Planning 4-dimensional Computational Tomography (4DCT) Cannot Adequately Represent Daily Intrafractional Motion of Abdominal Tumors

Abstract

To evaluate whether the planning 4DCT can adequately represent daily motion of abdominal tumors in regularly fractionated and stereotactic patients. Intrafractional tumor motion of 10 abdominal (4 pancreas-fractionated and 6 liver- stereotactic) patients with implanted fiducials were measured based on daily orthogonal fluoroscopic movies over 38 treatment fractions. Movies were taken immediately before and after treatment at 8 ± 3 and 19 ± 6 minutes apart for fractionated and stereotactic patients, respectively. For evaluating daily breathing pattern variation, the standard deviations (SD) of the breathing amplitude and the baseline were measured for each fraction. The needed internal margin for at least 90% of tumor coverage was calculated based on 95 and 5 percentile of daily 3-D tumor motion. The planning internal margin was generated by fusing 4DCT motion from all phase-bins. The disagreement between needed and planning internal margin was analyzed fraction by fraction in three motion axes (superior-inferior [SI], anterior-posterior [AP], and left-right [LR]). The 4DCT margin was considered as an over-/under-estimation of daily motion when disagreement exceeded at least 3 mm in SI, and/or 1.2 mm in AP and LR, which were equal to the 4DCT image resolution in corresponding axis. Finally the additional internal margin to account for breathing variation was suggested using the Van Herk margin formula. The breathing amplitude varied from breath to breath and day to day, with a mean SD ranging from 2 to 5 mm intrafractionally and 1 to 3 mm interfractionally in the primary motion axis of SI. The breathing baseline also shifted from breath to breath, with a mean SD ranging from 1 to 2 mm in the SI direction. The amplitude variation and baseline shift were also observed in AP and LR, but to a lesser extent. The needed internal margin disagreed with the 4DCT margin in 92% of the fractions. The 4DCT overestimated daily 3-D motion in 39% of the fractions in 7/10 patients, and underestimated in 53% of the fractions in 8/10 patients. The underestimated motion was up to 7, 9, and 3 mm in SI, AP, and LR. To account for change of breathing pattern from 4DCT to daily treatment, the additional margins needed in SI, AP and LR were 1.9, 1.3, and 1.9 mm for fractionated patients, and much larger values of 5.3, 4.5, and 1.9 mm for stereotactic patients. The internal margin derived from 4D-CT cannot encompass intrafractional breathing motion due to inter- and intra-fractional variability of breathing pattern, especially for stereotactic patients who tend to have longer treatment time and hence larger breathing variation. The additional anisotropic margins in all three motion axes were necessary to ensure an adequate treatment delivery.