Influence of air inhomogeneities in radiosurgical beams

The effect of air inhomogeneity on dose distribution in small diameter beams used in stereotactic radiosurgery (SRS) has been investigated. The measurements of the surface dose and central axis dose were made for a 6 MV photon beam with a diamond detector which has suitable radiologic properties for such measurements. Measurements made in a uniform density solid water phantom and in the presence of four air gaps indicate significant dose perturbation immediately beyond the air-solid water interface. The reduction in dose at the surface for a 12.5 mm diameter field is 11%, 17%, 23% and 33% for air gap thicknesses of 3, 4.6, 6 and 9.2 mm, respectively. The corresponding dose reduction for a 25 mm diameter field is 3%, 4%, 7% and 13%, respectively. The ratio of the dose with and without air inhomogeneity is highly dependent on field diameter and approaches 1.0 as the field diameter increases from 12.5 mm to 40 mm. The dose perturbation also increases with increase in air inhomogeneity thickness for all field diameters investigated. A dose buildup phenomenon is observed beyond the air gap with a shallow dmax of approximately 4-6 mm. Beyond the buildup region, a higher dose value compared to a homogeneous phantom is observed at all depths due to reduced photon attenuation in the air gap. The dose profiles beyond the air gap, measured with radiographic films, demonstrate no significant increase in the beam diameter but a pronounced broadening of the beam penumbra (80%-20% and 90%-10%) leading to enhanced dose outside the primary beam geometric edge and reduced dose inside the edge. The dose enhancement outside the beam edge increases with increase in air gap thickness.