Elsevier

Radiation Measurements

Volume 56, September 2013, Pages 285-289
Radiation Measurements

Spatial correlation between traversal and cellular response in ion radiotherapy – Towards single track spectroscopy

https://doi.org/10.1016/j.radmeas.2013.01.060Get rights and content

Abstract

Knowledge about the mechanisms linking physical energy deposition and biological response in ion cancer beam therapy and space research is still limited. We sought to develop a novel detector system based on fluorescent nuclear track detector (FNTD) covered with a viable cell-layer to gain spatial correlation between ion traversal and cellular damage on a μm scale. We investigated the possibility to gain, in addition to track location, information on ion type and energy from a single particle track by the FNTD read-out. To access both parameters we aimed to find surrogates for the local energy transfer (LET) of the incident ion and the maximum secondary electron range in the detector material as those quantities are related to the ion type and energy. We parametrized the intensity profile of the fluorescent signal of a single ion by the maximum intensity signal and full width half maximum (FWHM). The first seems to be a suitable surrogate for the LET of the incident particle whereas the latter is only a limited measure for the maximum secondary electron range in detector material.

Highlights

► We parameterized the intensity profile of a single ion gained by the FNTD read-out. ► Maximum intensity signal of acquired FNTD images seems to be a surrogate for LET. ► FWHM of intensity profile is a limited measure for the maximum sec. electron range. ► FNTD were read out by STED microscopy with a resolution beyond diffraction limit.

Introduction

To understand the fundamental molecular mechanisms of ion radiotherapy, colocalisation studies of energy deposition and cellular response are of essential importance. A lack of sensitive biocompatible detectors providing sufficient information on physical beam parameters and biological damage hampers these studies. We are therefore developing a novel cell/fluoresecent-crystal hybrid-detector. It is based on a biocompatible, fluorescent nuclear track detector (FNTD, Fig. 1A) (Akselrod and Sykora, 2011) and a cell layer on the inert detector surface. It enables a co-detection of the tracks of traversing ions and the subsequent biological damage in the cell layer after particle irradiation on a μm scale (Fig. 1B, unpublished results). The detector and the cell layer in situ are read-out by the very same commercial confocal laser scanning microscope (CLSM).

Here we present a first investigation on the feasibility to extract, in addition to track location and direction, information on ion type and energy from a single particle track. To obtain this information, simultaneous measurements of local energy transfer (LET) and the maximum range of the secondary electrons in the detector material seem to be most reasonable, as those quantities are related to the ion type and energy via the shape of the radial dose distribution.

In order to assess both LET and maximum range from acquired FNTD images we investigated in this paper the suitability of the following parameters: maximum intensity signal, full width half maximum (FWHM) and background of the intensity profile of the fluorescence signal in an ion track by a) direct measurement and b) by fitting a two-dimensional Gauss-curve to the track intensity data and extracting its parameters.

Section snippets

Al2O3:C,Mg based FNTDs

FNTD alumina crystal lattice is doped with magnesium and carbon ions creating fluorescent color centers F22+(2Mg) (Akselrod and Sykora, 2011). By exposing the detector to ionizing radiation these aggregate defects undergo radiochromic transformation. The altered color centers absorb light in the band centered at 620 nm, prompting fast 750 nm fluorescence. F22+(2Mg) color centers can be excited at 435 nm optical absorption band and emit fluorescent light at 515 nm. FNTDs are sensitive for ions

Direct measurement: ηˆ and FWHM

In Fig. 3 the mean maximum count-rate ηˆactual is correlated to the LET of the incident ion. In general the count-rate is increasing with rising LET – it follows an approximately exponential function (Sykora et al., 2008). Insert B shows the mean ηˆdetected, sorted in ascending order. Above 10 MHz significant saturation effects occur (Greilich et al., in this issue), leading to great discrepancy between ηdetected and ηactual. The detected and actual rates for Fe-56, Kr-84 and Xe-131 ions have

Discussion

Both maximum count-rate ηˆ and Gaussian peak amplitude Aˆ are rising with increasing LET of the incident particle (Figs. 3 and 5A). Due to a different read-out depth of 10 μm and 30 μm the actual ηˆ (and Aˆ)-LET dependency could differ slightly especially for the low energy particles. Within this depth the incident particles are interacting with the alumina and therefore loosing kinetic energy. Also, color center concentration inhomogeneity between FNTD crystals can cause small variations in ηˆ

Conclusion

We characterized the single ion's signature by its maximum fluorescence intensity (characterized by the APD count rate) and the track size (characterized by FWHM of the Gaussian fit of the track intensity distribution) to obtain a quantitative dependence for measuring ion's LET as well as maximum secondary electron range and hence retrieve information on ion type and energy. The maximum count-rate, whether detected or obtained by Gaussian fit seems to be a suitable parameter to measure the LET

Acknowledgments

The author would like to thank S. Brons for generously providing support and technical irradiation assistance at the Ion-Beam Therapy Center of Heidelberg University Hospital. We would also like to thank F. Bestvater and M. Brom of the DKFZ's light microscopy core facility for their enthusiasm and unflagging support and acknowledge B. Hartmann for her involvement in scientific discussions. We would like to thank M. Heinrich and J. Engelhardt for the read-out by STED microscopy. M.N. is funded

References (8)

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