with clinical data is presented for both conventional and hypofractionated radiotherapy.