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 Table of Contents  
REVIEW ARTICLE
Year : 2017  |  Volume : 13  |  Issue : 3  |  Page : 419-424

Optical computed tomography in PRESAGE® three-dimensional dosimetry: Challenges and prospective


1 Department of Medical Physics and Biomedical Engineering, School of Medicine, Tehran University of Medical Sciences, Tehran; Department of Radiology, Paramedical Science Faculty, Tabriz University of Medical Sciences, Tabriz, Iran
2 Department of Radiology, Paramedical Science Faculty, Tabriz University of Medical Sciences, Tabriz; Department of Optics and Photonics, Laser and Optics Research School, Nuclear Science and Technology Research Institute, Tehran, Iran
3 Department of Medical Physics and Biomedical Engineering, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran
4 Department of Optics and Photonics, Laser and Optics Research School, Nuclear Science and Technology Research Institute, Tehran, Iran
5 Department of Engineering, Science and Research Branch, Islamic Azad University Tehran, Tehran, Iran
6 Laser and Plasma Research Institute, Shahid Beheshty University, Tehran, Iran
7 Peter Mac Callum Cancer Centre, Melbourne, Australia

Date of Web Publication31-Aug-2017

Correspondence Address:
Hassan Ali Nedaie
Department of Radiation Oncology and Radiobiology Research Centre, Cancer Institute, and Department of Medical Physics and Biomedical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.202895

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 > Abstract 

With the advent of new complex but precise radiotherapy techniques, the demands for an accurate, feasible three-dimensional (3D) dosimetry system have been increased. A 3D dosimeter system generally should not only have accurate and precise results but should also feasible, inexpensive, and time consuming. Recently, one of the new candidates for 3D dosimetry is optical computed tomography (CT) with a radiochromic dosimeter such as PRESAGE®. Several generations of optical CT have been developed since the 90s. At the same time, a large attempt has been also done to introduce the robust dosimeters that compatible with optical CT scanners. In 2004, PRESAGE® dosimeter as a new radiochromic solid plastic dosimeters was introduced. In this decade, a large number of efforts have been carried out to enhance optical scanning methods. This article attempts to review and reflect on the results of these investigations.

Keywords: Gel dosimetry, optical computed tomography, PRESAGE®, three-dimensional dosimetry


How to cite this article:
Khezerloo D, Nedaie HA, Farhood B, Zirak A, Takavar A, Banaee N, Ahmadalidokht I, Kron T. Optical computed tomography in PRESAGE® three-dimensional dosimetry: Challenges and prospective. J Can Res Ther 2017;13:419-24

How to cite this URL:
Khezerloo D, Nedaie HA, Farhood B, Zirak A, Takavar A, Banaee N, Ahmadalidokht I, Kron T. Optical computed tomography in PRESAGE® three-dimensional dosimetry: Challenges and prospective. J Can Res Ther [serial online] 2017 [cited 2020 Jun 4];13:419-24. Available from: http://www.cancerjournal.net/text.asp?2017/13/3/419/202895


 > Introduction Top


With the advent of new radiotherapy techniques such as intensity modulated radiotherapy (IMRT), stereotactic radiosurgery, and volumetric modulated arc therapy (VMAT), the demand for an accurate, feasible three-dimensional (3D) dosimetry system has been increased. Recently, a new generation of plastic-based dosimeter was introduced which demonstrate a radiochromic response upon to ionizing radiation known as PRESAGE®.[1] Some special features of this solid dosimeter make it an attractive candidate to be used as a robust 3D dosimeter. It can be fabricated in any desired shape without any container. Diffusion is one of the disadvantages of ferric and some early polymer gels that cause varying of spatial resolution of dosimeter over the time; however, this problem in PRESAGE® is omitted. In megavoltage energies, PRESAGE® is also tissue equivalent; its response is independent of the wide range of energies, dose, dose rate, the room temperature, and oxygen.[1],[2],[3],[4],[5],[6],[7]

One of the major challenges in 3D dosimetry is scanning method of gels. However, magnetic resonance imaging is complex method widely used for ferric and polymer gels, but quantitative analysis is affected by a lot of inaccuracies.[8] The other advantage of PRESAGE® is optical tomographic scanning technique. Many efforts have been conducted to improve performance of the optical scanning systems to be used in the clinics, and therefore, various generations of optical computed tomography (CT) were introduced.[9],[10] In this study, first, PRESAGE® as a suitable dosimeter for optical scanning is evaluated and then principles of optical scanning and its historical are presented.


 > Why Presage® is a Potent Dosimeter for Optical Computed Tomography? Top


Structure and properties

PRESAGE® is a radiochromic plastic-based dosimeter. The main component of PRESAGE® dosimeter consists of polyurethane which used in medical equipment, construction of coating equipment, and adhesive. Three specific characteristics of polyurethanes that make it ideal for radiochromic-based dosimetry are its solid form, clear texture, and more importantly polymerization at a relatively low temperature (<80°C) which decrease thermal oxidation reactions in leuco dye. The polyurethane chemical formula that commonly used in previous studies consist of 20% oxygen, 61% carbon, 9% hydrogen, and 10% nitrogen, with Zeff= 6.6 and density of 1.05 g/cm.[11],[12],[13]

The radiochromic parts of PRESAGE® are a leuco dye and a free radical initiator (RI). For the radiochromic leuco dye, one of the derivatives of triphenyl methane, leucomalachite green (LMG), is more widely used. Upon irradiation, halocarbon radiolysis starts producing of free radicals and oxidizing LMG into the malachite green (MG). [Figure 1] shows the chemical formula of LMG and MG. Various kinds of organic peroxides, halocarbons, azo, and carbonyl and sulfur components can be used as a RI. Halocarbons such as chloroform, carbon tetrachloride, and methylene chloride can cause the oxidation of the leuco dye in water systems.[1],[2],[6],[11],[12],[14],[15],[16],[17] In summary, to design the optimal leuco dye and RIs concentration, four factors should be considered: stability during fabrication, overall radiation sensitivity, and pre- and post-irradiation dosimeter stability, so, by varying the amount and type of leuco dyes, RI, and probably catalyst, it is possible to fabricate the PRESAGE® with different dosimetric properties.[11],[12],[18]
Figure 1: Chemical formula of the radiochromic response. Reproduced with permission from Journal of Physics: Conference Series[1]

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Optical consideration of PRESAGE®

Because of PRESAGE®, maximum absorption wavelength is in the ranges of 625–635 nm; therefore, the light source for optical scanning system is red light such as helium–neon laser or red light-emitting diode (LED). Light absorption depends on the wavelengths of source, so care should be taken when using of calibration results that obtain from different wavelengths. To reduce this error, ideally the wavelength of light source of scanner should be the same as the spectrophotometer that used for calibration.[19],[20],[21] Another solution is to use a narrow bandwidth filter of 632 ± 5 nm which limited the bandwidth of light frequency; however, in this way, the noise of system and schlieren strings will be increased. Theoretical correction factor to reduce this challenge also was used by Thomas et al.[21]

Stability of the intensity of LED source is another factor affecting the accuracy and reproducibility of the optical measurements. Intensity of LEDs strongly depends on temperature; therefore, after turning on its intensity is highly reduced with time.[22] In the optical CT scanning method, the intensity instability of LED generates more noise and overestimates the dose near the internal region of PRESAGE®, so it is better to turn the source on before starting of scan to reduce the instability of source intensity. To reduce the instability of intensity, depending on the type of LED, it is recommended that the source turn on between 45 min to 2 h before scan.[23] Begg et al. showed that intensity drop for red light (633 nm) was lower than the orange light (590 nm). After turning on a certain LED, in the first 5 min, the intensity reduced to 3.6% and 13.3% for red and orange light, respectively, and in the first 2 h reduction was 9% and 29% for red and orange light, respectively. After 2 h, the intensity change was very little, it was 0.016% and 0.085% for red and orange light, respectively.[23]

A special characteristic of PRESAGE® is its high absorption proportion in comparison of scatter property.[3],[24],[25],[26],[27],[28],[29] Light absorption and scattering pattern depend on particle size inside the gel and wavelength of source. If the particles size is smaller than light wavelength, probability of scattering in low wavelengths, 380–495 nm (violet to blue) increases (Rayleigh scattering). However, if the particle sizes are approximately as same as light wavelength, probability of scattering in all visible wavelengths will be almost equal (Mie scattering). Therefore, particle size for red light scattering should be equal or larger than 600 nm, whereas particle size of PRESAGE® components even after oxidation of LMG is smaller than 600 nm, so probability of red scattering will be smaller than blue or violet wavelength.[30] Scattering behavior of PRESAGE® and BANG gel was compared by a 1 mm diameter laser beam. [Figure 2] illustrates the comparison of the intensity distributions from BANG, PRESAGE®, and water as the matching liquid. The scattering from the central axis to 2 mm for both of them were about 1% of the primary beam intensity, but beyond 2 mm, BANG gel scattering was about twice larger than PRESAGE®. In other words, BANG gel can be assumed as a scattered media, and PRESAGE® as an absorbent.[25],[31] Hence, the useful solution in reducing the effects of scattering is the reduction of the particle concentration in gel; however, another way is the use of algorithms to reduce the scattering. The scatter is very problematic in optical CT with the broad source such as LED and wide detectors such as charge-coupled device (CCD), but with the use of telecentric lens arrangement where light source into the object becomes parallel, and after passes through the object, focuses into the CCD, scattering effect can be reduced greatly. In addition, these lenses decrease aberration when light is focused on the CCD sensitive small area. However, in the formation of an image, both absorption and scattered light contribute, but the scattered photon distorts resolution and serves as the origin of noise in optical CT; however, in diffused optical tomography (DOT), the basis of image formation is scattered light although the reconstruction algorithms are very complex.[32],[33],[34],[35]
Figure 2: Comparison of the scattered light intensity distributions from the BANG gel dosimeter, the PRESAGE® dosimeter, and the matching liquid. Reproduced with permission from physics in medicine and biology[29]

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Different refraction and reflection of different material in the light path are the origins of artifacts in optical scanning.[9],[35],[36] The problem becomes more complex when the incident light is not perpendicular to PRESAGE® surface. According to the refractive index of the objects, light diverts from the primary path, and the optical data of a point in PRESAGE® matrix records on the wrong points in the image matrix. To reduce these effects and artifacts, and also to increase the signal-to-noise ratio and dynamic range, PRESAGE® should be placed into a liquid with a refractive index close to the its refractive index. PRESAGE® refractive index is approximately 1.5. In some studies, a combination of methyl salicylate and ethyl salicylate or glycerin and blue dye were used.[9],[36] However, such matching layer is essentially high viscose liquids. Mixing and filtering these liquids are very difficult. Furthermore, the cleaning of these oil-based liquids from aquarium is inconvenient and can cause degradation aquarium walls. It is also difficult to remove air bubbles and particulate impurities inside liquid. Moreover, during dosimeter rotation inside the aquarium, irregular motion of liquid around dosimeter causes local refractive index variation. This local variation creates schlieren band pattern around peripheral region of the image of dosimeter. To overcome this problem, in a study, water (n = 1.33) used as matching layer in cone beam optical CT. Then, two-dimensional (2D) trajectory pattern of a fiducial marker that attached at surface of gel obtained. This sonogram-like pattern used to correct mathematically peripheral rays that refract.[37] Schematic of fiducial-based ray path measurement technique is illustrated in [Figure 3]. Ramm et al. showed that the matching layer is effective when the dose distribution information around the edges of the PRESAGE is considered, but if the information of peripheral dose distribution be considered, matching layer is more effective. Any type of impurities with different refractive index and scratch in the wall of aquarium will reflect light, so it can be a source of star artifacts. However, PRESAGE® is plastic base so does not need holding container, therefore less problems of these artifacts are encountered in the edge.[9] [Figure 4] illustrates the principles behind the generation of schlieren effects.
Figure 3: Schematic of fiducial-based ray path measurement technique. (a) ray index of fiducial marker in θexit, (b) ray index of fiducial marker in θentry, (c) sinogram of fiducial marker. It should be noted that despite of the ray path deviation in θexit and θentry are equal, but ray index of them are different in sinogram pattern. Reproduced with permission from medical physics[37]

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Figure 4: (a) Illustration of the principle behind the generation of schlieren effects; (b) an extreme example of schlieren and other effects that degrade the quality of projection images. The dark region in the center is a brachytherapy irradiation. Reproduced with permission from Journal of Physics: Conference Series[10]

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The range of optical density or color change of irradiated PRESAGE® should be inside the dynamic range of optical CT system; otherwise, system estimates incorrect information of dose. “Dynamic range indicates the ability of an optical system to detect the maximum and minimum range of brightness.” In a comprehensive view, the dynamic range of a scanning system depends on the intrinsic PRESAGE® properties, detectors type, and image reconstruction methods.[36],[38]


 > Fundamental Principles in Optical Computed Tomography Top


One of the special characteristics of PRESAGE® in 3D dosimetry is its simple and feasible optical scanning method. The idea of using optical techniques such as spectrophotometry in medical dosimetry arose from the fact that irradiation can change optical density of some 3D gel dosimeters. Physical principle of optical imaging methods consists of Beer–Lambert law and Radon transform. According to Beer–Lambert law, there is a linear relationship between absorbance and concentration of an absorbing species or optical attenuation coefficient μ (x) of sample:





If the initial intensity is I0 and after transferring the line L the intensity at the detector is I, then:





Hence, with the line integrals of μ (x) obtaining from detectors, Radon transforms establish tomographic images.[38]

For the first time in 1996, the optical CT scan was used to scan of BANG polymer gels. After that, a variety of optical CT methods has been developed.[31],[39]

The first generation of optical CT that designed by Gore et al. was a single laser beam with a photodiode that scan the gel both rotational and translational movement. Although this generation is the golden standard, however, the main disadvantage is that the process of scan is time consuming. By this method, for instance, the scanning of each slice of a gel with 5 cm diameter takes about 12 min, and the whole gel scanning lasts for 4 h.[24],[40] [Figure 5] illustrates the schematic diagram of the different types of optical CT scanners. Several groups had focused extensively to improve designs for high-speed scanners. New designs proposed by Xu et al., van Doom et al., Conklin et al., and Krstajic and Doran with several movement mirrors to scan the gel.[19],[20],[35],[41],[42]
Figure 5: Schematic diagram of the different types of optical computed tomography scanners (a) first-generation laser system (Gore et al.); (b) fast laser scanner (Krstajic et al.); (c) cone-beam charge-coupled device scanner (Wolodzko et al.); (d) parallel-beam charge-coupled device scanner (Krstajic et al.). Reproduced with permission from Journal of Physics: Conference Series[10]

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The second generation based on the design of fast and accurate scanners, so broad cone laser beam with 2D array detectors such as CCD or complementary metal-oxide semiconductor introduced.[22],[43] Another fast type of scanner was provided by Doran et al. with a broad parallel beam with telecentric lens to reduce the effects of scattered radiation.[3] Despite the poor design of the cone-beam technique, one of its main advantages is the ability of scan of big samples without the need for expensive optical components. However, in the parallel broad beam with telecentric lens, scatter problem is manageable than in the cone mode scanner.[3],[22],[25],[26],[36],[41] However, no comprehensive studies have ever been focused, yet to compare these two conical and parallel techniques.


 > Clinical Use of Optical Computed Tomography Top


Feasibility and validation of PRESAGE® with optical CT scanner as a 3D dosimetry method were assessed with some radiotherapy techniques such as 3D-conformal radiation therapy (CRT), IMRT, and VMAT by several researches. The comparison results with film, treatment planning system, and thermoluminescent dosimeter showed that there are good agreements with 3 mm -3% gamma pass criteria.[44],[45],[46],[47],[48],[49],[50]

A parallel wide optical CT system recently has commissioned and benchmarked to clinical use in complex radiation treatments. This scanner consists of a matched telecentric source and imaging lens with 24 cm field of view and a 2/3” CCD array. The commissioning involved determining the dynamic range, spatial resolution, noise, temporal, and benchmarking test and consisted of delivering simple radiation treatments to PRESAGE® dosimeters, and comparing the measured 3D relative dose distributions with treatment planning system.

The spatial resolution was submillimeter modulation transfer function >0.5 for frequencies of 1.5 lp/mm and the dynamic range was 60 dB. Noise was low at <2% of maximum dose (4–12 Gy) for 2 mm reconstructions size. For comparison, normalized dose distribution (NDD) which has same concepts as gamma index was used. The mean 3D passing NDD rate 3% - 3 mm was 97.9 and 2% - 2 mm was 86%. This study was the most comprehensive step to the commissioning and use of wide parallel optical CT in clinics.[36] In a study, different techniques, i.e., 3D-CRT, IMRT, and VMAT in head and neck Radiologycal Physics Cennter (RPC) phantom were verified by PRESAGE®. For 3D-CRT, 96% of data, for IMRT 98%, and for VMAT 99% of data passed in 5% dose difference (DD) - 3 mm distance to agreement (DTA) gamma index criteria. Because of low maximum dose region in planning target volume (PTV) was create in VMAT, it illustrated better gamma results than others.[48] However, in another study, different results have been obtained, with gamma index criteria of 3% DD and 3 mm DTA, absolute measured dose were 99.6% and 94.5% for the IMRT and VMAT treatments, respectively.[49] In another study, VMAT-flattening filter-free, IMRT, and stereotactic body radiation therapy (SBRT) treatment techniques were verified with PRESAGE®. In SBRT, IMRT, and VMAT showed 94%, 96%, and 96.4% pass rate, respectively, with gamma index criteria of 3 mm DTA and 3% DD. Nevertheless, in all previous studies, a few millimeters (8–4 mm) of peripheral region of images of PRESAGE®, in which suffer from reconstruction due to mismatched refractive index (overestimated dose) was excluded from analysis.[27]

The ability of using the PRESAGE®-optical CT system for dosimetry of brachytherapy sources was also demonstrated recently. The results concluded that PRESAGE® optical CT system was suitable for relative dose measurement of brachytherapy sources. As previously mentioned, PRESAGE® with low halogen concentration shows more water equivalent properties, so it can show a good agreement with the results of treatment planning system dose distribution and Monte Carlo simulation. However, in high dose rate source, it is still necessary to investigate the heat-induced degeneration of leuco dye in PRESAGE®. It should be noted that for brachytherapy, high dynamic range scanning is required too. The reconstruction problem due to mismatched refraction index of PRESAGE® and hole to placement of source inside PRESAGE® that overestimate dose around peripheral region the hole should be considered.[51],[52],[53],[54]


 > Conclusion Top


Over the years, great steps have been undertaken to facilitate the usage of PRESAGE® in clinics. On the whole, previous results of studies have stated that PRESAGE® possess many advantages to introduce as a radiation dosimeter, especially in complex radiotherapy techniques, while it also has the ability to measure low energy ranges. An important salient point that should be considered is the energy range and type of radiation, sensitivity, and stability that is desired for appropriate combination.

Another way to improve the use of PRESAGE® is the possibility of utilizing of new methods of image processing to extract more information from the inside of it and produce images with better resolution. In this way, especially, the analysis of scattered photon such as DOT methods could be used. Up to date, all studies have focused on details of light absorption information of PRESAGE®, while the scattered light from the dosimeter could be a source of information to create image too. In principle, DOT is an optical CT imaging that is based on light scatter even from opaque media and tissues.

In the field of improvement of scan and image acquisition, peripheral overestimate of dose that rise from the variation of refractive index should be improved. Furthermore, the use of oil-based matching index is not easily feasible has some challenges, so scan without matching layer should be investigate.

Nowadays, radiation therapy which centers around the world fully equipped with modern radiotherapy machines which are able to implement techniques such as IMRT and VMAT suffers from the lack of a monolith standard 3D dosimetry protocol for treatment verifications. Many of the existing protocols are complicated and time consuming, and each radiation department employs a specific protocol corresponding to their own available hardware and software. In spite of robustness of PRESAGE® in 3D dosimetry, there is no monolith approach in fabrication process, storing condition, and readout techniques. Therefore, the establishment of a protocol by international competent authorities facilitates the application of PRESAGE® in clinics as a routine verification method.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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