SU‐E‐CAMPUS‐T‐02: Can Pre‐Treatment 4DCT‐Based Motion Margins Estimates Be Trusted for Proton Radiotherapy?

Abstract

Purpose:Radiotherapy motion margins are generated using pre‐treatment 4DCT data. The purpose of this study is to assess if pre‐treatment 4DCT is sufficient in proton therapy to provide accurate estimate of motion margins. A dosimetric assessment is performed comparing pre‐treatment margins with daily‐customized margins.Methods:Gold fiducial markers implanted in lung tumors of patients were used to track the tumor. A spherical tumor of diameter 20 mm is inserted into a realistic digital respiratory phantom, where the tumor motion is based on real patient lung tumor trajectories recorded over multiple days. Using “Day 1” patient data, 100 ITVs were generated with 1 s interval between consecutive scan start times. Each ITV was made up by the union of 10 tumor positions obtained from 6 s scan time. Two ITV volumes were chosen for treatment planning: ITVmean‐σ and ITVmean+σ. The delivered dose was computed on i) 10 phases forming the planning ITV (“10‐phase” – simulating dose calculation based on 4DCT) and ii) 50 phantoms produced from 100 s of data from any other day with tumor positions sampled every 2 s (“dynamic” – simulating the dose that would actually be delivered).Results:For similar breathing patterns between “Day 1” and any other “Day N(>1)”, the 95% volume coverage (D95) for “dynamic” case was 8.13% lower than the “10‐phase” case for ITVmean+σ. For breathing patterns that were very different between “Day 1” and any other “Day N(>1)”, this difference was as high as 24.5% for ITVmean‐σ.Conclusion:Proton treatment planning based on pre‐treatment 4DCT can lead to under‐dosage of the tumor and over‐dosage of the surrounding tissues, because of inadequate estimate of the range of motion of the tumor. This is due to the shift of the Bragg peak compared to photon therapy in which the tumor is surrounded by an electron bath.