Respiratory Gated CT-Imaging in Radiation Therapy of Lung Cancer
J. Dinkel, MD, A. Jensen, MD, U. Mende, MD, PhD, Department of Radiation Oncology, and J. Debus MD, PhD, Director, Department of Radiation Oncology, University of Heidelberg, Germany
A 62-year-old female patient under chemotherapy treatment for a non-small-cell lung cancer and cerebral metastases was examined using the SOMATOM Sensation Open with a 4D respiratory gated data acquisition protocol in order to determine the full range of motion of critical internal structures and the lung cancer during respiration. This method was used to achieve a more targeted radiation treatment.
Respiratory gating supplies information about tumor motion during the patient's breathing cycle. The introduction of the latest generation multislice CT systems with short acquisition times permits the evaluation of thoracic structures with a temporal resolution of 250 ms. Short acquisition times in this set-up are achieved by simultaneous acquisition of 24 or 40 transverse sections, half-second scanner rotation, and advanced respiratory-gated reconstruction algorithms. In this patient, the breathing frequency was over 12 cycles/min. CT data was collected in spiral mode, with simultaneous acquisition of 24 parallel sections using a 1.2 mm collimation and appropriate spiral pitch of 0.1. The respiratory signal from the patient was synchronized and simultaneously recorded during free-breathing CT data acquisition, using a chest-belt with a pressure sensor. Virtually correlated 4D phase volumes (with the time as the fourth dimension) were reconstructed after the scan to form a model of anatomic movement. 7 different reconstructions were performed corresponding to different phases of the breathing cycle. In these CT scans, a 4 x 3.7 x 3.8 cm lobular mass was clearly visible in the medial aspect of the left upper lobe extending to the left hilus. Various nodular calcified lymph nodes as well as an enlarged aorticopulmonary lymph node could be seen in the mediastinal region. Additionally, the CT scan showed an extrathoracic metastasis in the left adrenal gland. In our scans, the tumor mobility was about 2.1 mm in the x-axis (L-R), 6.2 mm in the y-axis (A-P) and 5.2 mm in the z-axis. The mass, however, did not show a deformation during the breathing cycle. Visualization of structure motion is possible with dedicated software syngo Inspace4D.
New approaches in radiation therapy with the use of more and more conformal dose application in combination with higher doses per fraction for irradiation treatment need accurate delineation of tumor and critical structures especially in areas where artifacts distorting the geometric shape and location of the organs cannot be tolerated. Motion artifacts usually occur at boundaries of anatomical structures (both target volumes and organs at risk), resulting in the image degradation and the inability to correctly delineate anatomical structures. This leads to erroneous position, shape and volume information for target volumes and other regions affected by motion. The respiratory gated data acquisition in CT allows the planning physician to visualize and study the organ and tumor motion in 3D coordinates and time, contributing to a better understanding of the target area and potential sparing of healthy tissue by minimization of treatment volume and reduction of side effects. Respiratory gating is a promising new tool to increase the quality of RT planning and patient treatment.
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