Program Information
Dosimetric Influence of Temporal Resolution Used by the PBS Proton 4D Dose Calculations
Y Zhang1*, F Mueller2 , D Weber3 , A Lomax4 , (1) Paul Scherrer Institut, Villigen-PSI, Aargau, (2) ETH Zurich, Zurich, Zurich, (3) Paul Scherrer Institut, Villigen-PSI, Aargau, (4) Paul Scherrer Institut, Villigen-PSI, Aargau
Presentations
TH-AB-605-4 (Thursday, August 3, 2017) 7:30 AM - 9:30 AM Room: 605
Purpose: This project aims to investigate the potential dosimetric impact on estimated 4D dose distributions as a result of the temporal resolution of 4D dose calculations.
Methods: For three liver patients (CTV volume: 122/403/264cc), 10-phase 4DCT datasets with ~15mm tumour motion were simulated for 7 different motion periods (2-8s). 4D dose distributions were calculated using dose warping between phases (temporal resolution 0.2-0.8s) and a deforming dose grid approach with 0.02s temporal resolution. Single scanned 4D plans for 7 fraction doses (0.7/2/4/6/8/10/12Gy) were investigated. Two Pencil Beam Scanning (PBS) delivery time-lines were considered: (1) layer-wise varying dose rate with raster scanning, (2) fixed dose rate with spot scanning. Dosimetric assessments were performed by analyzing dose distributions derived from the two 4D algorithms, their absolute differences and difference in D5-D95 (CTV) and Vdiff≥5% for all motion periods and fraction dose scenarios.
Results: Pronounced dose differences between the two 4D approaches (temporal resolutions) were observed for all patients, fraction doses and motion periods. Differences were however larger for timeline 2 than 1, due to the fixed dose rate where spot duration can significantly increase for high fraction doses. In this case, the effects of ignoring motion during spot delivery become considerable, especially for shorter motion periods where geometry changes occur at higher frequency. For fraction doses ≥4Gy and motion periods ≤4s, warping dose between 4DCT phases (longer temporal resolution) showed 3-10% increased D5-D95 (worsening dose homogeneity) and was associated with more than 20% of the CTV having absolute dose differences over 5% in comparison to the higher temporal resolution calculation (deforming dose grid).
Conclusion: The dosimetric effects of low temporal resolution 4D dose calculations by dose warping cannot be neglected for high dose plans and patients with short breathing periods (fast breathers), especially for PBS machine using constant dose rates during delivery.
Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by the research grant from Varian Medical Systems - Particle Therapy, Germany
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