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 Table of Contents 
ORIGINAL ARTICLE
Year : 2015  |  Volume : 38  |  Issue : 4  |  Page : 139-143  

Dosimetric study for the development of heterogeneous chest phantom for the purpose of patient-specific quality assurance


1 Department of Physics, Mewar University, Chittorgarh, Rajasthan; Roentgen-SAIMS Radiation Oncology Centre, Sri Aurobindo Institute of Medical Sciences, Indore, Madhya Pradesh, India
2 Department of Physics, Mewar University, Chittorgarh, Rajasthan; Department of Radiotherapy, Gajra Raja Medical College, Gwalior, Madhya Pradesh, India
3 Department of Radiotherapy, King George Medical College, Lucknow, Uttar Pradesh, India
4 Roentgen-SAIMS Radiation Oncology Centre, Sri Aurobindo Institute of Medical Sciences, Indore, Madhya Pradesh, India
5 Department of Physics, Mewar University, Chittorgarh, Rajasthan; Department of Radiotherapy, Dr. Ram Manohar Lohia Institute of Medical Sciences, Lucknow, Uttar Pradesh, India

Date of Web Publication11-Feb-2016

Correspondence Address:
Om Prakash Gurjar
Roentgen-SAIMS Radiation Oncology Centre, Sri Aurobindo Institute of Medical Sciences, Indore - 453 111, Madhya Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.176155

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  Abstract 

To analyze the dose absorption patterns of 6 Megavoltage (MV) photon beam using computed tomography (CT) slices of thorax of patient, slab phantom, and slab–kailwood–slab phantom. Single beam of 6 MV with field size of 10 × 10 cm 2 was put on CT images of chest wall, slab phantom, and slab–kailwood–slab phantom perpendicular to the surface. Dose was calculated using anisotropic analytical algorithm. Densities of each medium were calculated by Hounsfield units measured from CT images of each medium. The depths of isodose curves of 100%, 95%, 90%, 85%, 80%, 70%, 60%, and 50% were measured in all the three mediums. The densities measured for chest wall, lung, Soft tissue behind lung, slab phantom, and slab–kailwood-slab phantom were 0.89, 0.301, 1.002, 0.998, and 0.379 g/cc, respectively. The isodose depth (100%, 95%, 90%, 85%, 80%, and 50%) for patient (1.5, 2.76, 3.97, 5.33, 7.01, and 20.01 cm), slab phantom (1.5, 2.74, 3.92, 5.06, 6.32, and 15.18 cm), and slab–kailwood–slab phantom (1.5, 2.65, 3.86, 4.98, 5.95, and 20 cm) is approximately same for 100%, 95%, 90%, and 85% isodose curves. The isodose depth pattern in the chest is equivalent to that in slab–kailwood–slab phantom. The radiation properties of the slab–kailwood–slab phantom are equivalent to that of chest wall, lung, and soft tissue in actual human. The chest phantom mimicking the actual thoracic region of human can be manufactured using polystyrene and the kailwood.

Keywords: Isodose curves, slab–kailwood–slab phantom, solid phantom SP34


How to cite this article:
Gurjar OP, Mishra PK, Singh N, Bagdare P, Mishra SP. Dosimetric study for the development of heterogeneous chest phantom for the purpose of patient-specific quality assurance. Radiat Prot Environ 2015;38:139-43

How to cite this URL:
Gurjar OP, Mishra PK, Singh N, Bagdare P, Mishra SP. Dosimetric study for the development of heterogeneous chest phantom for the purpose of patient-specific quality assurance. Radiat Prot Environ [serial online] 2015 [cited 2022 Jan 19];38:139-43. Available from: https://www.rpe.org.in/text.asp?2015/38/4/139/176155


  Introduction Top


The thoracic cavity poses complexity in accurate dosimetry due to its curved topology and heterogeneity coupled with inner organ motions. The motions of lung and heart produce complex scenario for assimilating the dosimetric outcome of the radiotherapy planning algorithms.[1],[2] The electron motions consequent to photon interactions go through a complex process of energy deposition at the confluence of moving surfaces. The heterogeneity and the accurate assessment of equivalent radiological path length have been a matter of intense research by many workers.[3],[4],[5] Each method employed thus far has their own infirmities and limitations. Hence, it was proposed to develop a composite thoracic phantom which could encompass the complexity of the thorax and be close to radiation transport mechanism in the heterogeneous medium. A preliminary study on the isodose depths in computed tomography (CT) images of the chest, standard slab phantom, and slab–kailwood–slab phantom has been performed for better comprehension. The study was conducted to elucidate and establish density pattern and isodose depths patterns in the chest, slab phantom, and slab–kailwood–slab phantom. The purpose of this phantom is to perform patient-specific dose verification in a realistic clinical scenario to obtain reproducible dosimetry near to actual scenario in human thoracic cavity. The current study is to analyze the isodose absorption patterns of 6 megavoltage (MV) photon beam using CT slices of thorax of patient, slab phantom, and slab–kailwood–slab phantom.


  Materials and Methods Top


The CT images of three mediums were used for this study. The first one was the thoracic region of actual patient. The second one was slab phantom, “solid phantom SP34” (IBA Dosimetry GmbH, Schwarzenbruck, Germany). Each slab of this phantom is made up of polystyrene C8H8 (composition: 98% polystyrene + 2% TiO2) with effective atomic number 5.74. Thickness of each slice is 1 cm. The third one was SP34 slab–kailwood–slab phantom. The CT images with 3.00 mm slice thickness of these mediums were taken by scanning these on Siemens SOMATOM Definition Adaptive scanner (Siemens Medical Systems, Germany). The phantom SP34 was used as its electron density which is equivalent to that of different tissues (muscles and fats),[5] and the kailwood was used as a substitute of lung medium because it has equivalent radiological properties as that of actual lung volume.[6]

The CT images of all the mediums were imported on treatment planning system (TPS) Eclipse version 8.9 (Varian Medical Systems, Palo Alto, CA, USA). The single beam of 6 MV with field size of 10 × 10 cm 2 was put on the chest wall, slab phantom, and slab–kailwood–slab phantom perpendicular to the surface as shown in [Figure 1], [Figure 2], [Figure 3], respectively. The beam on chest wall surface [Figure 1] is in such a way that the central axis of beam is exactly perpendicular to the chest wall surface at the point of intersection. Dose was calculated using the anisotropic analytical algorithm version 8.9.08 with a grid size of 0.25 cc. The depths of isodose curves of 100%, 95%, 90%, 85%, 80%, 70%, 60%, and 50% were measured in all the three mediums. The depths in different three mediums measured on TPS are as follows;

  1. Actual patient thoracic region - Chest wall (5 cm at beam center, mean depth across beam ~5 cm), lung (16.64 cm at beam center, mean depth across field ~15 cm), and Soft tissue behind lung (~10 cm at beam center, medium back to lung has some bone portion and some part of contralateral lung)
  2. Slab phantom - Twenty slabs, each one of 1 cm thickness and 30 × 30 cm 2 surface area, and
  3. Slab–kailwood–slab phantom - Five slabs (5 cm) – kailwood (15 cm) – ten slabs (10 cm).
Figure 1: Isodose curves and their depths in computed tomography slice of actual patient at thoracic level. The isodose curve of 100% is not well visible as it has only few dots of very small size to maintain the 100% isodose at depth of maximum (Dmax = 1.5 cm) only

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Figure 2: Isodose curves and their depths in computed tomography slice at the middle of slab phantom. The straight lines in between the isodose curves are the lines drawn by scale tool to measure the depth of particular isodose curve from the surface of phantom

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Figure 3: Isodose curves and their depths in computed tomography slice at the middle of “slab (5 cm)-kailwood (15 cm)-slab phantom (10 cm)” phantom. The isodose curve of 100% is a very small dot at Dmax (1.5 cm) to maintain the 100% dose at Dmax only

Click here to view


The density of chest wall, lung, Soft tissue behind lung, slab and kailwood was measured with the help of Hounsfield units (HU) measured from CT images on TPS and HU-density conversion formula H = 1000 (ρ/ρw) − 1.0).[5] The HU was measured at arbitrary points in the particular volume; the measuring points were chosen in such a way that HU of all the parts of volume has been measured. [Figure 1],[Figure 2],[Figure 3] represent the isodose curves and their depths and also represent the effect of density variation on isodose curves. The straight lines in between the curves were drawn by scale tool for depth measurement.


  Results Top


The density of chest wall, lung, Soft tissue behind lung, slab phantom, and slab–kailwood–slab phantom was measured which is given in [Table 1].
Table 1: Hounsfield number and density measurement of chest wall, lung, Soft tissue behind lung, and SP34 slab phantom

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The depths of 100%, 95%, 90%, 85%, 80%, 70%, 60%, and 50% isodose curves in the chest wall, Soft tissue behind lung, slab, and slab–kailwood–slab were measured which are given in [Table 2].
Table 2: Isodose depths in computed tomography images of actual patient, slab phantom, and slab–kailwood–slab phantom

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  Discussion Top


This experiment was conducted to study the dose distribution pattern in chest level CT images which contains chest wall, lung, and soft tissue. The isodose depths were compared to the corresponding isodose depths in CT images of slab phantom and in CT images of the slab–kailwood–slab phantom. In the case of chest site, ~5 cm is chest wall, ~15 cm is lung, and ~10 cm is Soft tissue behind lung which make the heterogeneous medium. In the first ~5 cm, the soft tissue has a density of 0.89 g/cc, then ~15 cm lung medium has 0.301 g/cc, and then again ~10 cm soft tissue has 1.002 g/cc which is higher compared to that of the chest wall whereas slab phantom has almost homogeneous density throughout its medium which is 0.998 g/cc and is equivalent to the density of Soft tissue behind lung. Furthermore, the electron density of this slab phantom is equivalent to that of muscles.[5] Hence, using mentioned slab phantom as a substitute of chest wall muscles and back muscles is rational.

In the case of slab–kailwood–slab phantom, the first 5 cm (slab) has a density of 0.998 g/cc, then 15 cm (kailwood) has a density of 0.379 g/cc, and then again 10 cm (slab) has a density of 0.998 g/cc. Hence, the density pattern in slab–kailwood–slab phantom is equivalent to that in chest wall–lung–soft tissue medium. However, the density of lung (0.302 g/cc) is lesser than that of kailwood (0.379 g/cc), and this discrepancy can be removed using kailwood excluding knaggy portion which is present in the kailwood used for this study. The knaggy portion has comparatively higher density compared to that of kailwood portion between two joints. Second, the thicknesses of different densities in chest CT images are not uniform across the field size; these are varying from lesser to higher values than the above-mentioned thicknesses whereas the thicknesses in slab–kailwood–slab CT images are uniform across the field size.

The physical density of lung already noted in the literature varies between 0.2 and 0.5 g/cc during inhalation and exhalation, respectively.[7] The kailwood has equivalent radiological properties as that of lung medium, the relative electron densities of both these mediums are equivalent.[6] In addition, the physical densities of both these mediums measured in this study [Table 1] are equivalent. Hence, using the kailwood as a substitute of the lung is appropriate.

The field setup (field size, source to surface distance, and field orientation w.r.t. surface) in all the three cases is same. The isodose depths in all three cases are approximately same for 100%, 95%, 90%, and 85% isodose curves, however it is little higher in CT images of chest as the backscattering is low due to low density of lung and also the density of chest wall is little lesser than that of slab. The depth of corresponding isodose curves goes on increasing from 80% to 50%; at the same time, the differences between the depths of same isodose curve in all three cases vary significantly. The highest depth is in chest CT images of chest, then in CT images of the slab–kailwood–slab phantom, and then in CT images of slab phantom. The difference in depths of same isodose curves in the chest and in the slab–kailwood–slab phantom is due to the above-mentioned two reasons that is the higher density of kailwood compared to lung due to the presence of knaggy portion and the varying thickness of lung across the field whereas there is uniform thickness in the case of the slab-kailwood-slab phantom.

This study covers the density variation in the thoracic region and compares dose distribution pattern in actual patient's CT images and in the phantom with equivalent materials. Based on this study, the equivalent phantom in the exact shape as that of actual thoracic region can be manufactured, the authors of this study are doing work in that direction. Another most important issue in the case of thoracic region is breathing movements. Many new technologies have been introduced to treat tumors which move with the respiration. Use of active breath control for single breath hold cone beam CT guidance is an effective method to manage the breathing margins,[8] also managing the motion through real-time dynamic multileaf collimator tracking is well in practice.[9] Four-dimensional planning is in practice at many places, and many authors have presented the work on its implementation and advantageous authenticity.[10],[11],[12] However, apart from the planning and treating the patient with the breath management techniques, it is also equally important to do the patient-specific intensity modulated radiotherapy quality assurance (QA) in the similar kind of phantom which mimics the actual thoracic region density wise, internal structure wise, as well as breathing motion wise. Hence, the authors of the current study are working in that direction to develop the equivalent phantom which would represent the actual thoracic region of human. This study was conducted to check the radiobiological equivalence of the materials to be used in fabricating the chest phantom. Results of the study indicate that the mentioned materials can be used to fabricate the chest phantom.


  Conclusions Top


The density pattern in the chest is equivalent to that of slab–kailwood–slab phantom. In addition, the isodose depth pattern in the chest is equivalent to that in the slab–kailwood–slab phantom. It is concluded that the radiation properties of the slab–kailwood–slab phantom are equivalent to that of chest wall–lung–soft tissue in actual human. The chest phantom mimicking the actual thoracic region of human can be manufactured using polystyrene (with an adequate quantity of TiO2 to obtain the required density of the particular muscle part, for example, chest wall muscle, muscles back to lung, etc.) and the kailwood. This kind of phantom will definitely improve the patient-specific QA practices.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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Furuya T, Sugimoto S, Kurokawa C, Ozawa S, Karasawa K, Sasai K. The dosimetric impact of respiratory breast movement and daily setup error on tangential whole breast irradiation using conventional wedge, field-in-field and irregular surface compensator techniques. J Radiat Res 2013;54:157-65.  Back to cited text no. 1
    
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George R, Keall PJ, Kini VR, Vedam SS, Siebers JV, Wu Q, et al. Quantifying the effect of intrafraction motion during breast IMRT planning and dose delivery. Med Phys 2003;30:552-62.  Back to cited text no. 2
    
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Miften M, Wiesmeyer M, Kapur A, Ma CM. Comparison of RTP dose distributions in heterogeneous phantoms with the BEAM Monte Carlo simulation system. J Appl Clin Med Phys 2001;2:21-31.  Back to cited text no. 3
    
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Ueki N, Matsuo Y, Shibuya K, Nakamura M, Narabayashi M, Sakanaka K, et al. Differences in the dose-volume metrics with heterogeneity correction status and its influence on local control in stereotactic body radiation therapy for lung cancer. J Radiat Res 2013;54:337-43.  Back to cited text no. 4
    
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Kumar A, Sharma SD, Arya AK, Gupta S, Shrotriya D. Effect of low-density heterogeneities in telecobalt therapy and validation of dose calculation algorithm of a treatment planning system. J Med Phys 2011;36:198-204.  Back to cited text no. 6
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Zhong R, Wang J, Zhou L, Xu F, Liu L, Zhou J, et al. Implementation of single-breath-hold cone beam CT guided hypofraction radiotherapy for lung cancer. Radiat Oncol 2014;9:77.  Back to cited text no. 8
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Suh Y, Murray W, Keall PJ. IMRT treatment planning on 4D geometries for the era of dynamic MLC tracking. Technol Cancer Res Treat 2014;13:505-15.  Back to cited text no. 10
    
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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2]


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