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Susceptibility of SBRT to Delivery Uncertainties Using RapidArc Or Dynamic IMRT

G Betzel*, M Tejani, A Dhabaan, Department of Radiation Oncology and Winship Cancer Institute, Emory University, Atlanta, GA

Presentations

SU-E-T-307 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To determine if SBRT is more susceptible to delivery uncertainties when using either RapidArc or dynamic IMRT.

Methods: Systematic or random uncertainties in MLC leaf positions, gantry position and dose rate were incorporated into RapidArc (RA) and sliding-window IMRT (SW) treatment plans for retrospective SBRT lung and liver cases, which also included the use of FFF or SRS energies and therefore higher dose rates. Initial plan comparisons included homogeneity and conformity indices. Dose distributions and dose-value histograms of original and modified plans were compared using Gamma analysis and dose indices of planned treatment volumes (PTV) and organs at risk (OAR).

Results: SBRT lung PTVs using RA were more susceptible to systematic MLC delivery errors than SW (~6%/mm vs. ~4%/mm), in contrast to a previous study examining hyperfractionated prostate and head-and-neck treatments; esophagus and cord Dmin and Dmax values changed proportionately for both techniques; RA plans were superior globally but not locally (PTV) using Gamma analyses. Uniformly distributed random variations of 2 mm for active MLC leaves had a negligible effect on all PTV dose distributions (Dmin, Dmax < 0.4%). PTV coverage was affected equally when incorporating systematic gantry errors.

Conclusion: RapidArc deliveries were found to be more sensitive to systematic errors in MLC leaf positions, in contrast to a previous study on hyperfractionated treatments. However, this is reasonable as both studies correlate average segmental field size or mean leaf gap with adverse dosimetric effects due to such uncertainties; mean leaf gaps for either technique in this study were similar. Gantry angle variations had similar effects on both techniques.

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