Organs at risk

The organs at risk (OARs) are the healthy tissues / organs placed near the clinical target volume (CTV) whose irradiation could cause damage that would make changes to the radiotherapy treatment plan. The heart, for example, in radiotherapy with LINAC of the left breast cancer, is an organ at risk.

Tolerance to ionizing radiation from organs at risk (dose constraints), dependent on radiosensitivity and volume, is one of the main factors limiting the choice of radiation therapy dose. The maximum absorbed dose that the spinal cord can receive, in thoracic radiotherapy for example, is 45 Gy.

For this purpose, the therapeutic ratio is defined as the relationship between the probability of eradicating neoplastic disease, dependent on radiation therapy dose, and the toxicity of the OARs, dependent on dose constraints.