An in house inhomogeneous insert for use with ArcCHECK ™ was developed for dose calculation verification of Stereotactic Body Radiation Therapy (SBRT) lung plans. The inhomogeneous insert has various ion chamber inserts for different geometrical configurations (lung, soft tissue, bone, air). However, the insertion of an ion chamber in a low density medium perturbs the dose to that region by creating Charged Particle Disequilibrium (CPD), limiting the accuracy of ion chamber measurements. By simulating the ion chamber and phantom using Monte Carlo, a correction factor could be calculated and measured to verify the dose difference caused by CPD. BEAMnrc was used to generate a phase space input file for DOSXYZnrc with beam characteristics that matched clinical commissioning data. A model of the A1SL ion chamber geometry (shell, collector, stem, guard) was simulated in a simple water-lung-water slab phantom. Dose to the active area of the ion chamber was measured in several locations throughout the phantom. The active area of the ion chamber was replaced by the surrounding medium; i.e., water or lung within the phantom, and the dose to the same voxels was calculated. The dose was measured on a Linac and the results agreed within 3% and confirmed that the presence of the ion chamber in low density lung perturbs the dose measured in the field by over 31%.