Four-dimensional imaging of thoracic target volumes in conformal radiotherapy

Abstract

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