Methods and devices for calibrating a radiotherapy system are disclosed. The method includes providing a detection medium that responds to exposure to ionizing radiation, and preparing a calibration dose response pattern by exposing predefined regions of the detection medium to different ionizing radiation dose levels. The method also includes measuring responses of the detection medium in the predefined regions to generate a calibration that relates subsequent responses to ionizing radiation dose. Different dose levels are obtained by differentially shielding portions of the detection medium from the ionizing radiation using, for example, a multi-leaf collimator, a secondary collimator, or an attenuation block. Different dose levels can also be obtained by moving the detection medium between exposures. The disclosed device includes a software routine fixed on a computer-readable medium that is configured to generate a calibration that relates a response of a detection medium to ionizing radiation dose.