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The Dosimetric Consequences of 4DCT-Based Motion Margin Estimation for Proton Radiotherapy

O Koybasi

O Koybasi¹*, J Lewis², P Mishra³, S James⁴, J Seco⁵, (1)Mass General Hospital; Harvard Medical, Boston , MA ,(2) Brigham and Women's Hospital, Boston, MA, (3) Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts, (4) Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts, (5) Mass General Hospital; Harvard Medical, Boston , MA

SU-E-T-682 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Motion management in standard radiotherapy uses ITV margins obtained from pre-treatment 4DCT. The purpose of this study is to investigate the dosimetric effects of inaccuracies in motion margins obtained from pre-treatment 4DCT in proton radiotherapy.

Methods:We use the locations of gold fiducial markers implanted in lung tumors of patients to track the tumor. A 20 mm diameter spherical tumor 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 4D-CT) 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 a patient with similar breathing patterns on "Day 1" and the "other day", the 95% volume coverage (D95) for "dynamic" case was 8.13% lower than the "10-phase" case for ITVmean+σ. For a patient with relatively different breathing patterns on "Day 1" and the "other day", this difference was as high as 24.5% for ITVmean-σ.

Conclusion:Our preliminary results have shown that proton treatment planning based on 4DCT can lead to under-dosage of the tumor and over-dosage of the surrounding tissues. This is more severe in proton therapy due to possible shift of the Bragg peak compared to photon therapy in which the tumor is surrounded by an electron bath.

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