Cone-beam computed tomography internal motion tracking should be used to validate 4-dimensional computed tomography for abdominal radiation therapy patients

Purpose To demonstrate that fiducial tracking during pretreatment Cone-Beam CT (CBCT) can accurately measure tumor motion and that this method should be used to validate 4-dimensional CT (4DCT) margins before each treatment fraction. Methods and Materials For 31 patients with abdominal tumors and implanted fiducial markers, tumor motion was measured daily with CBCT and fluoroscopy for 202 treatment fractions. Fiducial tracking and maximum-likelihood algorithms extracted 3-dimensional fiducial trajectories from CBCT projections. The daily internal margin (IM) (ie, range of fiducial motion) was calculated for CBCT and fluoroscopy as the 5th-95th percentiles of displacement in each cardinal direction. The planning IM from simulation 4DCT (IM_4DCT) was considered adequate when within ±1.2 mm (anterior-posterior, left-right) and ±3 mm (superior-inferior) of the daily measured IM. We validated CBCT fiducial tracking as an accurate predictive measure of intrafraction motion by comparing the daily measured IM_CBCT with the daily IM measured by pretreatment fluoroscopy (IM_pre-fluoro); these were compared with pre- and posttreatment fluoroscopy (IM_fluoro) to identify those patients who could benefit from imaging during treatment. Results Four-dimensional CT could not accurately predict intrafractional tumor motion for ≥80% of fractions in 94% (IM_CBCT), 97% (IM_pre-fluoro), and 100% (IM_fluoro) of patients. The IM_CBCT was significantly closer to IM_pre-fluoro than IM_4DCT (P<.01). For patients with median treatment time t < 7.5 minutes, IM_CBCT was in agreement with IM_fluoro for 93% of fractions (superior-inferior), compared with 63% for the t > 7.5 minutes group, demonstrating the need for patient-specific intratreatment imaging. Conclusions Tumor motion determined from 4DCT simulation does not accurately predict the daily motion observed on CBCT or fluoroscopy. Cone-beam CT could replace fluoroscopy for pretreatment verification of simulation IM_4DCT, reducing patient setup time and imaging dose. Patients with treatment time t > 7.5 minutes could benefit from the addition of intratreatment imaging.