Patient immobilization and positioning is very important in radiotherapy (RT). There are 2 aspects to immobilizing and positioning a patient that we must be concerned about: intra-treatment (during the actual delivery) motion and immobilization and inter-treatment (reproducibility from one fraction to the next) motion and immobilization. To ensure that the patient is positioned and immobilized properly for every fraction, immobilization devices and imaging techniques are used.

The purpose of intra-treatment immobilization is to ensure that the patient is not moving during dose delivery and remains in the same position as simulation. An exception to this is when gating is used because the delivery is controlled by patient respiration so motion is expected. In a standard treatment plan, a margin to account for patient motion is added to the clinical target volume (CTV) to ensure that the CTV is always covered in treatment. The added margin is called the planned target volume (PTV). The PTV margin is determined based on the uncertainty in the patient positioning, the amount of motion the target experiences, and the proximity of critical structures. Without an immobilization device, e.g., a mask for head/neck treatments, the PTV margin would have to be very large to ensure that the CTV does not move outside of the PTV and receives the appropriate dose. Another way of reducing the uncertainty in patient setup is to use image guidance like orthogonal x-rays or cone beam CT (CBCT).

The purpose of inter-treatment immobilization is to ensure the reproducibility of every fraction of treatment. The role of the PTV is also to ensure CTV coverage during every fraction, and using an immobilization device or image guidance can help reproduce the patient’s position daily so that the CTV is covered by the prescription dose.

It is important to mention the procedures of post-planning simulation (so-called second simulation). After a plan is calculated and approved on the simulation CT, a patient will often undergo a second simulation and have external marks placed such as tattoos or BBs, to localize the treatment isocenter prior to treatment, as well as be able to reproduce the patient’s position at every fraction. This can be done in a simulator using fluoroscopy to visualize the patient’s anatomy and treatment area, or nowadays, most linear accelerators have KV imaging capabilities allowing for the second simulation to be done on the linac. In the case of image-guided radiotherapy (IGRT), target localization is also confirmed visually with imaging prior to each fraction. The choice to use image guidance is based on the complexity of the treatment and the proximity of OARs. An IMRT plan with steep dose gradients requires more accurate positioning to ensure the dose is being delivered to the correct area.