Systematic survey of the dose enhancement in tissue-equivalent materials facing medium- and high-Z backscatterers exposed to X-rays with energies from 5 to 250 keV

Abstract

The present study has been inspired by the results of earlier dose measurements in tissue-equivalent materials adjacent to thin foils of aluminum, copper, tin, gold, and lead. Large dose enhancements have been observed in low-Z materials near the interface when this ensemble was irradiated with X-rays of qualities known from diagnostic radiology. The excess doses have been attributed to photo-, Compton, and Auger electrons released from the metal surfaces. Correspondingly, high enhancements of biological effects have been observed in single cell layers arranged close to gold surfaces. The objective of the present work is to systematically survey, by calculation, the values of the dose enhancement in low-Z media facing backscattering materials with a variety of atomic numbers and over a large range of photon energies. Further parameters to be varied are the distance of the point of interest from the interface and the kind of the low-Z material. The voluminous calculations have been performed using the PHOTCOEF algorithm, a proven set of interpolation functions fitted to long-established Monte Carlo results, for primary photon energies between 5 and 250 keV and for atomic numbers varying over the periodic system up to Z = 100. The calculated results correlate well with our previous experimental results. It is shown that the values of the dose enhancement (a) vary strongly in dependence upon Z and photon energy; (b) have maxima in the energy region from 40 to 60 keV, determined by the K and L edges of the backscattering materials; and (c) are valued up to about 130 for "International Commission on Radiological Protection (ICRP) soft tissue" (soft tissue composition recommended by the ICRP) as the adjacent low-Z material. Maximum dose enhancement associated with the L edge occurs for materials with atomic numbers between 50 and 60, e.g., barium (Z = 56) and iodine (Z = 53). Such materials typically serve as contrast media in medical X-ray diagnostics. The gradual reduction in the dose enhancement with increasing distance from the material interface, owed to the limited ranges of the emitted secondary electrons, has been documented in detail. The discussion is devoted to practical radiological aspects of the dose enhancement phenomenon. Cytogenetic effects in cell layers closely proximate to surfaces of medium-Z materials might vary over two orders of magnitude, because the dose enhancement is accompanied by the earlier observed about twofold increase in the low-dose RBEM at a tissue-to-gold interface.

Nanoscale radiation transport and clinical beam modeling for gold nanoparticle dose enhanced radiotherapy (GNPT) using X-rays

“…58 In this respect, gadolinium contrast agent was proposed for microbeam radiotherapy (MRT). 41,42 Gadolinium, by virtue of its greater atomic number (Z 5 64) than iodine (Z 5 53) has dosimetric properties closer to those of gold (Z 5 79).…”

“…Simulation of radiation transport and energy deposition as well as the associated radiobiological quantities have been performed in various ways [46][47][48][49][50][51][52][53][54][55][56][57][58] using a plethora of radiation transport codes, simulation beams GNP geometries, GNP sizes and concentrations, X-ray sources and energies, as well as normalization schemes and metrics. Table 1 summarizes the literature on simulations using nanoscale MC and deterministic radiation transport.…”

“…58 There are several caveats of GNP radiotherapy (GNPT), which may not be completely valid in reality. First of all that the nanoparticles accumulate primarily in the tumour microvasculature (passive agents) or in the tumour cells (active agents) and that normal tissues, organs or cells have negligible concentrations of nanoparticles and therefore their direct impact on NTCP is negligible.…”

“…For convenience, we arrange the literature according to the following order: general radiation transport aspects relevant for nanoscale simulations, [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] macroscopic dose enhancement in contrast agents and dose perturbation at high-Z material interfaces, radiation transport for nanoscopic dose enhancement of GNP, [46][47][48][49][50][51][52][53][54][55][56][57][58] radiobiological and clinical aspects of GNPT, [59][60][61][62][63][64][65][66][67] application of Monte Carlo (MC) simulations to cellular environment, [68][69][70] modification of linear accelerator spectra to maximize dose enhancement 71 and GNPT using other than X-ray therapeutic beams (proton, electron). …”

Thin‐film CdTe detector for microdosimetric study of radiation dose enhancement at gold‐tissue interface

“…The interplay between problem details such as difference in Z, radiation energy, and interface geometry lead to different amounts of primary beam attenuation and scatter, calling for a dosimetric evaluation in each specific case. While attempts have been made for a comprehensive coverage of extended energy range, (1) there is no universal method for accurate quantitative description of the interface dosimetry. However, a high‐resolution microdosimetry approach should be employed for such studies since most of the dose perturbation effects occur over a very short () distance from the interface.…”