Surgery remains the main procedure for rectal cancer therapy. However, in the past decades various radiation therapy modalities were used to improve outcomes. The optimization of the clinical results of radiotherapy alone with concomitant boost or hyperfractionated, accelerated radiotherapy or chemoradiation requires a better understanding of biological evidences on RT effect, considering the possible presentation of this tumor: "visible" unresectable, "visible" resectable and subclinical disease. Furthermore, parameters as tumor volume, hypoxic fraction, intrinsic radiosensitivity, doubling time and clonogen proliferation are factors shown to have an impact on tumor control probability.