Quantifying three-dimensional displacement of cardiac implantable electronic devices and implications for safety margins in radiotherapy.

This study aimed to quantitatively evaluate the three-dimensional (3D) displacement of cardiac implantable electronic devices (CIEDs) during radiotherapy and derive safety margins. Sixteen treatment courses (13 patients, 262 fractions) were retrospectively analyzed. Daily device displacement was quantified using orthogonal 2D kV images relative to clinical bone-matching. To evaluate manual registration uncertainty, the intraclass correlation coefficient and minimal detectable change (MDC) were calculated. To assess the impact of patient positioning, displacements were compared between arm-up and arm-down groups. Planning organ at risk volume (PRV) margins were derived from systematic and random errors. A maximum 3D displacement of 24 mm was observed. Manual registration demonstrated good to excellent reproducibility for the primary translational axes. The maximum MDC was 3.6 mm for the 3D displacement vector. While overall displacement distributions showed no significant differences between arm positions, extreme medial displacement significantly increased in the arm-up group (p < 0.05). The calculated PRV margins reached a maximum of 7.3 mm in the longitudinal direction for the arm-up group. Throughout the radiotherapy course, CIEDs can undergo displacement independently of the skeletal structure. The PRV margins derived here provide a quantitative basis for pre-treatment dose evaluation. The observed range of displacement can inform treatment planning and supports verification of the device position at each fraction via image-guided radiotherapy.