Stereotactic synchrotron microbeam radiotherapy.

Abstract

Highly collimated synchrotron x-ray beams with high fluence rate may be used in stereotactic radiotherapy of brain tumours. Several monochromatic x-ray beams having uniform microscopic thickness ie (microplanar beams) are directed to the center of the tumour from varying directions, delivering lethal dose to the target volume while sparing normal cells. This proposed technique takes advantage of the hypothesised repair mechanism of capillaries between closely spaced microplanar beam zones. The sharply dropping lateral dose profile of a microplanar beam provides low scattered dose to the off-target interbeam volume. In close proximity to the target volume, relatively high secondary electron doses close to the edge of the beams overlap and produce a high dose region between angled beams. This allows precise targeting and prevents gradual blurring of the higher and lower dose margins in the target volume. The advantages of stereotactic microplanar beam radiotherapy will be lost as the dose between microplanar beams exceeds the tolerance dose of the dose limiting tissues. Therefore to minimize the risks of delayed radiation damage it is essential to optimize the interbeam doses inside a human head phantom. The EGS4 Monte Carlo code is used to calculate the lateral dose profiles and depth dose of a 100 keV single microplanar beam in the phantom. A general equation for absorbed dose as a function of depth and lateral distances is derived for the single beam. Several microplanar beams are directed into the target volume at the center of the phantom. Using the equation, maximum dose on the beam axis (primary + total scattered dose) and the minimum interbeam dose (total scattered dose) are calculated at different depths and an isodose map of the phantom is obtained. A stereotactic microplanar beam radiotherapy model is proposed for a 10 mm diameter (approximately spherical) tumour at the center of the phantom.