Using kV Triggered Imaging and Liver Dome Position to Reduce the Dosimetric Error Caused by Breath Hold Variability for Liver Stereotactic Body Radiation Therapy

Abstract

In a previous study, we demonstrated that manual gating using kV triggered imaging and liver dome position can reduce targeting errors caused by breath hold variability for liver stereotactic body radiation therapy (SBRT). In this study, we quantified the dosimetric error caused by breath hold variability and investigated the effect of liver dome gating on reducing dosimetric error. Twenty-five liver SBRT patients treated with deep inspiration breath-hold were included in this IRB approved study. Volumetric modulated arc therapy was used to deliver 30-60 Gy in 1-5 fractions. To verify the breath-hold reproducibility during treatment, a KV triggered image was acquired at the beginning of each breath-hold. The liver dome position was visually compared with the expected upper/lower liver boundaries created by expanding/contracting the liver contour 5mm in the superior-inferior direction. If the liver dome position was within the boundaries, delivery continued; otherwise, beam was held manually and the patient was instructed to take another breath hold until the liver dome position was within boundaries. To calculate delivered dose, for each fraction, the treatment plan was divided into sub-beams, each corresponding to one breath hold using delivery log files. The triggered images were registered to the planning CT to determine the liver position during each breath hold. Dose delivered during each breath hold was calculated by shifting the isocenter of the sub-beam according to the liver position. Breath holds discarded by gating were excluded since no dose was delivered during these breath holds. Delivered fractional doses were compared with planned fractional doses using GTV D99 and liver Dmean. To estimate delivered dose without gating, the first "corrective" breath hold taken after the discarded breath holds was replaced with the prior discarded breath hold and dose calculation was repeated. Seven hundred eleven triggered images from 91 treatment fractions were analyzed. Without gating, in 11 of the 91 fractions from 7 of the 25 patients, delivered GTV D99 reduced > 0.50 Gy from planned value (range 0.51-1.68 Gy, 3-10% of planned fractional GTV D99). Liver dome gating was able to detect/exclude irreproducible breath holds in 8 of the 11 fractions, increasing the delivered GTV D99 by 0.70 Gy per fraction on average (range 0.21-1.63 Gy). With liver dome gating, delivered fractional GTV D99 was comparable to planned value for all fractions (12.96 +/- 5.19 Gy vs 13.04 +/- 5.18 Gy, p > 0.05). Liver mean dose was not affected by breath hold variability or gating. Fractional liver Dmean was 2.26 +/- 1.19 Gy from plan, 2.27 +/- 1.21 Gy for delivery with gating and 2.27 +/- 1.20 Gy for delivery without gating. Breath hold variability may cause tumor underdose. Liver dome gating using kV triggered imaging reduces dosimetric error and ensures tumor coverage for liver SBRT.