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Potential Dosimetric Impact of Two-Isocenter Treatments for Pelvis and Nodal Irradiation Using Halcyon Platform: A Robustness Analysis

M Kim*, R Scheuermann , L Dong , T Li , University of Pennsylvania, Philadelphia, PA


(Sunday, 7/29/2018) 2:05 PM - 3:00 PM

Room: Karl Dean Ballroom A2

Purpose: The Varian Halcyon has a maximum field size of 28x28cm2, which requires a two-isocenter technique to cover the extended field length for some pelvis+nodal irradiations. Auto-feathering is a newly-introduced feature in the treatment planning system to generate smooth dose transitions in the S/I direction between superior and inferior beam groups. This study compares the effectiveness of the auto-feathering function, and quantifies plans robustness to inter-isocenter shifts for IMRT and VMAT plans.

Methods: VMAT and IMRT plans were retrospectively generated for four patients with IRB approval. Distances between two isocenters were 8cm and 10cm. To simulate patient drift/motion between two isocenters, inferior-isocenter beams were perturbed in L/R, A/P, and S/I directions by ±0.5cm and ±1cm with dose-recalculation afterwards. To assess the effectiveness of auto-feathering, plans optimized without auto-feathering were also generated for S/I direction perturbations. Perturbed inferior-isocenter beams were summed with superior-isocenter beams, and the total doses were compared to original non-shifted plans for PTV-D95, Global-Dmax, and DVH parameters to organs-at-risk.

Results: Auto-feathering significantly reduced variation to PTV-D95 and Global-Dmax when S/I-direction perturbation were introduced. This effect was more pronounced for IMRT than VMAT technique. For plans generated with auto-feathering, in 3D perturbation simulation PTV-D95 varied by -1.56±1.10% for 0.5cm-perturbations, and -7.34±2.44% for 1cm-perturbations. Global-Dmax varied by 1.58±1.36% for 0.5cm-perturbations, and 2.89±2.29 for 1cm-perturbations. For key OARs, Bladder-D2cc, Rectum-D2cc, Small-Bowel-D0.1cc, Large-Bowel-D0.1cc were relatively stable with 0.5cm perturbations (0.71±1.40% difference) compared to 1cm-perturbations (0.90±3.25% difference). In paired comparison, VMAT showed significantly lower variability (p<0.05) with perturbations than IMRT for PTV-D95, Global-Dmax, and Bladder-D2cc, except for PTV-D95 with 0.5cm-perturbation.

Conclusion: For pelvis+nodal irradiation when PTV exceeds 28cm, auto-feathering improved plan’s robustness to S/I direction perturbations . In general, key dosimetric parameters varied within 2% for 0.5cm perturbations in any direction between isocenters with AF turned on, but increased substantially for 1.0cm perturbations.

Funding Support, Disclosures, and Conflict of Interest: This research is partially supported by Varian Medical Systems.


Dose, Setup Errors, Treatment Planning


TH- External beam- photons: Development (new technology and techniques)

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