  The XCAT phantom is a realistic 4D digital torso phantom that is widely used
in imaging and therapy research. However, lung mass is not conserved between
respiratory phases of the phantom, making detailed dosimetric simulations and
dose accumulation unphysical. A framework is developed to correct this issue by
enforcing local mass conservation in the XCAT lung. Dose calculations are
performed to assess the implications of neglecting mass conservation, and to
demonstrate an application of the phantom to calculate the accumulated
delivered dose in an irregularly breathing patient. Monte Carlo methods are
used to simulate conventional and SBRT treatment delivery. The spatial
distribution of the lung dose was qualitatively changed by the use of mass
conservation; however the corresponding DVH did not change significantly.
Comparison of the delivered dose with 4DCT-based predictions shows similar lung
metric results, however dose differences of 10% can be seen in some spatial
regions. Using this tool to simulate patient treatments reveals differences
between the planned dose and the calculated delivered dose for the full
treatment. The software is freely available from the authors.
