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ORIGINAL ARTICLE
Year : 2012  |  Volume : 37  |  Issue : 1  |  Page : 27-31
 

Determination of the tissue inhomogeneity correction in high dose rate Brachytherapy for Iridium-192 source


1 Department of Radiotherapy, Government General Hospital, Kakinada, Andhra Pradesh, India
2 Department of Nuclear Physics, Andhra University, Visakhapatnam, Andhra Pradesh, India

Date of Submission16-Apr-2011
Date of Decision17-Sep-2011
Date of Acceptance18-Oct-2011
Date of Web Publication8-Feb-2012

Correspondence Address:
Barlanka Ravikumar
Department of Radiotherapy, Government General Hospital, Kakinada - 53 3001, Andhra Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0971-6203.92717

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   Abstract 

In Brachytherapy treatment planning, the effects of tissue heterogeneities are commonly neglected due to lack of accurate, general and fast three-dimensional (3D) dose-computational algorithms. In performing dose calculations, it is assumed that the tumor and surrounding tissues constitute a uniform, homogeneous medium equivalent to water. In the recent past, three-dimensional computed tomography (3D-CT) based treatment planning for Brachytherapy applications has been popularly adopted. However, most of the current commercially available planning systems do not provide the heterogeneity corrections for Brachytherapy dosimetry. In the present study, we have measured and quantified the impact of inhomogeneity caused by different tissues with a 0.015 cc ion chamber. Measurements were carried out in wax phantom which was employed to measure the heterogeneity. Iridium-192 (192 Ir) source from high dose rate (HDR) Brachytherapy machine was used as the radiation source. The reduction of dose due to tissue inhomogeneity was measured as the ratio of dose measured with different types of inhomogeneity (bone, spleen, liver, muscle and lung) to dose measured with homogeneous medium for different distances. It was observed that different tissues attenuate differently, with bone tissue showing maximum attenuation value and lung tissue resulting minimum value and rest of the tissues giving values lying in between those of bone and lung. It was also found that inhomogeneity at short distance is considerably more than that at larger distances.


Keywords: Brachytherapy, dosimetry, inhomogeneity, Iridium-192 source


How to cite this article:
Ravikumar B, Lakshminarayana S. Determination of the tissue inhomogeneity correction in high dose rate Brachytherapy for Iridium-192 source. J Med Phys 2012;37:27-31

How to cite this URL:
Ravikumar B, Lakshminarayana S. Determination of the tissue inhomogeneity correction in high dose rate Brachytherapy for Iridium-192 source. J Med Phys [serial online] 2012 [cited 2019 Mar 19];37:27-31. Available from: http://www.jmp.org.in/text.asp?2012/37/1/27/92717



   Introduction Top


Brachytherapy is a form of radiotherapy in which small, sealed radioactive sources are placed inside or near the tissue to be irradiated. With this form of treatment, a high dose can locally be delivered to the small tumor volume, with a rapid dose fall-off in the surrounding healthy tissues as a result of the inverse square law. In contrast to external beam radiotherapy (EBRT), in which high-energy X-rays are directed at the tumor from outside the body, Brachytherapy involves the precise placement of radiation source directly at the site of the cancerous tumor.

A key feature of Brachytherapy is that the irradiation only affects a very localized area around the radiation sources. Exposure to radiation of healthy tissues farther away from the sources is therefore reduced. In addition, if the patient moves or if there is any movement of the tumor within the body during treatment, the radiation sources retain their correct position relative to the tumor. These characteristics of Brachytherapy provide advantages over EBRT. The tumor can be treated with very high doses of localized radiation, whilst reducing the probability of unnecessary damage to the surrounding healthy tissues.

While calculating dose at a point in high dose rate (HDR) Brachytherapy, homogeneous water medium is assumed. [1] But, in practice, human body is not homogenous. It consists of different types of tissues like bone, liver, lung, spleen, muscle, air cavities, etc. (in between source and the point of interest). As such, the final isodose distribution is affected by the inhomogeneity around the source. Recently, three-dimensional computed tomography (3D-CT) based treatment planning for Brachytherapy applications has been popularly adopted as it shows more realistic dosimetric outcome on patient anatomy. However, very few studies aimed to study the impact of tissue inhomogeneity in Brachytherapy. [2],[3],[4]

Most of the current commercially available planning systems do not provide the heterogeneity corrections for Brachytherapy dosimetry [5-7] for quantifying the effect of inhomogeneity raised [2],[3],[4] due to different types of tissues.

The present work was carried out to quantify the impact of inhomogeneity caused by different tissues for HDR Brachytherapy treatment with Iridium-192 ( 192 Ir) source. The data could be potentially helpful for developing future Brachytherapy dose calculation formalisms.


   Materials and Methods Top


Inhomogeneity measurements were carried out in wax phantom [8],[9] (30 cm × 30 cm × 15 cm) with different types of tissues, using Micro Selectron HDR 192 Ir source (Nucletron corporation The Netherlands) Brachytherapy machine installed in the Department of Radiotherapy. This machine also serves as the basis for calculation and measurements. We chose five different types of tissues, i.e. bone, spleen, liver, muscle and lung, [10] such that electron densities vary from 4.8 × 1023 to 0.5 × 1023 per cc. Fresh human tissues were collected from the Department of Forensic Medicine. These tissues were processed and cut to the required dimensions. Doses at different distances (4 cm, 6 cm, 8 cm and 9.9 cm) were calculated by Plato planning system. These were also cross-checked by manual calculations at the center of the detector point. Treatment time/dwell time was also calculated and transported to Treatment Control Station (TCS) of HDR Brachytherapy machine. Both the homogenous wax medium and the inhomogeneous tissue were irradiated for this same dwell time. A 0.015cc ion chamber Physikalisch Technische Wertstatten (PTW make) [11] was used to measure the dose, and four flexible catheters were embedded in wax phantom at distances 4 cm, 6 cm, 8 cm and 9.9 cm from the center of the ion chamber. While placing flexible catheters at different distances, utmost care was maintained as the dose is dependent upon distance. All the catheters were placed in pre-measured and cut grooves and sealed with wax so that they were immobilized. The same catheters were used throughout our experiments and the inhomogeneity due to these catheters was not taken into account. 192 Ir source from HDR machine was allowed to dwell at a point such that the center of the chamber, and the center and transverse axis of the source [12],[13],[14] were collinear. A 2 cm 2 cm× 2 cm hole/slot was made in the wax phantom between the ion chamber and source dwell point to accommodate tissues of different thicknesses (0.5 cm, 1.0 cm, 1.5 cm and 2.0 cm). The experimental set-up is depicted in [Figure 1] and [Figure 2].
Figure 1: Schematic diagram of the experimental set-up

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Figure 2: Pictorial view of the micro HDR connected with wax phantom and 0.015cc PTW ion chamber

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The 0.015cc ion chamber was coupled to PTW Unidos-E electrometer to record doses and was kept in dose accumulation mode is shown in [Figure 3]. The instrument was allowed to stabilize before taking the readings. The machine was switched on and the readings (dose measured in terms of charge in picocoulombs) were noted down for the homogeneous medium by keeping 2-cm-thick wax block made up of same material as that of the wax phantom in the slot. After noting down the dose values, homogeneous medium, i.e. the 2-cm-thick wax block was removed. Subsequently, 0.5 cm, 1.0 cm, 1.5 cm and 2 cm thick bone, spleen, liver, muscle and lung tissues were kept in the same pre-cut slot one after the other and readings were taken for different distances. While using the tissues of thickness less than 2 cm, the vacant slot was filled with wax. Tissue homogeneity was measured as the ratio of dose measured with inhomogeneous medium (bone, spleen, liver, muscle and lung tissues) to the dose measured with homogeneous medium for a given distance.
Figure 3: Shows the Treatment control station of HDR and PTW electrometer

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The tissue homogeneity in terms of dose reduction is given by the equation:




   Results and Discussion Top


In the present experiment, we tried to establish the relation between percentage homogeneity and the distance from the source for different tissues of various thicknesses. The experimental results are given in [Table 1],[Table 2],[Table 3],[Table 4] and [Table 5] for different tissues. They give the variation of the percentage homogeneity with distance from source for tissues of different thicknesses. The variation of homogeneity is also shown graphically in [Figure 4],[Figure 5],[Figure 6],[Figure 7] and [Figure 8]. The absorbed dose at the point of interest is as a result of the primary beam that passes through the tissue and the scattering components from the tissue and the homogeneous phantom. At short distances from the source, the dose at the point of measurement is predominantly due to primary beam than the scattered contribution. On the other hand, at larger distance, it is mainly due to scattering component.
Figure 4: % Dosimetric impact of inhomogeneity as a function of the distance from the source for bone tissue

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Figure 5: % Dosimetric impact of inhomogeneity as a function of the distance from the source for spleen tissue

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Figure 6: % Dosimetric impact of inhomogeneity as a function of the distance from the source for liver tissue

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Figure 7: % Dosimetric impact of inhomogeneity as a function of the distance from the source for muscle tissue

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Figure 8: % Dosimetric impact of inhomogeneity as a function of the distance from the source for lung tissue

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Table 1: Variation of % homogeneity as a function of distance from source for bone tissues of different thicknesses

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Table 2: Variation of % homogeneity as a function of distance from source for spleen tissues of different thicknesses


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Table 3: Variation of % homogeneity as a function of distance from source for liver tissues of different thicknesses

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Table 4: Variation of % homogeneity as a function of distance from source for muscle tissues of different thicknesses

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Tables 5: Variation of % homogeneity as a function of distance from source for lung tissues of different thicknesses

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In the present investigation, bone tissue shows maximum dose reduction at 4 cm distance from the source for a tissue of 2-cm thickness and minimum value at a distance of 9.9 cm for 0.5-cm-thick bone tissue. The percentage homogeneity increases linearly with distance from the source. We also observe that the percentage inhomogeneity increases linearly with the thickness of the tissue. This can be attributed to the presence of high Z [15],[16] elements in bone tissue and also its large physical density and electron density values [17] compared to the constitution of other tissues.

Spleen tissue results in lesser inhomogeneity compared to bone, but gives distinctly higher values than those of liver and muscle tissues. This is more significant for increasing thickness. It is also observed that the percentage homogeneity decreases linearly with thickness of the tissue.

Liver and muscle tissue show similar behavior as the electron densities and physical densities of liver and muscle are nearly equal. [17] The dose reductions are more significant for large tissue thickness at shorter distances from the source.

Lung tissue shows a behavior contrary to other tissues. The homogeneity increases with increasing tissue thickness for all distances from the source. This behavior can be understood as the lung is porous and filled with air. Its physical density varies between 0.2 and 0.5 gm/cm 3 during inhalation and exhalation, respectively, taking density of water as 1 gm/cm 3 . [17]


   Conclusions Top


The impact of inhomogeneity caused by different tissues has been measured. It is clear that different tissues attenuate to different extents. Bone tissue shows the maximum inhomogeneity and lung tissue shows the minimum value. Rest of the tissues give values of inhomogeneity lying in between those of bone and lung. It is also observed that inhomogeneity at short distances is considerably more than that at larger distances. The percentage homogeneity increases linearly with the distance from the source. The percentage inhomogeneity increases linearly with the thickness of the tissue. So, while calculating dose at a point, it is necessary to incorporate heterogeneity to arrive at correct dose distribution without cold or hot points.


   Acknowledgments Top


The authors are thankful to Dr. Sivasankar Kotne, Professor, Department of Radiotherapy, for his valuable sugesstions, Dr. P. Sarth Kumar Babu, Professor and Dr. Umamaheswara Rao, Assistant Professor of Department of Forensic Medicine, for providing tissue materials, and Dr. Prasanth for processsing the tissue materials. We would also like to express our gratitude to Dr. S. D. Sharma of RP & AD of BARC and Ritu RAJ Upreti for their valuable suggestions and encouragement.

 
   References Top

1.Nath R, Anderson LL, Luxton G, Weaver KA, Williamson JF, Meigooni AS. Dosimetry of interstitial brachytherapy sources: Recommendation of the AAPM Radiation Therapy Committee Task Group No. 43. Med Phys 1995;22:209-34.  Back to cited text no. 1
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2.Daskalov GM, Kirov AS, Williamson JF. Analytical approach to heterogeneity correction factor calculation for Brachytherapy. Med Phys 1998;25:722.  Back to cited text no. 2
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3.Wang R, Sloboda RS. Brachytherapy scatter dose calculation in heterogeneous media: II. Empirical formulation for the multiple-scatter contribution. Phys Med Biol 2007;52:5637-54.  Back to cited text no. 3
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4.Meigooni AS, Nath R. Tissue in homogeneity correction for Brachytherapy sources in a heterogeneous phantom with cylindrical symmetry. Med Phys 1992:19: 401.  Back to cited text no. 4
    
5.Cho SH, Muller-Runkel R, Hanson WF. Determination of the tissue attenuation factor along two major axes of a high dose rate (HDR) 192Ir source. Med Phys 1999;26:1492-7.  Back to cited text no. 5
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6.Russell KR, Tedgren AK, Ahnesjö A. Brachytherapy source characterization for improved dose calculations using primary and scatter dose separation. Med Phys 2005;32:2739-52.  Back to cited text no. 6
    
7.Kirov AS, Williamson JF. Two-dimensional scatter integration method for brachytherapy dose calculations in 3D geometry. Phys Med Biol 1997;42:2119-35.  Back to cited text no. 7
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8.Meli JA, Meigooni AS, Nath R. On the choice of phantom material for the dosimetry of 192Ir sources. Int J Radiat Oncol Biol Phys 1988;14:587-94.  Back to cited text no. 8
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9.Serago CF, Houdek PV, Pisciotta V, Schwade JG, Abitbol AA, Lewin AA, et al. Scattering effects on the dosimetry of iridium-192. Med Phys 1991;18:1266-70.  Back to cited text no. 9
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10.Kim YS. Human tissue: Chemical composition and photon dosimetry data radial res 1974; 57:38-45.   Back to cited text no. 10
    
11.PTW detectors including code of practice Pinpoint chambers type 31014,31015 manual 2010; 22-93.  Back to cited text no. 11
    
12.Muller -Runkel R, Cho SH. Anisotropy measurements of a high dose rate Ir-192 source in air and in polystyrene. Med Phys 1994;21:1131-4 .  Back to cited text no. 12
    
13.Kocher DC. Radioactive Decay Tables DOE/TIC 11026.13C. Thomason, Mackie T R, Lindstrom M J, and Higgins P D, The dose distribution surrounding 192Ir and 137Cs seed sources. Phys Med Biol 1991;36:475-93.  Back to cited text no. 13
    
14.Thomason C, Higgins P. Radial dose distribution of 192Ir and 137Cs sources. Med Phys 1989;16:254-7.  Back to cited text no. 14
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15.Hubbell JH, Veigele WJ, Briggs EA, Brown RT, Cromer DT, Howerton RJ. Atomic form factors, incoherent scattering functions, and photon scattering cross sections. J Phys Chem Ref Data 1975;4:471.  Back to cited text no. 15
    
16.Anand S, Sood BS. Measurement of Z dependence of elastic scattering cross section of gamma rays. Nucl Phys 1965;73:368-78.  Back to cited text no. 16
    
17.Computerized Imaging Reference Systems inc(CIRS) Tissue simulation and Phantom Technology manual. CIRS Inc; 2010.  Back to cited text no. 17
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]


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