Reproducibility and stability of spirometer-guided deep inspiration breath-hold in left-breast treatments using an optical surface monitoring system.

Abstract

The aim of this study was to evaluate the reproducibility and stability of left breast positioning during spirometer-guided deep-inspiration breath-hold (DIBH) radiotherapy using an optical surface imaging system (AlignRT). The AlignRT optical tracking system was used to monitor five left-sided breast cancer patients treated using the Active Breathing Coordinator spirometer with DIBH technique. Treatment plans were created using an automated hybrid-VMAT technique on DIBH CTs. A prescribed dose of 60Gy to the tumor bed and 50Gy to the breast in 25 fractions was planned. During each treatment session, the antero-posterior (VRT), superior-inferior (LNG), and lateral (LAT) motion of patients was continuously recorded by AlignRT. The intra-breath-hold stability and the intra- and inter-fraction reproducibility were analyzed for all breath-holds and treatment fractions. The dosimetric impact of the residual motion during DIBH was evaluated from the isocenter shifts amplitudes obtained from the 50%, 90%, and 100% cumulative distribution functions of intra-fractional reproducibility. The positional variations of 590 breath-holds as measured by AlignRT were evaluated. The mean intra-breath-hold stability during DIBH was 1.0±0.4mm, 2.1±1.9mm, and 0.7±0.5mm in the VRT, LNG, and LAT directions, with a maximal value of 8.8mm in LNG direction. Similarly, the mean intra-breath-hold reproducibility was 1.4±0.8mm, 1.7±1.0mm, and 0.8±0.5mm in the VRT, LNG, and LAT directions, with a maximal value of 4.1mm in LNG direction. Inter-fractional reproducibility showed better reliability, with difference in breathing levels in all fractions of 0.3mm on average. Based on tolerance limits corresponding to the 90% cumulative distribution level, gating window widths of 1mm, 2mm, and 5mm in the LAT, VRT, and LNG directions were considered an appropriate choice. In conclusion, despite the use of a dedicated spirometer at constant tidal volume, a non-negligible variability of the breast surface position has been reported during breath-holds. The real-time monitoring of breast surface using surface-guided optical technology is strongly recommended.