Margin Reduction with Active Breathing Control in Image Guided Gastric Cancer Radiotherapy

Abstract

Treatment of target in abdominal region often requires a large planning margin especially in the cranial-caudal direction to account for daily setup variations and target motion caused by respiration, peristalsis, and organ filling. This study was to investigate the use of active breathing control (ABC) technique for potential margin reduction in image-guided radiation therapy in patients with gastric cancer. Five patients were immobilized in Vac-loc and planed to receive 45 Gy of postoperative radiation. After 5 ABC training sessions, planning CT images were acquired using ABC with threshold set to 80% of the maximal inspiration volume. Anterior and lateral digital fluoroscopy video images were recorded weekly with free breathing (FB) and ABC on Ximatron simulator (Varian Medical, Palo Alto, CA). The setup error was determined by registering the vertebral body on fluoroscopy images to DRRs. The surgical clips placed near the tumor bed were tracked on each image frame, and trajectory of the centroid of the clips as function of time was computed to quantify the FB target motion and ABC inter and intra breath hold motion. For each patient, planning margins were calculated for FB treatment and for image-guided ABC treatment, respectively, and IMRT plans with corresponding margins were generated. The DVHs of target, liver and kidneys of these two plans were compared. All patients were able to hold their breaths for at least 40 seconds after ABC training. The planning CT was acquired with one breath hold. A total of 63 series of fluoroscopy images (33 FB and 30 ABC) were analyzed. The mean/STD of setup errors was 1.3/3.5 mm (CC), -1.5/3.2 mm (AP) and 0.0/1.8 mm (RL). For FB treatment, the centroid of clips had maximal motion excursion ranging from 6.0 to 18.0 mm (CC), 1.3 to 12.2 mm (AP) and 0.5 to 6.2 mm (RL). The ABC reduced the range of the intra-breath hold residual motion to 0.4-4.0 mm (CC), 0.3-3.3 mm (AP), and 0.2-0.9 mm (RL). For 5 patients in this study, the occurrence of combined inter- and intra-breath hold target motion exceeding 5 mm and 7 mm was 5% and 1%, respectively. A 3 mm margin was added to account for online correction residual error. A total of 8 mm margin was used for CTV expansion for image-guided ABC treatment. The FB margin was typically 5 to 10 mm (AP and RL) and 10 to 25 mm (CC). Comparison of IMRT plans for FB and ABC treatment showed that PTV had similar coverage. However, ABC with online correction, on average, could reduce liver V40 by 10% and V18 for kidneys by 7%. The ABC was well tolerated in the selected gastric cancer patients. An ABC can effectively limit the respiratory induced target motion and may reduce the planning margin with image guidance in gastric cancer radiotherapy which may reduce the treatment-related toxicity and lead to possible dose escalation.