Fundamental characterization of surface temperature effects on ExacTrac Dynamic monitoring accuracy: A phantom study

ExacTrac Dynamic (EXTD) is an advanced surface-guided radiotherapy system that integrates thermal imaging with optical surface tracking. However, the effect of surface temperature on EXTD positional uncertainty has not been sufficiently characterized. This study aimed to fundamentally characterize the impact of surface temperature changes on EXTD positional errors along individual axes.

Simultaneous monitoring was performed using EXTD and an external thermal camera on geometric and thoracic phantoms during surface cooling. Tracking areas were defined for each phantom. Linear regression was used to assess the relationship between surface temperature and positional errors, including the overall translational error (ΔD_Trans) and translational and rotational errors along each axis.

ΔD_Trans increased as surface temperature decreased, with greater temperature dependence in smaller tracking areas. The maximum regression slopes of ΔD_Trans were -0.867 mm/°C and -0.984 mm/°C in the geometric and thoracic phantoms, respectively. In the geometric phantom, consistent positive correlations were observed in the vertical and pitch directions, suggesting systematic temperature-dependent behavior. In the thoracic phantom, the longitudinal direction showed weak negative correlations, whereas the roll direction showed the greatest temperature dependence, indicating stronger effects of surface geometry. For rotational errors, the standard errors decreased as the tracking area increased. Overall, the influence of surface temperature varied according to axis. No significant differences were found in the temperature changes at which tracking was lost among the tracking areas.

Accurate EXTD monitoring requires an understanding of these characteristics and the definition of appropriate tracking areas.