Four-dimensional imaging of thoracic target volumes in conformal radiotherapy

The goal of conformal radiotherapy (CRT) is to deliver the prescribed dose to a volume that closely conforms to the three-dimensional (3D) target volume while the dose to adjacent healthy tissues or organs at risk is minimized. Because the position of the target volume can change substantially both within and between radiation treatment fractions the fourth dimension, namely time, needs to be addressed as well. The consideration of time in the 3D treatment process is referred to as fourdimensional (4D) radiotherapy. Variations in the target volume position with time are mainly due to organ motion and patient and beam set-up deviations. Changes in the target volume position that occur within a treatment fraction are referred to as intra-fraction variation. Respiratory and cardiac motion are the main contributors to intra-fraction positional variations of thoracic and abdominal target volumes. In routine clinical practice thoracic and abdominal tumors are irradiated while the patient breathes freely. To account for target volume variations in size, shape and position and patient and beam set-up deviations, an empirical 3D margin is added to the clinical target volume to obtain the planning target volume (1, 2). The 3D margin is often derived from respiratory motion measurements in patients representative of the general population. Such a margin is not tailored to the individual patient and will therefore be suboptimal in most cases. Alternatively, the tumor motion in a specific patient can be determined as part of the treatment planning procedure. Fluoroscopy is most widely used for this purpose. However, tumors are often poorly visualized using this imaging modality. In addition, fluoroscopic data cannot directly be related to the treatment planning computed tomography (CT) data.

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