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ORIGINAL ARTICLE
Year : 2019  |  Volume : 15  |  Issue : 8  |  Page : 97-102

Water equivalent radiological properties of Gafchromic external beam therapy and external beam therapy 2 film dosimeters


Department of Physics, School of Advanced Science, VIT University, Vellore, Tamil Nadu, India

Date of Web Publication22-Mar-2019

Correspondence Address:
Mr. K Srinivasan
Department of Physics, School of Advanced Sciences, VIT University, Vellore, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jcrt.JCRT_965_16

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 > Abstract 


Background: Water equivalent property of any clinical dosimeter is important. Water has the approximately similar radiation absorption and scattering properties to soft tissue. Film dosimeter plays a significant role in radiotherapy quality assurance and treatment plan verification.
Aims: In this study, we are evaluating the water equivalent radiological properties of Gafchromic electronic benefit transfer (EBT) and EBT2 film dosimeters.
Materials and Methods: Radiological properties such as number of electrons per gram (ne), electron density (ϼe), and effective atomic number (Zeff) are calculated using Mayneord formula. Mixture rule is used to calculate the mass absorption coefficient (μen/ϼ) and mass attenuation coefficient (μ/ϼ), and data are generated using Win-XCom over 10 KeV to 20 MeV. Electron stopping power data are generated with the help of ESTAR database over 10 KeV to 30 MeV. Those results are compared with water and deviations are found.
Results: Our results suggest that Zeff, ne, and ϼe of EBT is showing deviations <8.83%, 4.39%, and 16.18% and for EBT2 is 4.26%, 2.82%, and 19.41% with respect to water. Deviation in μen/ϼ and μ/ϼ of EBT and EBT2 film is ≤5% and ≤6%, respectively, with respect to water >100 KeV. Electron stopping power properties are also close in agreements with water having deviations ≤5%.
Conclusion: Presence of high atomic number element chlorine, potassium, and bromine may disturb the water equivalent properties in the lower energy range <100 KeV and similarly enhance the dose sensitivity because of the strong photoelectric absorption process.

Keywords: Effective atomic number, film dosimeter, mass attenuation coefficient, water equivalency


How to cite this article:
Srinivasan K, Jabaseelan Samuel E J. Water equivalent radiological properties of Gafchromic external beam therapy and external beam therapy 2 film dosimeters. J Can Res Ther 2019;15, Suppl S1:97-102

How to cite this URL:
Srinivasan K, Jabaseelan Samuel E J. Water equivalent radiological properties of Gafchromic external beam therapy and external beam therapy 2 film dosimeters. J Can Res Ther [serial online] 2019 [cited 2019 Nov 23];15:97-102. Available from: http://www.cancerjournal.net/text.asp?2019/15/8/97/243506




 > Introduction Top


Modern radiotherapy techniques such as intensity modulated radiation therapy, sterotactic radiosurgery, and stereotactic radiotherapy deliver very highly conformal radiation dose to the tumor.[1] Use of Linac-based radiosurgery has significantly increased, which delivers narrowly collimated radiation beams with the field size smaller than 4 cm × 4 cm to the target volume. The aims of these advanced techniques are delivering the high dose to the tumor and fewer doses to the normal tissue, with an accuracy of ±5% level.[2] In this sense, to know the planned radiation dose distribution before applied to the patient is a challenging task. A dosimetric technique plays an important role in advanced radiotherapy practices and also is very complicated to perform due to steep dose gradient and lateral electronic disequilibrium. In general, there are 2 types of tissue interfaces which are important in radiotherapy such as heterogeneity/soft tissue and bone/soft tissue. These interfaces create the perturbation which leads to deceptive dose localization in the target.[3] Radiochromic films have been successfully used for the determination of interface effect which was important consideration when treating the head-and-neck site. Dosimetric techniques have various dimensional tools to measure the radiation dose distribution, for example, one dimensional (1D) (ion chamber, diode), 2D (film dosimetry), and 3D (gel dosimeter).[4] The ionization chamber has good accuracy for absolute dose measurement, but it is not correct choice when measuring the radiation dose from steep dose gradient, electronic disequilibrium condition, and volume of detector also affect the accuracy.[5] Fricke-gel, free air ion chamber, and colorimeter are currently used absolute dosimeters. Radiation dose responses of those absolute dosimeters are chemical extinction in Fricke gel, charge collection in free air ion chamber, and temperature changes in the colorimeter. The Gafchromic film is the 2D relative dosimetry which has been made using specialty chemicals capable of pixel-by-pixel dose measurement.[6] It is better compared with conventional silver halide radiographic film because of the number of advantages such as self-developing, near tissue equivalence, low fading, high spatial resolution, and weak energy dependence.[7] Radiochromic film is light blue in color with transparent in nature before irradiation. Postirradiation it changes its color to dark blue due to the polymerization process.[8] Radiation absorbed dose is directly proportional to the optical density or degree of darkness. Dose rate dependence of the film is less in real-time measurement. The dose range of the film mainly depends on the thickness of the sensitive layers, if the layer is thick sensitive should be high.[9] The radiation absorption and scattering properties of radiochromic dosimetric films are similar to water.[10] Tissue equivalent materials play the vital role in diagnostic radiology, radiotherapy, radiation biology, tissue engineering, space research, etc.[11],[12],[13] Radiotherapy code practice Technical report series (TRS)-398 recommends that water-or-water equivalent materials may be used as a phantom for determining the absorbed dose.[14] Water (tissue) equivalency is much important in clinical dosimetry which is used to avoid the perturbation correction factor and also absorbed dose is similar to the soft tissue. Water is a tissue equivalent medium, which is used to know the photon characteristics in clinics.[12] Tissue equivalent materials are used to mimic the human tissue or organ and many of the researchers studied the tissue/water equivalent properties of the materials and radiation detector used in radiotherapy practice based on their radiological properties, for gel dosimeter,[12] thermoluminescence compound,[13] waxes, plastics,[15] abiological tissue,[13],[15] etc. In this study, we will elaborate the water equivalent properties of electronic benefit transfer (EBT) and EBT2 radiochromic film dosimeters in terms of their radiological properties such as effective atomic number (Zeff), electron density (ϼe), number of electrons per gram (ne), mass absorption coefficient (μen/ϼ), mass attenuation coefficient (μ/ϼ), and electron total stopping power properties.


 > Materials and Methods Top


Radiation interaction probabilities such as photoelectric effect, pair production, and Compton effects depend on the atomic number and ϼe of the materials.[12] Mass percentage of the element present in each layer of Gafchromic EBT and EBT2 film is calculated from the published data.[16] Single Z number cannot use to describe the atomic number for the multielement materials; hence, Zeff is introduced in terms of its equivalent elements.[17] Mayneord formula is used to calculate the Zeff.[18] μ/ϼ is a parameter used to determine the penetration ability of the X-rays. It is defined as the linear attenuation coefficient divided by the density of the materials. Photon μen/ρ and μ/ρ are calculated using mixture rule, and data are generated through NIST X-ray attenuation databases over 10 KeV to 20 MeV.[19] For electron energies >10 KeV, Bothe theory provides a good prediction about electron stopping power in a solid. In this method, Bothe theory is used to calculate the stopping power of electron in the energy range of above 10 KeV to 30 KeV clinical range.[20] Collisional and radiative stopping powers are generated using ESTAR database.[21]

Effective atomic number Zeff= (a1Z12.94 + a2Z22.94 + a3Z32.94 +…………+anZn2.94)1/2.94

Where a1, a2, a3..... an and Z1, Z2, Z3..... Zn are the fractional contribution of each element to the total number of electrons in the mixture and its atomic number, respectively.

Number of electrons per gram (e/g) = NA ∑i(Zi/Awi) × Wi)

Where, NA,Zi, and Awi are the Avogadro's number, atomic number, and atomic weight of the ith element, respectively.

Electron density ϼe= ϼ NA ∑iWi(Zi/Ai)

Where, ϼe-is the electron density.

Mass energy absorption coefficient (μen/ρ)film= ∑iWien/ρ)i

Mass energy attenuation coefficient (μ/ρ)film= ∑iWi(μ/ρ)i

Where, Wi, (μen/ρ)i, and (μ/ρ)i are the atomic weight fraction, mass energy absorption coefficient, and μ/ϼ of the ith element.

Total Stopping power (S/ϼ)tot= (S/ϼ)col+ (S/ϼ)rad where,

(S/ϼ)col – is the collision stopping power and

(S/ϼ)rad– is the radiative stopping power


 > Result and Discussion Top


Effective atomic number, number of electrons per gram, and electron density

[Table 1] shows the Effective atomic number (Zeff), the ne, and ϼe of the EBT and EBT2 film layers. To know the reliability of our methods, we performed the calculation for water Zeff= 7.416 which is in close agreement with 7.42.[18] Our results are compared with previously published data and variation was observed within <3% which may be due to different methods of calculation[16] Vishwanath et al. proposed four different methods for the calculation of Zeff and results suggest that variation was found among the methods, and its depends on the photon energy and methods of calculation.[22] Our results were little higher compared with Bekerat et al. He found Zeff of EBT (lot EBT-48022-07I) film active layer (Zeff= 6.98), surface layer (Zeff= 7.99) and EBT2 (lot EBT2-A08161006) film active layer (Zeff= 9.38), an adhesive layer (Zeff= 6.26).[23] It may be due to the higher elemental mass percentages. For example, in our case, EBT film[16] consists of chlorine (Cl) = 11.54% (active layer) and Cl = 63.7% (surface layer) and EBT 2 film[16] contains bromine (Br) =38.5% and Cl = 35.3% (Active layer (lots-020609)) higher compared with Bekerat et al. studies.[23]
Table 1: Number of electrons per gram, electron density, and effective atomic number of the external beam therapy and external beam therapy 2 dosimetric film layers

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Stopping power properties

Electron beams are conventionally used to treat the superficial target. American Association of Physicists in Medicine recommended that film dosimeter is a good candidate for 2D electron dose measurement due to the high spatial resolution and permanent record of the film dosimeter. Su et al. found that electron energy dependency of the EBT film is ±4% over the energy range of 6–22 MeV and their results were in good agreement with our calculated values.[24]

The electron stopping power properties of EBT and EBT2 film are close to water (<5%) over the energy range of 10 KeV to 30 MeV. The ratio of stopping power properties of EBT and EBT2 film layers to water is shown in [Figure 1] and [Figure 2], respectively. The rate of electron energy loss depends on the ϼe of the target material, which will be higher for low Z material compared to high atomic number material. High atomic number materials have few electrons per gram and tightly bound electron then that of low Z material.[25] Electron beam is conventionally used in radiotherapy for the treatment of cancer with energy up to 22 MeV generated by the medical linear accelerator. It has some limitations such as deep-seated tumor treatment due to dose distribution of lateral and longitudinal plane are attenuated.
Figure 1: Stopping power ratio of the different layers of the external beam therapy film and water

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Figure 2: Stopping power ratios of the different layers of the external beam therapy 2 film and water

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Photon mass absorption coefficient

[Figure 3] and [Figure 4] show the mass energy absorption coefficient ratio of film layers with water. We found that, for energies, >100 KeV EBT and EBT2 film can be considered as water equivalent in terms of their absorption properties. Below <100 KeV significant variation was observed due to the alteration in the photon interaction probabilities. Strong absorbed dose energy dependence was observed over the energy range of 25 KeV to 4MV X-rays. It may be due to the alteration of mass energy absorption coefficient[7] which is directly proportional to the radiation absorbed dose. The radiochromic film contains high Z material for improving the dose sensitivity, which may destroy water equivalent properties at lower energy range. Actually, both EBT and EBT2 films consist of the active layer and a surface layer which contains moderate atomic number elements such as Cl (Z = 17), potassium (K, Z = 19), and Br (Z = 35). This is shown in [Table 2]. Bekerat et al. found that removal of bromine eliminated the over-response at 40 KeV, and the presence of chlorine makes the film more hygroscopic which may affect the stability of the readout.[23] The addition of aluminum in the active layer is improved the under response for energies ≤30 KeV. K-edge absorption value of Br is 13.5 KeV; hence, absorption has considerable increases about 90 at 40 KeV compared to water.[23] The photoelectric effect is dependent on the atomic number; hence, low energy photon readily interacts with high atomic number material.[26] EBT2 film contains yellow dye which decreases sensitive to ambient light.
Figure 3: Photon mass absorption coefficients ratio of the external beam therapy film layers and water

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Figure 4: Photon mass absorption coefficient ratios of external beam therapy 2 film layers and water

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Table 2: Elemental composition (percentage weight fraction) of each layer of the external beam therapy and external beam therapy 2 film dosimeters

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Photon mass attenuation coefficient

The ratio of photon attenuation properties of polyester layer of EBT to water is one over the photon energy range of 10 KeV to 20 MeV. Other layers such as active layer and surface layers show the deviation in lower energy range below 100 KeV but in close agreement with water for higher energy range from 0.1 to 20 MeV.

Active layer is much thicker than others and it contains high Z element, due to this photon is attenuated more predominantly through the photoelectric effect.[26] EBT2 film layers such as active 2, adhesive, and polyester are very close with respect to water and others such as active 1 and surface layer peaks are observed at lower energy range due to more attenuation and above 100 KeV; those layers are also close relative to water. [Figure 5] and [Figure 6] show these result. μen/ϼ and μ/ϼ may increase Temperature of the scanner glass plate increased while doing multiple scan with ultraviolet fluorescence light. It may create additional polymerization of film during the scanning, resulting in increase in optical density which leads to dosimetric inaccuracy.
Figure 5: Photon mass attenuation coefficient ratio of external beam therapy film layers and water

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Figure 6: Photon mass attenuation coefficient ratio of external beam therapy2 layers and water

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 > Conclusion Top


Water equivalent radiological properties of Gafchromic EBT and EBT2 films are verified. The presence of high Z element may disturb the water equivalent properties such as Cl (Z = 17), K (Z = 19), and Br (Z = 35). The Gafchromic film is easy to expand 2D into 3D reference dosimeter by keeping the dosimetric film between the water equivalent square slabs' phantom or human phantom (inhomogeneous), for example, Rando phantom. However, systematic studies are needed for comparing the precision between 3D film reading with one of the practically possible 3D dosimeters such as gel dosimetry system. Furthermore, there is a possibility to make water equivalent real 3D gel dosimeter using corresponding film dosimeter chemicals. It may be possible to readout the gel through optical and other methods (computed tomography, magnetic resonance imaging, etc.,) also.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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Singh VP, Badiger NM, Kucuk N. Assessment of methods for estimation of effective atomic numbers of common human organ and tissue substitutes: Waxes, plastics and polymers. Radio Prot 2014;49:115-21.  Back to cited text no. 15
    
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

  [Table 1], [Table 2]



 

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