Gopal Subramanian


Purpose To demonstrate that during pretreatment Cone-Beam CT (CBCT) can accurately measure tumor motion and that this method should be
used to validate margins during treatment.
Methods and Materials For 26 patients with abdominal tumors and implanted fiducial markers, tumor motion was measured daily with CBCT and
fluoroscopy for 186 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. We validated CBCT fiducial tracking as an accurate predictive measure of intrafraction
motion by comparing the daily measured IM CBCT with the daily IM.
Results Four-dimensional CT could not accurately predict intrafractional tumor motion for ≥84% of fractions in 96% (IMCBCT ), 94% (IM prefluoro
), and 100% (IM fluoro ) of patients. For patients with median treatment time t < 7.6 minutes, IM CBCT was in agreement with IM fluoro for
94% of fractions (superior–inferior), compared with 67% for the t > 7.8 minutes group, demonstrating the need for patient-specific intratreatment
Conclusions Cone-beam CT could replace fluoroscopy for pretreatment verification of simulation IM, reducing patient setup time and imaging
dose. Patients with treatment time t > 7.6 minutes could benefit from the addition of intratreatment imaging.


CBCT, Radiotherapy, Marker, Quality Assurance

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