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

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Dosimetry

Background:

• Air cavities and gaps pose significant challenges in dose calculation for head and neck radiotherapy using flattening filter medical linear accelerators (Linacs).
• Accurate dose calculation is critical for effective and safe radiation delivery in complex anatomical regions.

Purpose of the Study:

• To evaluate the impact of air-gap dose calculation on flattening-filter-free (FFF) small field irradiation.
• To compare dose calculations in the presence of heterogeneities using different algorithms and Monte Carlo simulations.

Main Methods:

• Comparative study using experimental and Monte Carlo (MC) simulations to assess air-gap effects.
• Dose distribution simulated on virtual phantoms and patient CT images for open small fields and modulated photon beams.
• Dose ratios calculated using Analytical Anisotropic Algorithm (AAA) and MC for air-gaps versus tissue-equivalent material.

Main Results:

• A linear correlation was observed between the air-to-tissue-equivalent dose ratio and field size (slope: -0.198±0.001 for AAA, -0.161±0.014 for MC).
• Dose ratios below air-gaps were field size-dependent, with AAA to MC dose calculation discrepancies ranging from 1.57% to 5.35%.
• Significant dose discrepancies were noted for patient skin and oral cavity in head and neck cases due to air gaps.

Conclusions:

• The air-to-tissue-equivalent dose ratio decreases with smaller air gaps and larger field sizes.
• Dose correction for AAA calculations is recommended for open small fields following small air gaps.
• Utilizing multiple gantry angles in clinical cases can mitigate the impact of air-gap effects.