Dose perturbation caused by high-density inhomogeneities in small beams instereotactic radiosurgery

The influence of high-density tissue heterogeneities in small-diameter beam s used in stereotactic radiosurgery has been investigated. Dose perturbatio n immediately behind aluminium sheets, used to simulate a high-density tiss ue inhomogeneity such as bone, was studied in a solid water phantom. Dose r eduction factors (DRFs), which are the ratios of the dose in the presence o f the inhomogeneity to dose in a uniform density solid water phantom, were measured with a diamond detector for three thicknesses of aluminium. DRFs e xhibit dependence on both the inhomogeneity thickness and the beam diameter . The DRF decreases with inhomogeneity thickness. The DRF initially decreas es with increase in the beam diameter from 12.5 to 25 mm. For fields greate r than 25 mm, the DRFs are nearly constant. The commonly used algorithms su ch as the TAR ratio method underestimate the magnitude of the measured effe ct. A good agreement between these measurements and Monte Carlo calculation s is obtained. The influence of the high-density inhomogeneity on the tissu e maximum ratio (TMR) was also measured with the inhomogeneity at a fixed d epth d(max) from the entrance surface. The TMR is reduced for all detector- inhomogeneity distances investigated. The dose build-up phenomenon observed in the presence of low-density air inhomogeneity is absent in the presence of a high-density inhomogeneity. The beam width (defined by 50% dose point s) immediately beyond the inhomogeneity is unaffected by the high-density i nhomogeneity. However, the 90%-10% and 80%-20% dose penumbra widths and the dose outside the beam edge (beyond the 50% dose point) are reduced. This r eduction in dose outside the beam edge is caused by the reduced range of th e secondary radiation (photons and electrons) in the high-density medium.