Comparison of Air-Gaps Effect in a Small Cavity on Dose Calculation for 6 MV Linac

Background: Many authors stated that cavities or airgaps were the main challenge of dose calculation for head and neck with flattening filter medical linear accelerator (Linac) irradiation. Objective: The study aimed to evaluate the effect of airgap dose calculation on flatteningfilterfree (FFF) small field irradiation.Material and Methods: In this comparative study, we did the experimental and Monte Carlo (MC) simulation to evaluate the presence of heterogeneities in radiotherapy. We simulated the dose distribution on virtual phantom and the patient’s CT image to determine the airgap effect of open small field and modulated photon beam, respectively. The dose ratio of airgaps to tissueequivalent was calculated both in Analytical Anisotropic Algorithm (AAA) and MC. Results: We found that the dose ratio of air to tissueequivalent tends to decrease with a larger field size. This correlation was linear with a slope of 0.198±0.001 and 0.161±0.014 for both AAA and MC, respectively. On the other hand, the dose ratio below the airgap was field sizedependent. The AAA to MC dose calculation as the impact of airgap thickness and field size varied from 1.57% to 5.35% after the gap. Besides, patient’s skin and oral cavity on head and neck case received a large dose discrepancy according to this study. Conclusion: The dose air to tissueequivalent ratio decreased with smaller air gaps and larger field sizes. Dose correction for AAA calculation of open small field size should be considered after small airgaps. However, delivered beam from others gantry angle reduced this effect on clinical case.