Treatment Verification in Radiation Oncology: portal imaging in clinical practice

In radiation oncology, the treatment aims include cure of cancer (curative radiotherapy), a prolongation of the progression- or symptom free interval, and/or the palliation of symptoms (palliative radiotherapy). In the curative setting, the purpose of radiotherapy is to eradicate a patient's cancer, while preserving normal anatomy and function. Ideally, this would be achieved by treating the tumor volume to a radiation dose offering a maximum probability of tumor control, without causing damage to the surrounding healthy tissue. Realistically, however, the dose level will be chosen as a result of balancing the probability of tumor control and the risk of serious complications, thus accepting a certain level of minor complaints and a small risk of major complications. The radiation oncologist defmes the target volume to be irradiated from the known tumor extension and/or the tissues at risk of containing tumor cells (subclinical disease), which is called the clinical target volume (CTV) (61). A margin is then added to compensate for the effects of organ and patient movement and inaccuracies in beam and patient setup: the planning target volume (PTV). The radiotherapy treatment fields are planned tightly around the PTV, in order to ensure treatment of the PTV to a high dose, while reducing the dose delivered to the surrounding normal tissues. The volume in the patient which will receive ;:" 95 % of the prescribed dose is called the treated volume, which ideally matches the PTV closely. The volume which will receive a dose that is considered significant in relation to normal tissue tolerance is called the irradiated volume (61). Often the 20% or 30% dose level is chosen to represent the irradiated volume.