Calculation of dose profiles in stereotactic synchrotron microplanar beam radiotherapy in a tissue-lung phantom.

Synchrotron x-ray beams with high fluence rate and highly collimated may be used in stereotactic radiotherapy of lung tumours. A bundle of converging monochromatic x-ray beams having uniform microscopic thickness i.e. (microplanar beams) are directed to the center of the tumour, delivering lethal dose to the target volume while sparing normal cells. The proposed technique takes advantage of the hypothesised repair mechanism of capillary cells between alternate microbeam zones, which regenerate the lethally irradiated endothelial cells. The sharply dropping lateral dose of a microbeam provides low scattered dose to the off-target interbeam volume. In the target volume the converging bundle of beams are closely spaced, and relatively high primary and secondary electron doses overlap and produce a high dose region between the beams. This higher and lower dose margins in the target volume allows precise targeting. The advantages of stereotactic microbeam radiotherapy will be lost as the dose between microbeams exceeds the tolerance dose of the dose limiting tissues. Therefore, it is essential to optimize the interbeam doses in off-target volume. The lateral and depth doses of 100 keV microplanar beams are investigated for a single beam and an array of converging microplanar beams in a tissue, lung and tissue-lung phantoms. The EGS5 Monte Carlo code is used to calculate dose profiles at different depths and bundles of beams. The maximum dose on the beam axis (peak) and the minimum interbeam dose (valley) are compared at different energies, depths, bundle sizes, heights, widths and beam spacings. The interbeam dose is calculated at different depths and an isodose map of the phantom is obtained. An acceptable energy region is found for tissue and lung microbeam radiotherapy and a stereotactic microbeam radiotherapy model is proposed for a 4 cm diameter and 1 cm thick tumour on the lung phantom.