Cone-Beam CT assessment of inter-fraction and intra-fraction motions during lung stereotactic body radiotherapy with and without abdominal compression

Purpose/Objective(s): The purpose of this study is to assess the effect of abdominal compression on the inter-fraction and intra-fraction variations in lung stereotactic body radiotherapy (SBRT) using cone-beam computed tomography (CBCT). Materials/Methods: Sixty-nine lung stereotactic body radiation therapy (SBRT) patients were investigated in this study. The patients were separated into two groups, one with abdominal compression, and the other without abdominal compression. Seven to nine conformal beams were used for SBRT treatment plans. The prescriptions doses were 50 Gy in 5 fractions or 48 Gy in 4 fractions. At each treatment, two CBCTs were performed before and during the treatment. Set-up errors were measured on the first CBCT, which were recorded as inter-fraction variation. Tumor movements during treatment were measured by the second CBCT during treatment, which were recorded as intra-fraction variation. Results: For intra-fraction variation, the shifts 3 mm for "abdominal compression" group were 12.0% in AP direction, 4.6% in SI direction, 5.0% in LR direction, corresponding to 17.3% in AP direction, 21.2% in SI direction and 11.5% in LR direction respectively, for the group "without abdominal compression" For inter-fraction variation, The shifts 5 mm for "abdominal compression" group were 22.8% in AP direction, 22.8% in SI direction, 24.9% in LR direction, corresponding to 15.4% in AP direction, 48.1% in SI direction and 15.4% in LR direction, respectively for the group "without abdominal compression". Abdominal compression reduced breathing organ motion during SBRT lung treatment in all three directions. However, setup error was just reduced significantly in SI direction for abdominal compression group. There is a slightly increase for setup error in AP and LR direction for abdominal group. Both inter-fraction and intra-fraction variations had been greatly reduced in SI direction after abdominal compression was applied. Conclusion: Abdominal compression reduced the amplitude of intra-fraction lung breathing motion in all directions during lung SBRT treatment. However, the use of abdominal compression seemed to increase the inter-fraction variation in AP and LR directions although the inter-fraction variation in SI direction was reduced significantly. Therefore, target matching is required to localize the inter-fraction variation.