  Purpose: To validate the accuracy of 4D Monte Carlo (4DMC) simulations to
calculate dose deliveries to a deforming anatomy in the presence of realistic
respiratory motion traces. A previously developed deformable lung phantom
comprising an elastic tumor was modified to enable programming of arbitrary
motion profiles. 4D simulations of the dose delivered to the phantom were
compared with the measurements. Methods: The deformable lung phantom moving
with irregular breathing patterns was irradiated using static and VMAT beam
deliveries. Using the RADPOS 4D dosimetry system, point doses were measured
inside and outside the tumor. Dose profiles were acquired using films along the
motion path of the tumor (S-I). In addition to dose measurements, RADPOS was
used to record the motion of the tumor during dose deliveries. Dose
measurements were then compared against 4DMC simulations with
EGSnrc/4DdefDOSXYZnrc using the recorded tumor motion. Results: The agreements
between dose profiles from measurements and simulations were determined to be
within 2%/2 mm. Point dose agreements were within 2{\sigma} of experimental
and/or positional/dose reading uncertainties. 4DMC simulations were shown to
accurately predict the sensitivity of delivered dose to the starting phase of
breathing motions. We have demonstrated that our 4DMC method, combined with
RADPOS, can accurately simulate realistic dose deliveries to a deforming
anatomy moving with realistic breathing traces. This 4DMC tool has the
potential to be used as a quality assurance tool to verify treatments involving
respiratory motion. Adaptive treatment delivery is another area that may
benefit from the potential of this 4DMC tool.
