Imaging for high-precision thoracic radiotherapy

The motion of intra-thoracic tumors and radiosensitive normal organs is an important consideration when planning curative radiotherapy. An individualized assessment of such motion is preferable over use of standard 'planning margins', which may be based upon tumor location or patient characteristics (1-3). This is primarily because many tumors move less than 1 cm in any direction (3), thereby requiring smaller treatment fields. In addition, lung tumors may exhibit complex motions patterns, including hysteresis and be influenced by cardiac motion (4). Furthermore, patients with locally advanced lung cancer may exhibit motion in metastatic mediastinal nodes that exceeds that of the primary tumor or move maximally in a different phase (2,5). Identifying such variations in tumor motion can minimize the risk of compromising the effectiveness of radiotherapy delivery. Based on these studies, the European Organization for Research and Treatment of Cancer guidelines recommend use of respiration-correlateDCT or 4-Dimensional CT (4DCT) scans for planning (6).