Andrew Wu1, Hungcheng Chen2 ,
and Edward Brandner2
1 Thomas
Jefferson University, Department of Radiologic
Science, Philadelphia, PA, 19014, USA, 2 University of Pittsburgh Cancer Institute,
Department of Radiation Oncology, Pittsburgh, PA 15232, USA
Abstract: The purpose of this study is to investigate
and to evaluate the effects of target motion resulting from respiration on its dosimetric parameters such as absolute dose, intensity
profiles, and dose distributions delivered by respiratory-gated intensity
modulated radiation therapy (IMRT). A moving phantom was designed and built to
simulate the cycle of normal breathing motion. A commercial respiratory gating
system was used to automatically control the beam on and off based on the
motion of an external marker placed on the phantom. For the gated beam
delivery, the gated window was determined by examining the 4D-CT images of the
moving phantom. Different widths of the gated windows representing different
length in time scale were selected so that the motion of the phantom during the
beam-on period within the window would range from 1 to 3 cm. We delivered IMRT
fields to expose ready-pack radiographic films placed in three various
conditions: a stationary phantom, a moving phantom with the gated controling beam and a moving phantom without the gated
controlling beam. The IMRT dose distributions obtained from the stationary
phantom would serve as a reference for comparison with other two cases with
moving phantom. Our results showed significant dose variations in dose
distributions between the cases of stationary phantom and the moving phantom
without gating. However, the absolute doses and dose profiles delivered with
IMRT fields to a moving target of 1 cm maximum displacement had shown no
significant differences from those of a stationary target. With maximum
displacement of 2 and 3 cm, the overdosed and under-dosed areas seemed to
appear as large as 1.0 and 2.2 cm long in the superior margin and 1.2 and 2.7
cm long in the inferior direction, respectively. With the gating system of beam
delivery, the results have shown that significant reduction of the level of
variations and deviations in dose distribution and absolute dose measurement in
spite of the target motion.