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ORIGINAL ARTICLE
Year : 2015  |  Volume : 11  |  Issue : 4  |  Page : 775-779

Evaluation of dose perturbation at the interface of two different density medium using GAFCHROMIC film EBT2 and Monte Carlo code EGSnrc for Co-60 beam


1 Department of Radiotherapy, King George Medical University, Lucknow, India
2 Department of Radiotherapy, Jaypee Hospital, Noida, Uttar Pradesh, India
3 Medical Physicist, M.S. Patel Cancer Center, Shree Krishna Hospital and Research Centre, Karamsad, Gujarat, India
4 Associate Professor, Shri Ramswaroop Memorial University, Lucknow, Uttar Pradesh, India
5 Department of Radiotherapy, King George Medical University, Lucknow; Vice Chancellor, Hemwati Nandan Bahuguna University, Srinagar, Uttrakhand, India

Date of Web Publication15-Feb-2016

Correspondence Address:
Nirmal Kumar Painuly
Department of Radiotherapy, King George Medical University, Lucknow - 226 003, Uttar Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.147727

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 > Abstract 

Introduction: Accurate dosimetry at the interface of two different density medium (e.g., air cavity in the head and neck cancers and lungs in thoracic region) is a major cause of concern in external beam radiation therapy. It has been observed that there is dose variation in and around air cavities, which occur as a result of the loss of both longitudinal and lateral electronic equilibrium. Heterogeneous structures with spatial differences in functionality and sensitivity for radiation pose challenge to radiation dosimetry. This study is an attempt to evaluate the dose perturbations produced at the interface of two medium for C0-60 gamma radiation.
Materials and Methods: Low density polyethene foam has been used to mimic air cavity. GAFCHROMIC EBT2 dosimetry film was used for the measurement of dose at different locations. Simulation studies were performed using DOSRZnrc user code that comes with EGSnrc V4 2.4.0. Cylindrical geometry is used for all the simulations.
Results and Discussion: We observed significant variation in dose for smaller fields. There is a dose build down in the backward region and a dose build up in the forward direction. In the region of electronic disequilibrium, dose reduction near interface (proximal end) will have negative impact if target region is embedded there, on the contrary, it would be beneficial if there is normal tissue/critical organ adjacent to it.

Keywords: Film dosimetry, inhomogeneity, Monte Carlo simulation


How to cite this article:
Painuly NK, Singh N, Verma TR, Chairmadurai A, Chaudhari LN, Saily A, Pant MC. Evaluation of dose perturbation at the interface of two different density medium using GAFCHROMIC film EBT2 and Monte Carlo code EGSnrc for Co-60 beam. J Can Res Ther 2015;11:775-9

How to cite this URL:
Painuly NK, Singh N, Verma TR, Chairmadurai A, Chaudhari LN, Saily A, Pant MC. Evaluation of dose perturbation at the interface of two different density medium using GAFCHROMIC film EBT2 and Monte Carlo code EGSnrc for Co-60 beam. J Can Res Ther [serial online] 2015 [cited 2019 Sep 20];11:775-9. Available from: http://www.cancerjournal.net/text.asp?2015/11/4/775/147727


 > Introduction Top


Radiation absorbed dose is the fundamental physical quantity of interest and measurement of absorbed dose is vital for the efficacy of radiation treatment. However, direct measurement of absorbed dose in vivo is not always feasible except in few cases where miniature thermo luminescence dosimeters and tiny metal-oxide-semiconductor field-effect transistors (MOSFETs) etc., can be placed within the body cavities to measure the delivered dose. Even these measurements have limitations and they cannot produce dose maps. Further, accurate dosimetry at the interface due to presence of heterogeneity (e.g., air cavity in the head and neck cancers) is a major cause of concern in external beam radiation therapy. It has been observed that there is dose variation in and around air cavities, which occur as a result of the loss of both longitudinal and lateral electronic equilibrium. The magnitude of this variation is, however, inconsistent owing to differences in the geometry and location of air cavities, the beam energy, radiation field size, and the measuring methods. [1],[2],[3]

Apart from macro inhomogeneity, malignant tumor in itself is a non-homogeneous mass that consists of regions that differ in clonogenic cell density, normal tissue involvement, vasculature, hypoxia, proliferation, and gene expression, etc., This biological heterogeneity results in large differences in radio-sensitivity between different regions of the tumor. Heterogeneous structures with spatial differences in functionality and sensitivity for radiation pose challenge to radiation dosimetry. [4]

The dose distribution becomes even more complicated especially at the interface of two media of varying density and atomic number. In order to deliver the correct dose of radiation to the point of interest, correction in the dose calculating programs must be made to account for the tissue differences. The American Association of Physicists in Medicine (AAPM) Task Group 65 (2004) reviewed the clinical need for inhomogeneity corrections in the light of the available methodologies for tissue inhomogeneity corrections in photon beams. [5] The task group recognized that proper accounting for tissue heterogeneity is an essential component of dose optimization and the objective analysis of clinical results.

The objective of the present work is to evaluate the dose perturbations produced by inhomogeneity in the path of the cobalt-60 (Co-60) gamma radiation. Co-60 gamma rays were used for this study as this is still the common mode of treatment for majority of the centers across India.


 > Materials and methods Top


We chose low-density polyethene foam [6],[7],[8],[9] to mimic heterogeneity in head and neck malignancy. The Hounsfield scale (CT) number for the low-density polyethene foam is - 977 Hounsfield unit (HU). Using the equation [10],[11]

Relative Electron Density =

the relative electron density is obtained equal to 0.023 g/cm 3 . We used 2 cm thick foam as air cavities in this region are usually 2 to 3 cm in diameter.

International specialty Product (ISP) GAFCHROMIC EBT2 dosimetry film was used for the measurement of dose (s) at different locations. [12] It is tissue equivalent and thickness of the film is in microns [Table 1], which is unlikely to introduce any significant perturbation when sandwiched between the two solid water phantom plates. The film has a high spatial resolution and does not require any chemical, physical, or thermal process for the development. It is useful for the measurement of 2D dose distribution and has got almost flat energy response in the energy range under consideration.
Table 1: Description of GAF CHROMIC Film EBT2 (Zeff=6.84)


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Experimental set-up is shown in [Figure 1]. Briefly, a rectangular piece of the polyethene foam with an area of 10 × 10 cm 2 and a thickness of 2 cm was sandwiched between 4 cm tissue equivalent solid water phantom from top (2 slabs each with an area of 30 × 30 cm 2 and thickness of 1 cm) and 10 cm tissue equivalent solid water phantom below it (2 slabs each with an area of 30 × 30 cm 2 and thickness of 0.5 cm and 9 slabs each with an area of 30 × 30 cm 2 and thickness of 1 cm). It is to mimic the case of open-ended longitudinal air gap in head and neck region. GAFCHRONIC film EBT2 was cut into small pieces of size 4 × 4 cm 2 and to maintain the original symmetry of film for irradiation and scanning, all films were marked at one corner. These films were then placed between subsequent slabs as shown in [Figure 1] to measure the dose at various points along the depth. The field sizes selected for the study were 3 × 3 cm 2 , 5 × 5 cm 2 , 8 × 8 cm 2 , and 10 × 10 cm 2 at 80 cm Source to surface distance (SSD) (4 cm above the polyethene foam). A dose of 5 Gy was delivered at dmax using single anterior field (Gantry angle 0 degree).
Figure 1: Experimental setup

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Films were scanned in a single scan using EPSON EXPRESSION 10000 × L flatbed color scanner with EPSON scan software. Scanner warm-up effects were reduced by doing five successive pre-scans before the final reading of the film. The scanned films were analyzed using MEPHYSTO mc2 software. To ensure the reproducibility and accuracy of the results, films were selected from the same batch and were stored in light tight envelopes when not in use. For the procedure followed in this study, our statistical uncertainty of film dosimetry is estimated to be around 2.0%.

Simulation study

The experimental set-up for simulation is shown in [Figure 2]. The structure and composition of the multi-layered film was modeled as per the data provided by the vendor. Monte Carlo (MC) simulation was performed using DOSRZnrc user code that comes with EGSnrc V4 2.4.0. [13],[14],[15] The low density foam and solid water phantom is replaced with air and water for simulation purpose. Low density polyethene foam is modeled as air cylinder of radius 10 cm and height 2 cm, sandwiched between water cylinders of radius 10 cm and heights 4 cm and 10 cm, respectively [Figure 2]. Electron Range Rejection, a variance reduction technique was used with parameter ESAVIN = 2 MeV. PRESTA-II was enabled for all electron transport. The particles were transported with a cut-off energy of AP = ECUT = 10 keV for photons and AE = ECUT = 521 keV for the electrons. Photon and electron interaction cross-section data (PEGS data set 521icru.pegs4dat) was used for this work. co60.spectrum which comes inbuilt with EGSnrc code system was used as photon spectrum incident perpendicular to the phantom. [16],[17],[18],[19] Dose at different depths were calculated using circular fields with radius of 1.692 cm, 2.820 cm, 4.513 cm, and 5.641 cm equivalent to 3 × 3 cm 2 , 5 × 5 cm 2 , 8 × 8 cm 2 , and 10 × 10 cm 2 square field.
Figure 2: Simulation Model

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Equivalent square field size has been calculated using the equation [20]

d/σ =1.123 - 0.00067σ

where σ is the side of the square field and d is the equivalent diameter corresponding to σ.

In each simulation 1.9 × 10 10 histories were used to get statistical accuracy of 0.5% and below in total dose in the dose calculation region at the central axis of 0.25 cm radius and 0.0006 cm height. Calculation time for each simulation is approximately 42 hours.


 > Results Top


The results obtained using MC simulations for field size 10 × 10 cm 2 in a homogeneous medium have been compared with reference data (BJR Supplement 25) for the Co-60 beam. There is good accordance (within ± 1%) between the reference data and the calculated data [Figure 3] which validates the input parameters for the simulation.
Figure 3: BJR vis~a~vis simulated data

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Outside the inhomogeneity, the calculated and the measured results are in agreement within the statistical uncertainty. However, within the inhomogeneity, the deviations are large. There is a dose build down in the backward region and a dose build up in the forward direction. This region, where electronic equilibrium is lost, is extremely important from therapeutic point of view.

The results obtained using GAFCHROMIC film and MC simulations for field size 3 × 3 cm 2 , 5 × 5 cm 2 , 8 × 8 cm 2 , and 10 × 10 cm 2 with inhomogeneity are plotted as depth dose curves [Figure 4] [Figure 5] [Figure 6] [Figure 7]. Simulated depth dose data match with the measured data (Film) within ± 3%. The secondary build-up effect is observed for all the fields at foam-tissue interface. A higher depth dose is observed beyond this interface due to reduced attenuation of the radiation beam in the foam.
Figure 4: PDD data for 3 × 3 field size

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Figure 5: PDD data for 5 × 5 field size

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Figure 6: PDD data for 8 × 8 field size

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Figure 7: PDD data for 10 × 10 field size

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The secondary build-up effect near distal surface is more pronounced for smaller fields then larger fields. For 3 × 3 cm 2 field, the dose increases from 23% to 67% in a range of 1 cm beyond the distal surface. For 5 × 5 cm 2 field, this increase is from 37% to 70% in the range of 1 cm. For larger field sizes, 8 × 8 cm 2 and 10 × 10 cm 2 , the dose increases from 49% to 74% and 54% to 76%, respectively in a range of 0.5 cm beyond the interface.

At the proximal end the dose reduction with foam for 3 × 3 field size is 33%, for 5 × 5 field size dose reduction is 29%, for 8 × 8 field size dose reduction is 26%, and for 10 × 10 field size dose reduction is 24%. The dose reduction at the distal surface with foam for 3 × 3 field size is 43%, for 5 × 5 field size dose reduction is 32%, for 8 × 8 field size dose reduction is 23%, and for 10 × 10 field size dose reduction is 19%.

The relation between the data obtained by simulation with and without foam and measured data (film) were analyzed by means of one-way analysis of variance (ANOVA) [Table 2]. Highly significant associations were found between the MC data (simulated) with foam and measured film data (P value 0.01) for all field sizes. For MC data with foam and MC data without foam do not have any significant relationship (P value 1.7 E-12) for all field sizes.
Table 2: Statistical analysis ANOVA


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 > Discussion Top


The inhomogeneity produces significant dose perturbations at tissue-foam (air) interface due to loss of electronic equilibrium. It causes sudden fall of dose near the interface and an increase in photon fluence along the low density foam [Table 3]. Dose reduction near interface (proximal end) will have negative impact if target region is embedded there, however, it would be beneficial if there is normal tissue/critical organ adjacent to it.
Table 3: Percentage variation of dose inside inhomogeneity and near interface


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Results obtained using GAFCHROMIC films are in good agreement with simulation data. MC simulation, though applied with few approximations, could predict the variation accurately. Under dosing and over dosing depends whether the beam is entering the interface from low density medium to high density medium or vice versa. In case of low density foam (air), there is underestimation of dose at the proximal end and overestimation of dose at the distal end. This increases the potential risk for recurrence of malignancy in head and neck region e.g., maxillary sinuses, pharynx (nasopharynx, oropharynx, hypopharynx), larynx, and trachea, etc., where there is possibility of cancer tissue be embedded in the interface region.


 > Conclusion Top


It is therefore vital for safe and better tumor control to closely monitor the under-dosing and dose build-up regions following an air cavity. It can be concluded that MC code provides accurate estimate of interface dose, which can be exploited for the treatment of most head and neck cancers which encompass heterogeneity in the beam path. MC-based treatment planning will be quiet useful for treatment with small fields in regions of low-density tissues near air cavities.

 
 > References Top

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Rosenblum LJ, Mauceri RA, Wellenstein DE, Thomas FD, Bassano DA, Raasch BN, et al. Density patterns in the normal lung as determined by computed tomography. Radiology 1980;137:409-16.  Back to cited text no. 11
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Kawrakow I, Rogers DWO. The EGSnrc Code System: Monte Carlo simulation of electron and photon transport, Technical Report No. PIRS-701. National Research Council of Canada, Ottawa, Canada; 2006.  Back to cited text no. 15
    
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Kawrakow I, Mainegra-Hing E, Rogers DW, Tessier F, Walters BRB. The EGSnrc Code System: Monte Carlo simulation of electron and photon transport, Technical Report No. PIRS-701. National Research Council of Canada, Ottawa, Canada; 2006.  Back to cited text no. 17
    
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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