Radiochromic Gel Research Articles

Sci-Fri PM: Planning-08: Zero diffusion radiochromic genipin-gelatin dosimeter.

To develop a zero diffusion radiochromic hydrogel by cross-linking gelatin polymers with genipin, for three-dimensional (3-D) radiation dosimetry. Gelatin and genipin were dissolved in distilled water and continuously stirred at 40C for thee hours and then sulfuric acid was acid prior to placing gel in containers. Concentrations of genipin, gelatin and sulfuric acid were varied to determine the optimal gel sensitivity for optical computed tomography (CT). Photon beams (4 MV x-rays, 1×1, 2×2, 3×3 cm) irradiated a 10 cm diameter cylinder of gel. Using a cone beam optical CT scanner at λ = 590 nm, zero diffusion was observed by recording sequential 3D scans separated by 18 hours. Additional evidence was provided by sequential transmission photographs of a cuvette containing a sample that had been exposed to a 0.8 mm diameter, 1 mW, 594 nm, He-Ne laser beam and photobleached. Radiochromic bleaching is a fast process, complete within seconds of termination of irradiation. Dose sensitivity increases with cross- linking density and sulfuric acid concentration. The initial optical density limits dose sensitivity, requiring approximately 30 Gy for 10 cm diameter samples to have reasonable contrast. More sensitive but darker gels require smaller samples. A comparison of profiles through 3D reconstructed dose distribution showed identical profiles for a time interval of 18 hours. Visually the sample maintained this image for six months when stored at 4C in a dark refrigerator. Transmission photographs of the laser beam image recorded by photobleaching remained constant when stored a 23 C in the dark for ten months. Radiochromic genipin gelatin gels form a permanent and stable image when protected from bright visible light. This result is a new, non-toxic material for 3D dosimetry.

Gel dosimetry measurements and Monte Carlo modeling for external radiotherapy photon beams: Comparison with a treatment planning system dose distribution

Gel dosimetry has proved to be useful to determine absorbed dose distributions in radiotherapy, as well as to validate treatment plans. Gel dosimetry allows dose imaging and is particularly helpful for non-uniform dose distribution measurements, as may occur when multiple-field irradiation techniques are employed. In this work, we report gel-dosimetry measurements and Monte Carlo (PENELOPE ®) calculations for the dose distribution inside a tissue-equivalent phantom exposed to a typical multiple-field irradiation. Irradiations were performed with a 10 MV photon beam from a Varian ® Clinac 18 accelerator. The employed dosimeters consisted of layers of Fricke Xylenol Orange radiochromic gel. The method for absorbed dose imaging was based on analysis of visible light transmittance, usually detected by means of a CCD camera. With the aim of finding a simple method for light transmittance image acquisition, a commercial flatbed-like scanner was employed. The experimental and simulated dose distributions have been compared with those calculated with a commercially available treatment planning system, showing a reasonable agreement.

Dosimetric properties of a radiochromic gel detector for diagnostic X-rays

The gel dosimetry method was found to be capable of addressing complicated issues related to dose measurements particularly in modern sophisticated radiotherapy applications. The Ferrous-sulphate Xylenol-orange and Gelatin (FXG) radiochromic gel dosemeter is one of the systems used for such applications. Some chemical dosemeters show different responses for low- and medium-energy X-rays in comparison with high-energy γ-photons. The energy and dose rate dependence of the FXG dose response was examined. In addition to the detector response, other important dosimetric properties of the system were investigated for different X-ray beam qualities with tube voltages in the range 100–300 kV. An orthovoltage X-ray therapy unit was used to irradiate standard sized samples of FXG from different batches for radiation doses in the range 0–20 Gy.

Behavior of oxidation in the radiochromic gel dosimeter through photoacoustic technique measurements

Natural oxidation is an undesirable process suffered by gel dosimetry systems as it changes the values and spatial distribution of absorbed doses. Natural oxidation of Fricke xylenol gel (FXG) dosimeter was inferred from optical absorbance values measured for the first time using the photoacoustic technique. An acoustic detector measures the intensity of the light transmitted by the FXG, which is directly proportional to the ionizing radiation dose absorbed by the samples. In this work, we demonstrated the behavior, the rates, and the dependence of natural oxidation on storage temperature, time, and active components through measurements by photoacoustic technique.

Small, medium and large optical cone beam CT

Optical cone beam CT is a versatile geometry for scanning low scatter objects, such as transparent, radiochromic gels and plastics. Stray light is a significant factor for this geometry and this paper investigates some techniques to compensate for its effects.

Advances in optical CT scanning for gel dosimetry

Optical computed tomography (CT) is physically similar to x-ray CT but is more versatile since many powerful light sources exist and optical elements such as mirrors, lenses, polarizers and efficient detectors are available. There are many potential forms of optical CT. Attenuation, fluorescence or scatter, polarization and refractive index spatial changes are all examples of optical CT. To date, optical CT for gel dosimetry has been limited to attenuation measurements that are the sum of scatter and absorption along defined lines. Polymerization gels turn white with absorbed dose and attenuation is due to scatter. Radiochromic gels also form a dose image due to changes in visible absorption.This short review concentrates on the papers published since the DOSGEL 2001 meeting and highlights experimental results and issues that are important for obtaining good quality input data for reconstruction. The format involves selected highlights from the papers and associated points from our experience with optical CT experimentation. The comments are intended to assist researchers unfamiliar with optical measurements to obtain high quality transmission data, a necessary step in quantitative gel dosimetry.

Scanning water phantom to optical laser CT scanner conversion kit

Optical computed tomography (CT) scanners have been developed for quantitative reading of dose images written into radiation sensitive gels. Laser equipped, single ray CT scanners, similar to the first generation x-ray CT scanner geometry, provide high quality data because of efficient scatter rejection. These systems are effective for quantitative transmission measurements with both polymerization and radiochromic gels. Widespread research into optical CT gel dosimetry is limited by the lack of readily available commercial instruments. This project demonstrates how scanning water phantoms, which are common equipment in radiotherapy institutions, can easily be modified to perform optical laser CT scanning.

High-resolution optical tomography for 3-D radiation dosimetry with radiochromic gels

Modern radiotherapy methods require sophisticated treatment planning and radiation dose delivery routines, which in turn demand stringent quality assurance. This paper describes recent advances in three-dimensional optical computed tomography (OCT) readout for radiation dosimetry in which a tissue-equivalent radiochromic gels is used to achieve sub-millimetre spatial resolution for (512)3 voxel arrays in readout times of about half an hour.

Dose response of ferrous-xylenol orange gels: the effects of gel substrate, gelation time and dose fractionation

Investigations of the dose dependent change in optical transmission, dose response, for radiochromic ferrous-xylenol orange-gelatin gels (FXG) 3D optical CT scanning has revealed that gelation time, temperature, and dose fractionation affect the dose response (Δμ/Δdose). Correction for these factors is important for developing a reproducible dosimeter that can be reliably calibrated and used clinically. The purpose of this report is to examine trends in dose response changes for the following parameters: gelation time-temperature, concentrations of ferrous ion and xylenol orange (XO), dose range and dose fractionation.

Effects of ambient temperature on the FXG radiochromic gels used for 3-D dosimetry

Environmental effects on the optical properties of a sensitive radiochromic gel dosemeter; in particular storage, irradiation and measurements temperature were studied. Knowledge of light temperature and other ambient effects help to optimise working conditions and minimize errors. A ferrous-sulphate dosemeter with xylenol orange ion indicator incorporated in a gelatin gel matrix (FXG) was prepared under normal working conditions, and the samples were then kept in closed storage area at different temperature ranging from 5°C up to the gel melting temperature about 35°C. The samples optical absorbance was then measured quantitatively using double beam spectrophotometry. There is a small and steady increase in the absorbance 0.3×10-3 /°C with increasing temperature until about 30°C when we observe a big jump in the gel absorbance. Finally, additional important behaviour of FXG material was noticed, that is the changes occurred under the influence of rising temperature are reversible which is different from the permanent radiation caused changes.

Development of a three-dimensional radiation dosimetry system

A radiochromic gel dosemeter in which ionizing radiation causes a color change in the visible wavelength region was produced. The roles of the dosemeter’s active components were quantified with special consideration given to their effect on the system sensitivity and stability. Optimal composition was found to be 0.5 mM ferrous sulfate, 0.1 mM xylenol orange, and 25 mM sulfuric acid. The dose response is linear in the range 0.1–30 Gy and the dosemeter sensitivity was ΔA=0.084±10% cm −1 per Gy , where A is the optical absorbance measured at a wavelength of 585 nm. An optical tomographyscanner with parallel light beam and charged coupled device detector was designed, built, and used to interrogate cylindrical samples of the dosemeter. The current version of the scanner can cope with samples up to 10 cm in diameter limited by the width of the light beam. A maximum of 512×512 pixels per slice images are obtainable with this scanner. Slice thickness as small as 0.14 mm was achieved that is much narrower than the value of about 6 mm achieved when using the MRI method of three-dimensional (3D) readout. The time to acquire a stack of 100 cross-sectional images with a resolution of 128×128 pixels per slice is about 20 min. The 3D imaging performance was tested with collimated x-ray beams, and the scanner high-resolution presentation demonstrated.

A CCD-based optical CT scanner for high-resolution 3D imaging of radiation dose distributions: equipment specifications, optical simulations and preliminary results

Methods based on magnetic resonance imaging for the measurement of three-dimensional distributions of radiation dose are highly developed. However, relatively little work has been done on optical computed tomography (OCT). This paper describes a new OCT scanner based on a broad beam light source and a two-dimensional charge-coupled device (CCD) detector. A number of key design features are discussed including the light source; the scanning tank, turntable and stepper motor control; the diffuser screen onto which images are projected and the detector. It is shown that the non-uniform pixel sensitivity of the low-cost CCD detector used and the granularity of the diffuser screen lead to a serious ring artefact in the reconstructed images. Methods are described for eliminating this. The problems arising from reflection and refraction at the walls of the gel container are explained. Optical ray-tracing simulations are presented for cylindrical containers with a variety of radii and verified experimentally. Small changes in the model parameters lead to large variations in the signal intensity observed in the projection data. The effect of imperfect containers on data quality is discussed and a method based on a ‘correction scan’ is shown to be successful in correcting many of the related image artefacts. The results of two tomography experiments are presented. In the first experiment, a radiochromic Fricke gel sample was exposed four times in different positions to a 100 kVp x-ray beam perpendicular to the plane of imaging. Images of absorbed dose with slice thickness of 140 μm were acquired, with ‘true’ in-plane resolution of 560 × 560 μm2 at the edge of the 72 mm field of view and correspondingly higher resolution at the centre. The nominal doses measured correlated well with the known exposure times. The second experiment demonstrated the well known phenomenon of diffusion in the dosemeter gels and yielded a value of (0.12 ± 0.02) mm2 s−1 for the diffusion coefficient of the xylenol orange/iron complex. Finally, the overall implications of the above findings for dosimetry using OCT are discussed.