SU‐E‐T‐528: Robustness Evaluation for Fiducial‐Based Accelerated Partial Breast Proton Therapy

Abstract

Purpose:To investigate the robustness of the proton treatment plans in the presence of rotational setup error when patient is aligned with implanted fiducials.Methods:Five Stage I invasive breast cancer patients treated with the APBP protocol (PCG BRE007‐12) were studied. The rotational setup errors were simulated by rotating the original CT images around the body center clockwise and counterclockwise 5 degrees (5CW and 5CCW). Manual translational registration was then performed to match the implanted fiducials on the rotated images to the original dataset. Patient contours were copied to the newly created CT set. The original treatment plan was applied to the new CT dataset with the beam isocenter placed at the geometrical center of PTV. The dose distribution was recalculated for dosimetric parameters comparison.Results:CTV and PTV (D95 and V95) coverages were not significantly different between the two simulated plans (5CW and 5CCW) and the original plan. PTV D95 and CTV D95 absolute difference among the three plans were relatively small, with maximum changes of 0.28 CGE and 0.15 CGE, respectively. PTV V95 and CTV V95 absolute differences were 0.79% and 0.48%. The dosage to the thyroid, heart, contralateral breast and lung remained zero for all three plans. The Dmax and Dmean to the volume of ipsilateral breast excluding CTV were compared, with maximum difference values of 1.02 CGE for Dmax and 3.56 CGE for Dmean. Ipsilateral lung Dmean maintained no significant changes through the three plan comparison, with the largest value 0.32 CGE. Ipsilateral lung Dmax was the most sensitive parameter to this simulation study, with a maximum difference at 20.2 CGE.Conclusion:Our study suggests that fiducial‐based Accelerated Partial Breast Proton Therapy is robust with respect to +/− 5 degree patient setup rotational errors, as long as the internal fiducial markers are used for patient alignment.