Determination of dosimetrical quantities used in microbeam radiation therapy (MRT) with Monte Carlo simulations
The two irradiation geometries simulated in this work are reported here. In (a) a multislit collimator (MSC) producing the planar microbeams, and in (b) a collimator producing the cylindrical beams, are shown. Subsequently the beams propagate through the cylindrical water phantom, parallel with the cylinder axis. Note that the microbeams were assumed to start on the phantom surface with a common direction in the MC simulations performed in this work.
The measured x-ray spectrum as used for MRT studies at the ESRF.
In (a) depth-dose curves obtained from MC simulations are shown for two different dose-scoring geometries; in one case for a narrow geometry around the center of the microbeam and in the second case for a wide geometry extending over the whole cylindrical disc. The beam size was constant, , and the x-ray energies were sampled from the ESRF spectrum. In (b) a detailed study of the surface-dose buildup in the microbeam field is shown, when using the central-axis scoring geometry, for a 100-keV beam compared to the ESRF beam. The error bars for the ESRF beam, showing the statistical uncertainty, correspond to three standard deviations.
Comparison of microbeam lateral-dose profiles when using different beam energy/geometry.
A comparison is shown of the (a) spectra and (b) angular distributions of secondary electrons leaving a planar microbeam with either monoenergetic x-ray energies (50, 100, or ) or with energies sampled from the ESRF spectrum.
A comparison is shown of the spectra (a) and angular distributions (b) of secondary photons leaving a planar microbeam with either monoenergetic x-ray energies (50, 100, or ) or with energies sampled from the ESRF spectrum.
The curves 1–4 demonstrate the impact on the lateral-dose profile of suppressing different types of interactions.
The lateral-dose profile for a 100-keV monoenergetic beam and cylindrical-beam geometry is compared with calculations done by Stepanek et al. (Ref. 11) and Slatkin et al. (Ref. 3).
Lateral-dose profiles for three different planar microbeam widths are shown. The beam height is constant, at .
(a) Lateral-dose profiles for planar microbeams of three different heights but of constant width are presented. (b) A detailed view is shown of the dose at distances which, for a composite dose distribution as used in MRT, would correspond to the distance to the valley.
The three curves show simulated lateral-dose profiles for different beam sizes while suppressing electron transport. A larger valley dose (between 25 and ) is obtained when the beam height is increased due to additional scattered photons.
The variation of peak and valley doses in the center of the planar microbeam array is shown for two different microbeam spacings. The doses were calculated for a array of planar microbeams (each microbeam of size ). The x-ray energies were sampled from the ESRF spectrum.
The variation of PVDR’s with depth in water in the center of a planar microbeam array (each microbeam of size ) is shown for two different microbeam spacings and for four different microbeam energies.
The variation of PVDR’s with depth in water is shown for three arrays of planar beams of sizes , , and . The sizes of the corresponding microbeam arrays were , , and . The ctc microbeam spacing used was and the photon energies were sampled from the ESRF spectrum.
MC calculated relative maximum doses for different microbeam field sizes (output factors). The beam height was constant, , while the width was varied in the simulations. The doses were calculated in water at 7– depth.
PVDR’s calculated at the center of a matrix of evenly spaced cylindrical microbeams traveling along the axis of a 16-cm-long, 16-cm-diam, cylindrical phantom.
PVDR’s calculated at the center of a matrix for evenly spaced planar microbeams traveling along the axis of a 16-cm-long, 16-cm-diam, cylindrical phantom.
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