(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: Constant dose rate and constant gantry speed arc therapy (CDR-CAS-IMAT) offers constant dose rate delivery in the form of arcs while dynamically varying gantry position, leaf positions. The aim of the present study is to investigate the effects of CDR-CAS-IMAT simulated delivery errors for brain tumors.
Methods: All CDR-CAS-IMAT plans were calculated with the Monte Carlo engine of MONACO TPS, the dose rate was constant (set at 500MU/min), and the gantry rotation velocity was constant set at 6°/s. With an in-house software, we modified original plans to induce specific errors within and outside manufacturer specifications while keeping the rest of the treatment information unchanged. We introduced the following errors: (i) Both sets of leaf banks were shifted by same distance in the same direction by 1mm, and 2mm. (ii) gantry angle was shifted by 0.5°, 1°, 2°. All these modified plans were imported into MONACO and dose distributions were recalculated. The parameters of the target volume and the organs at risk were compared and analyzed.
Results: For plans with errors in both leaf banks, maximum deviations were (-2.10±3.2)%, (2.63±6.61)%, (-0.7±1.45)%, (0.56±1.91)% and (-0.6±0.75)% for PTVmin, Lensmax, chiasmmax, optic-nervemax and brainstemmax respectively. With gantry angle errors, the deviations for PTVmin, Lensmax, chiasmmax, optic-nervemax and brainstemmax were (0.82±0.87)%, (1.8±9.53)%, (-0.62±1.51)%, (0.95±1.32)% and (0.45±0.52)% respectively compared to the original plans.
Conclusion: Patients with brain tumors treated with CDR‑CAS‑IMAT using Varian Clinical 23EX, any change in the aperture shape (like error in one leaf bank) showed largest deviations followed by plans with errors in two sets of leaf banks and gantry angle. Therefore, to avoid any deleterious clinical consequences, accurate calibrations of machine parameters, predominantly of leaf position, are indispensable for CDR-CAS-IMAT.
Funding Support, Disclosures, and Conflict of Interest: This work was funed by the Natural Science Foundation of Hebei Province of China (Grant No. H2018206135).
Keywords
Absolute Dosimetry, Linear Accelerator, Quality Assurance
Taxonomy
TH- External beam- photons: Quality Assurance - VMAT
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