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 60 Gy to the tumor bed and 50 Gy 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 ± 0.4 mm, 2.1 ± 1.9 mm, and 0.7 ± 0.5 mm in the VRT, LNG, and LAT directions, with a maximal value of 8.8 mm in LNG direction. Similarly, the mean intra-breath-hold reproducibility was 1.4 ± 0.8 mm, 1.7 ± 1.0 mm, and 0.8 ± 0.5 mm in the VRT, LNG, and LAT directions, with a maximal value of 4.1 mm in LNG direction. Inter-fractional reproducibility showed better reliability, with difference in breathing levels in all fractions of 0.3 mm on average. Based on tolerance limits corresponding to the 90% cumulative distribution level, gating window widths of 1 mm, 2 mm, and 5 mm 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.

Keywords: ABC; SGRT; VMAT; breast; breath-hold; spirometer; surface-guided.

FIGURE 1

Screenshot of the AlignRT software control showing the region of interest for DIBH monitoring of a breast target for a representative patient.

FIGURE 2

Graphical representations of (a) reproducibility and (b) stability parameters. Black line represents the deviation in mm over time during one DIBH cycle and red line represents the linear fit plot of the black signal.

FIGURE 3

(a) The spirometer signal is shown in volume units (L). (b) The continuous signals of patient monitoring by the surface‐guided system for the vertical (VRT, anterior‐posterior in blue), longitudinal (LNG, superior‐inferior in red), and lateral (LAT, in green) coordinates. In particular, this patient displays some breast relaxation during intra breath‐hold time, most in the LNG direction, not reflected in the perfectly flat spirometer signal.

FIGURE 4

Box plots of stability and intra‐ and inter‐fraction reproducibility in the VRT (red), LNG (blue), and LAT (black) directions during all breath‐holds for the five evaluated patients (STAB = Stability, INTRA = intra‐fraction reproducibility, and INTER = inter‐fraction reproducibility).

FIGURE 5

Cumulative distribution functions for deviations in millimeters along the three directions for the stability metric.

FIGURE 6

Differences between breathing maneuvers of all patients along the three spatial directions.

FIGURE 7

Plots of the main dosimetric metrics for PTV, heart and lung of the isocenter‐shifted plans versus the original plans. The dashed lines correspond to the bisector, that is, the locus of points where the dosimetric metrics have the same values for both plans. Each point represents an individual patient. Blue, red, and green colors correspond to the 50%, 90%, and 100% cumulative distribution levels.