Interfraction and Respiratory Organ Motion During Conformal Radiotherapy in Gastric Cancer

Purpose: To quantify the interfraction and breathing organ motion during adjuvant radiotherapy for gastric cancer and assess organ stability in different breathing states. Methods and Materials: A planning computed tomography (CT) scan and serial study CT scans in free breathing, voluntary inhale and exhale were performed in weeks 1, 3, and 5 of radiotherapy for 22 resected gastric patients. All data sets were fused to register the vertebral bodies. The regions of interest (kidneys, stomach, liver, pancreas, celiac axis, and porta hepatis) or points of interest (POIs; left dome of diaphragm, splenic hilum) were identified. For each region of interest, a POI was automatically placed at the center of mass. The interfraction displacement and breathing amplitude were assessed in the craniocaudal (CC), anteroposterior (AP), and right-left (RL) directions. Results: Comparison of the serial free-breathing CT scans with the planning CT scan showed a median displacement of all POIs of 5.6, 2.2, and 1.8 mm in the CC, AP, and RL directions, respectively. Comparison of the serial inhale scans with the first inhale scan showed a displacement of 4.9, 2.6, and 1.8 mm in the CC, AP, and RL directions, respectively. The comparable values for the exhale scans were 5.1, 2.0, and 1.8 mm. The displacements of the organs were similar in the free breathing, inhale, and exhale states. The median respiratory amplitude in the CC, AP, and RL direction was 14, 4.8, and 1.7 mm, respectively. Conclusion: The median interfraction displacement of the POIs relative to the vertebral bodies was about 6 mm in the CC direction and 2 mm in the other directions. The planning target volume margins need to account for these shifts. Individual assessment of respiratory motion is recommended to identify patients with unusually large respiratory amplitude.