The aim of this work was to validate a 4D Monte Carlo (MC) dose calculation tool (4DdefDOSXYZnrc/EGSnrc) by comparing dose calculations with measurements in rigid and deforming anatomies. This 4DMC approach, that is based on the voxel warping method, calculates the cumulative dose by including continuous motion/deformation of anatomy and motion of linac components. We present how motion recorded during dose delivery combined with deformation/displacement vectors and delivery log files can be used to incorporate motion of the anatomy and treatment units into simulations. Dose deliveries were performed on an Elekta Infinity linac with Agility MLC for static and volumetric modulated arc therapy treatments on the stationary and breathing states of the phantoms. The dose was measured with film (point dose and dose profile) and the RADPOS 4D dosimetry system (point dose).
Preliminary validations were done using a phantom with a rigidly moving lung insert. To evaluate the performance of 4DMC simulations in a more realistic condition a programmable deformable tissue-equivalent lung phantom was developed. The lung inserts in both phantoms served the purpose of holding film as well as a spherical tumor to hold RADPOS. RADPOS detectors were also placed outside the tumor in the deformable phantom. Sinusoidal (both phantoms) and irregular (deformable phantom) respiratory traces were tested. Point dose agreements between simulations and measurements were within 2σ of experimental and/or positional/dose reading uncertainties. The agreement between dose profiles from measurements and simulations were determined to be within 2%/2 mm.
We also present Monte Carlo modeling of the Elekta linac used in this work. A detailed model was created in BEAMnrc/EGSnrc and model parameters were adjusted through appropriate measurements. We demonstrate how using a simple virtual source model of a linac can help study the sensitivity of the isocentric photon fluence to several beam parameters for small radiation fields. Finally, we present how we can use MC simulations and the virtual source model to explain dose discrepancies observed between film measurements and calculations obtained from the Monaco treatment planning system. The results show that proper model parameters can heavily impact the accuracy of dose calculations of small fields.