Delivery Uncertainty Estimation Using Daily Breath-Hold Cone-Beam CTs For Liver Proton Stereotactic Body Radiotherapy

Abstract

To estimate for proton stereotactic body radiotherapy (SBRT) plan delivery uncertainty with daily cone-beam CT (CBCT) imaging for liver patients under breath-hold with active breath coordinator (ABC). Images from 11 liver patients previously treated with photon SBRT with ABC at our institution are used for this study. The prescribed dose for the proton SBRT plan is 75 Gy (RBE) in 5 fractions. There is a total of 11 patients with 113 setup CBCT images collected during the patient’s previous photon SBRT treatment. The planning CT and all CBCT images are imported into Raystation 9A. A CT relative linear stopping power (RLSP) curve is calibrated for CBCT and used for proton dose calculation. A four beams proton SBRT plan is optimized with the single field optimization (SFO) technique. All 113 CBCT images are registered to planning CT with bony anatomy (rib cage and vertebrae) or liver to evaluate the delivery uncertainty. The delivery dose is estimated from the proton dose calculation on each CBCT image with the same proton plan. The dose coverages for targets are evaluated for: gross tumor volume (GTV), gtv_multABC_2mmRad_5mmSI, and ptv_5mmRad_7mmSI. Liver positions under DIBH with ABC are not consistent under bony alignment as measured. The inter-fractional and intra-fractional variation of DIBH for liver are analyzed. Proton plan delivery uncertainty can dramatically reduce under well liver alignment (up to 5% prescription dose). Daily CBCT images could be potentially used to estimate proton plan delivery uncertainty for liver patients.