Dosimetric impact of baseline drift in volumetric modulated arc therapy with breath holding

Abstract

Background: We investigated the change of dose distributions in volumetric modulated arc therapy (VMAT) under baseline drift (BD) during breath holding.

Materials and methods: Ten VMAT plans recalculated to a static field at a gantry angle of 0° were prepared for measurement with a 2D array device and five original VMAT plans were prepared for measurement with gafchromic films. These measurement approaches were driven by a waveform reproducing breath holding with BD. We considered breath holding times of 15 and 10 s, and BD at four speeds; specifically, BD0 (0 mm/s), BD0.2 (0.2 mm/s), BD0.3 (0.3 mm/s), and BD0.4 (0.4 mm/s). The BD was periodically reproduced from the isocenter along the craniocaudal direction and the shift during breath holding (Shift_BH) ranged 0-6 mm.The dose distribution of BD0.2, BD0.3 and BD0.4 were compared to that of BD0 using gamma analysis with the criterion of 2%/2 mm.

Results: The mean pass rates of each Shift_BH were 99.8% and 98.9% at 0 mm, 96.8% and 99.4% at 2 mm, 94.9% and 98.6% at 3 mm, 91.5% and 98.4% at 4 mm, 70.8% and 94.1% at 4.5 mm, and 55.0% and 83.6% at 6 mm for the array and film measurements, respectively.

Conclusion: We found significant differences in Shift_BH above 4 mm (ρ < 0.05). Hence, it is recommended that breath holding time should be shortened for patients to preserve the reproducibility of dose distributions.

Keywords: Baseline drift; Breath holding; Dosimetry; Pancreatic cancer; Volumetric modulated arc therapy.