Definition of a moving gross target volume for stereotactic radiation therapy of stented coronary arteries

Purpose : To measure the effect of cardiac motion on coronary artery stent position during the cardiac cycle as a first step toward exploring the feasibility of stereotactic external beam radiation therapy targeted at restenotic stented coronary arteries. Methods and Materials : The three-dimensional (3D) position of eight coronary artery stents in 8 patients immobilized in a stereotactic body frame was studied noninvasively by single-breathhold ECG-gated multislice spiral computed tomography (MSCT) during 10 retrospectively selected phases, equally distributed throughout the R-R interval of consecutive cardiac cycles. The volume encompassing all measured 3D positions of the stent was measured. Results : Stent volumes measured by MSCT closely agreed with measurements by quantitative coronary angiography ( r > 0.99). The mean of the maximum 3D stent center of mass displacement between any two phases during the cardiac cycle for all eight coronary arteries was 7.5 mm (range 3.3–20.5 mm) in the lateral direction, 8.6 mm (range 2.7–21.6 mm) in the ventrodorsal direction, and 8.2 mm (range 2.5–19.7 mm) in the craniocaudal direction. As was anticipated, the volume encompassing all measured 3D positions of the stent represented only a fraction of the whole heart volume in all patients, i.e., less than 0.6%. Conclusions : ECG-gated MSCT allowed the measurement of the volume encompassing multiphase 3D positions of coronary artery stents during the cardiac cycle. This volume, a measure of the cardiac motion effect on coronary artery stent position during the cardiac cycle, represents a moving gross target for stereotactic external beam radiation therapy.