|Year : 2012 | Volume
| Issue : 4 | Page : 555-560
Experimental and Monte Carlo study of the effect of the presence of dry air, cortical bone inhomogeneities and source position on dose distribution of the mHDR-v2 source
Rakesh M Chandola1, Samit Tiwari2, Manjula Beck1, Pradeep Kumar Chandrakar1, Suresh Kumar Thakur1
1 Department of Radiotherapy, Pt. J. N. M. Medical College & Dr. B.R.A.M. Hospital, Raipur, India
2 Department of Applied Physics, Bhilai Institute of Technology, Durg, Chhattisgarh, India
|Date of Web Publication||29-Jan-2013|
Rakesh M Chandola
Department of Radiotherapy, Pt. J.N.M. Medical College and Dr. B.R.A.M. Hospital, Raipur, Chhattisgarh
Source of Support: The necessary accessory/equipment for research had been provided by Deptt. Radiotherapy, Pt. J. N. M. Medical College, Raipur, C.G., India and Deptt. of Applied Physics, Bhilai Institute of Technology, Durg, C.G., India., Conflict of Interest: None
Background: Recently it was data wise established that there is a considerable dose difference due to source position from the surface of the patient, and due to the presence of inhomogeneities.
Aim: It aims at to find out the dose difference due to source position, and inhomogenieties in water phantom of high dose rate (HDR) 192 Ir mHDR-v2 source by experiment and by Monte Carlo (MC) simulation GEANT4 code.
Materials and Methods: The measured study of the source was done using an in-air ionization chamber, water phantom while the calculated study was done by modeling the water phantom and its water, inhomogeneities, position of source, and points of calculation.
Results: The measured and calculated dose differences are 5.48 to 6.46% and 5.43 to 6.44% respectively higher in the presence of dry air and 4.40 to 4.90% and 4.38 to 4.88% respectively lower in the presence of cortical bone. However, for the study of the effect of source position on dose distribution, when the source was positioned at a 1 cm distance from the surface of water phantom, the near points between 1 cm and 2 cm are 2 to 3.5% and 2.1-3.7% underdose and for distant points from 3 cm to 8 cm from the source are 4 to 15% and 4.1 to 15.8% underdose for measured and calculated studies, respectively, to the dose when the source was positioned at midpoint of water phantom.
Conclusion: These results can be used in the treatment planning system.
Keywords: Brachytherapy, GEANT4, HDR, inhomogeneity, Monte Carlo
|How to cite this article:|
Chandola RM, Tiwari S, Beck M, Chandrakar PK, Thakur SK. Experimental and Monte Carlo study of the effect of the presence of dry air, cortical bone inhomogeneities and source position on dose distribution of the mHDR-v2 source. J Can Res Ther 2012;8:555-60
|How to cite this URL:|
Chandola RM, Tiwari S, Beck M, Chandrakar PK, Thakur SK. Experimental and Monte Carlo study of the effect of the presence of dry air, cortical bone inhomogeneities and source position on dose distribution of the mHDR-v2 source. J Can Res Ther [serial online] 2012 [cited 2019 Sep 19];8:555-60. Available from: http://www.cancerjournal.net/text.asp?2012/8/4/555/106536
| > Introduction|| |
The main aim of radiotherapy is the delivery of maximum lethal dose to target with minimum dose to the surrounding healthy tissues. To achieve this goal, the HDR brachytherapy treatment has emerged as an important technique in clinical radiotherapy. The main advantage of the brachytherapy technique is the high conformal energy delivery to the malignant tissues volume and sparing of the healthy tissues due to the law of inverse square on the dose distribution around the source. MC has become an accepted dose calculation methodology in brachytherapy treatment planning. ,,
Brachytherapy involves the precise placement of radiation sources 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 further 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 in relation to the tumor.
It is advocated in American Association of Medical Physicists (AAMP) Task Group TG-43U  and TG-43U1  that before delivering dose to the patient, the dosimetry should be done either by experiment or by MC simulation using the appropriate code (s). Hence in this study GEANT4 code of MC have been taken.
In dry air, oxygen, nitrogen, carbon dioxide, hydrogen, argon, neon, helium, krypton, and xenon are present with their volume ratio 0.2095, 0.7809, 0.0003, 0.0000005, 0.00933, 0.000018, 0.000005, 0.000001, and 0.09 × 10 -6 respectively. The molecular mass (kg/kmol) of oxygen, nitrogen, carbon dioxide, hydrogen, argon, neon, helium, krypton, and xenon are 32, 28.02, 44.01, 2.02, 39.94, 20.18, 4, 83.8, 131.29; hence the molecular mass of oxygen, nitrogen, carbon dioxide, hydrogen, argon, neon, helium, krypton, and xenon with their volume ratio comes to be 6.704, 21.88, 0.013, 0, 0.373, 0, 0, 0, 0. By adding all these molecular masses of components of dry air with their volume ratio comes to be 28.97. However, the effective density of dry air can be calculated with the ideal gas law ρ = p/(R T), where, p = pressure (kPa), R = 286.9 = individual gas constant (J/kg °K), T = absolute temperature (°K), so the density of dry air at STP can be calculated as 1.294 kg/m 3 .
Bone is a living tissue. It is a highly organized composite material comprising minerals, organic matrix, cells, and water. Cortical bone is much denser with maximum 5-10% porosity than the trabecular bone with 50-90% porosity. By mass the cortical bone is made by water (12%), organic compound (28.1%), and ash (59.9%). However, by volume the cortical bone is made by water (23.9%), organic compound (38.4%), and ash (37.7%).  Cortical bone is highly ordered into units called osteons. These osteons gradually increase in density from 0.7 to 0.95 g/cm 3 .
The absorption of radiation energy by any matter or biological system depends upon the attenuation of radiation passing through it. Four factors determine the degree of attenuation of a radiation beam as it passes through it. One involves the nature of radiation and three involves the composition of matter or biological system, i.e., density, atomic number, and electrons per gram of it. Increasing the radiation energy increases the number of transmitted radiations, hence decreasing the attenuation, while increasing the density, atomic number, or electrons per gram of the absorber decreases the number of transmitted radiation, hence increasing attenuation.
In general, the absorbers with atomic numbers are denser than elements with low atomic numbers. But, the definite relationship between density and atomic number is complex, and no simple rule covers all situations.
Density determines the number of electrons that will be present in a given thickness, and this is what determines radiation attenuation as it is thought in clinical radiotherapy. In clinical radiotherapy 1 cm thickness of the patient means 1 cubic centimeter. Hence, the number of electrons per cubic centimeter is the most important factor for the attenuation of radiation, when it passes through the air, fat, muscle, or bone. The greater the number of electrons per cubic centimeter of any biological system, the higher the attenuation of the radiation beam passing through it: (e/g) × (g/cm 3 ) = e/cm 3 . 
The number of electrons per gram can be calculated by the equation N 0 = NZ/A where, N 0 = number of electrons per gram, N = Avagadro's number = 6.02 × 10 23 , Z = atomic number, A = atomic weight = sum of the weights of the protons and neutrons in the atom's nucleus.
The dry air has effective atomic number 7.64, density (g/cm 3 ) 0.00129, electrons per gram 3.01 × 10 23 , and electrons per cubic centimeter 0.0039 × 10 23 . 
However, the compact bone has the effective atomic number 13.8, effective density (g/cm 3 ) 1.85, electrons per gram
3.0 × 10 23 , and electrons per cubic centimeter 5.55 × 10 23 . 
In fact the number of Compton reactions depends on the number of electrons. The number of electrons is usually expressed in the unit e/g (a mass unit) rather than e/cm 3 (a volume unit).
Previous studies done
Pantelis et al.,  and Anagnostopoulos et al.  studies show that spinal cord dose and breast dose respectively are overestimated of the order of 13% and 10% respectively, if the patient's inhomogeneities are not taken in to consideration. On this line recently a systematic study of Chandola et al.  using EGSnrc MC code on mHDR-v2 source was presented, which shows that the dose behind the dry air inhomogeneity in the body increases and behind the cortical bone inhomogeneity decreases, and when the source is implanted very close to the surface, the rest points behind the source are underdose. The study of Chandola et al.  is taken here for comparison of the results of the present study.
| > Materials and Methods|| |
The geometric diagram of the source is derived here from Daskalov et al.  and is illustrated in [Figure 1]. It has an active core made of 192 Ir of effective density of 22.42 g/cm 3 and active length of 3.6 mm with active diameter of 0.65 mm. The active core is surrounded by the stainless steel AISI 316 L encapsulation of density 8.02 g/cm 3 with composition by weight Fe 68%, Cr 17%, Ni 12%, Mn 2%, and Si 1%. With this encapsulation the total length of the source becomes 4.5 mm. with total diameter of 0.9 mm. This distal capsule tip has rounded borders with radius of curvature of 0.4 mm. The source is welded on a flexible woven stainless steel cable with a diameter of 0.7 mm. As the portion of the cable near the source remains in a straight line, the dose around the cable in this length is not critical; therefore, the length of simulation is not taken more than 5 mm.
|Figure 1: Geometric diagram of the Nucletron mHDR-v2 brachytherapy source|
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Water phantom modeled and used for measurement was having a dimension 30 × 30 × 30 cm 3 which acts as unbounded phantom up to 20 cm.  The density of water used in the simulation and measurement was 0.997 g/cm 3 at 22°C, as is recommended in TG-43U1. 
Applicator used for modeling and measurement was of stainless steel 1.4401 (equivalent to AISI/SAE 316) of density 8.0 g/cm 3 and with wall thickness of 0.15 mm. The inner and outer diameters of the applicator were 1.35 ± 0.02 mm and 1.65 ± 0.02 mm respectively, and the value of effective attenuation coefficient was taken as 0.030 ± 0.002 cm 2 /g (ANSI 303/304). 
The in-air ion chamber used for measurement in this study was a 0.1 cc model PTWM-23322 Freiburg with an active length of 1.2 cm and effective diameter of 0.35 cm. The wall material was made of PMMA with thickness 0.12 g/cm 3 and no cap. The calibration factor was 3.597 E ± 08 Gy/C for 60 Co beam. The chamber should have a wall thickness of about 0.31 g/cm 2 to provide charge particle equilibrium for the 192 Ir source emitting a photon spectrum in the energy range from 9-885 keV.  The wall thickness correction factor (A w ) was determined using a formula: A w = 1 – γt, where 'γ' is the attenuation and scattering fraction per wall thickness (cm 2 /g), and 't' is the total thickness (g/cm 2 ) of the wall material.  By calculation, the A w value equals 1.072. Further, the beam quality correction factor K Q , which accounts for the difference in the energy spectrum of the photon beam (usually 60 Co) for which chamber has been calibrated, was determined for the 192 Ir source by the interpolation method. The charge of the chamber was measured and corrected for nonuniformity, displacement, temperature, and pressure.
The gamma spectrum of the Ir HDR brachytherapy source used in this study has been obtained from NuDat database.  The beta spectrum of the 192 Ir source has not been considered in simulation and measurement, since its contribution to the dose rate distribution for distances greater than 1 mm from the source is negligible due to the encapsulation of the source and the catheter in which the source is introduced.  The cross-section tabulation was taken from the EPDL 97. 
Dry air inhomogeneity was of the composition and density as recommended in TG-43U1  with relative humidity of 40% of volume 1 × 2 × 2 cm 3 , and cortical bone inhomogeneity was with composition and density as recommended in International Commission of Radiological Protection (ICRP) of volume 1 × 2 × 2 cm 3 . In dry air, oxygen, nitrogen, carbon dioxide, hydrogen, argon, neon, helium, krypton, and xenon are present. However, as per ICRP 1995a, the ash of cortical bone have 37.5% calcium by mass. As per ICRP publication 1989, the specific gravity, and therefore the density of hydrated cortical bone, increases with age up to 40 years, which is given by the formula CD (A) = –0.000156 A 2 + 0.0125A + 1.65 for A ≤ 40 where CD (A) is the density of cortical bone and A is the age in years. As per ICRP 1995a, the mean surface to volume ratio of adult compact or cortical bone has a value 30 per cm. The matrix of cortical bone contains various organic and inorganic compounds. The bulk of organic material is made of protein called as collagen, and other organic materials are carbohydrates, lipids. The inorganic matter of bone consists mainly of microscopic deposits of forms of calcium phosphate. Bone minerals mainly are hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 , amorphous calcium phosphate Ca 9 (PO 4 ) 6 (var), and octacalcium phosphate Ca 4 H(PO 4 ) 3 .
For the inhomogeneities effect study, firstly the source was put fixed in the center of said water phantom along the Y axis with the tip of the source toward the +Y axis. The center of the active source acts as the center of the co-ordinate axes. For the measurement study, the chamber was put along the Z axis direction with the tip of the chamber along the +Z direction at different points of measurement at the transverse axis of the source. The dose was measured and calculated without any inhomogeneity at different points from the source at its transverse axis. Then, the said inhomogeiniety was positioned at the transverse axis at a 1 cm distance from the center of the source and the dose was measured and by simulation MC calculated at different points from the source at its transverse axis.
For the study of the effect of source position in the said water phantom, the source was put at a 1 cm distance from the surface of water phantom along the Y axis with the tip of the source toward the +Y axis. Then, the dose was measured and by simulation MC calculated at different points at the transverse axis of the source.
The air kerma strength needed for calculation of dose or energy deposited was determined in a separate simulation of 10 8 histories. For this, the said source position was modeled at the center of an air volume 4 × 4 × 4 m 3 with composition and density of air recommended in TG43-U1  with 40% relative humidity. It was calculated at 0 cm to 150 cm using cylindrical voxels/cells of 1 cm thick and 1 cm high. These scoring voxels/cells assure volume averaging artifacts <0.1% for distances greater than 5 mm from the source. 
| > Results|| |
In [Table 1] and in [Figure 2] and [Figure 3], the data and graph of measured and calculated % relative differences between dose in the presence of air and cortical bone inhomogeneity and dose in the absence of inhomogeneities respectively vs. the distance from the source at its transverse axis of air and bone respectively are presented. Decreased attenuation by air due to its less density increases the measured and calculated dose by 5.48 to 6.46% and 5.43 to 6.44% respectively behind air inhomogeneity. However, increased attenuation by cortical bone due to its high density decreases the measured and calculated dose by 4.4 to 4.9% and 4.38 to 4.88% respectively behind cortical bone inhomogeneity. These results were found in good agreement with literature data of Chandola et al. 
|Figure 2: Graph showing the effect on dose distribution due to the presence of dry air inhomogeneity|
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|Figure 3: Graph showing the effect on dose distribution due to the presence of cortical bone inhomogeneity|
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|Table 1: % Relative difference between dose in the presence of inhomogeneity and dose in the absence of inhomogeneity|
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The data and graph between the ratio of the dose when the source is at a 1 cm distance from the surface of water phantom to the dose when the source is at the center of the said water phantom of the measured and calculated study vs. the distance from the source at its transverse axis are illustrated in [Table 2] and [Figure 4] respectively. As the primary dose may overlap missing scatter function for near points from the source, the near points between 1 and 2 cm from the source undergo 2 to 3.5% and 2.1 to 3.7% underdose respectively of measured and calculated study respectively. However, far points between 3 and more distant points undergo underdose between 4 to 15% and 4.1 to 15.8% of measured and calculated study respectively. These results were found in good agreement with literature data of Chandola et al. 
|Figure 4: Graph showing the effect on dose distribution due to source position|
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|Table 2: Ratio of dose due to the effect of source position in water phantom of the mHDR-v2 source|
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| > Discussion|| |
In the calculated study, for the validation of source geometry, the deposited energy was determined by using 10 9 histories in GEANT4 simulation, yielding a statistical uncertainty of dose distribution of 0.22% near the source and 0.51% for distances greater than 4 cm. To estimate the calculated uncertainty due to variation of the geometry from one source to another in the manufacturing process, the worst possible situation of first thinner core and thickest rest of capsule and second thickest core and thinner rest of capsule is MC GEANT4 simulated, which shows that 0.52% variation is to be considered compared to the case of normal dimension. For evaluation of the effects of inhomogeneities and position of the source in water phantom on dose distribution, 10 8 histories are traced. As the rotational symmetry around the source axis is discontinued by inclusion of dry air and cortical bone inhomogeneities and by placing the source very near to the surface of water phantom; therefore, the energy scored was taken to be in cubiodal 2 × 2 × 2 cm 3 scoring voxels, as very small voxels increases the uncertainty.
The sources for uncertainty may arise from chamber, electrometer, temperature, pressure, measurement set-up, and primary calibration of ionization chamber. Although ionization chambers are energy independent and show a linear response to the dose, the steep dose gradient inside the active volume of the ionization chamber nearer to the brachytherapy source gives large uncertainties in the measurement. The nonuniform photon fluence caused by the divergence of the incident photon is greatest for ionization chambers in the vicinity of the brachytherapy sources.
The accuracy in determining the positioning of the chamber center was ± 0.2 mm from the absolute center. The outer diameter of the source applicator was 1.35 mm, and the outer diameter of the source was 0.9 mm. This means that the source could be displaced a maximum of ± 0.215 mm from the central axis of the applicator. The uncertainty in the positioning the applicator was ± 0.1 mm. Thus, the maximum uncertainty in positioning chamber, source, and applicator may be taken as ± 0.515 mm. The uncertainty due to positional error was performed for reference distance 10 cm. Thus, the overall uncertainty including temperature and pressure, stability of the chamber, leakage, and chamber calibration was measured to be ± 1.32 mm.
To estimate the uncertainty due to variation of the geometry from one source to another in the manufacturing process, the worst possible situation of first thinner core and thickest rest of capsule and second thickest core and thinner rest of capsule are MC simulated, which shows that 0.5% variation is to be considered compared to the case of normal dimension. The cross section uncertainties of the EPDL 97 library is considered here as 0.5%. 
The calculated uncertainties of dose rate have been evaluated according to the recommendations of TG-43U1 considering the type A or statistical uncertainty due to the MC simulation of photon histories in dose rate and air kerma strength simulation, and the type B uncertainty due to the contribution of underlying cross section data and that arising from the geometrical model of the source. In the simulation of water, the uncertainty in dose rate along the transverse axis is approximately equal to 0.5% except along the longitudinal axis where it has reached to more or less 1%.
Simulation of air kerma strength yielded an uncertainty of roughly 0.5%. The average type A uncertainty in the calculated study may be taken as 0.667% for all points. To estimate the uncertainty by MC due to the variations of the geometry from one source to another in the manufacturing process, the uncertainty is evaluated keeping the worst possible situation for the core and capsule dimensions of the source. First, the thinnest capsule and thickest core possible and second, the thickest capsule and thinnest core possible are assumed here, which showed 0.5% variation to be considered.
Although the air and bone have more or less same number of electrons per gram but density and atomic number of the air is very lower than bone, the degree of decreased attenuation by air may be higher than the degree of increased attenuation by bone.
However, by placing the source close to the surface of water phantom, a lack of scattering photons may result in a significant drop of dose due to the absence of backscattering material.
| > Conclusion|| |
The current study results clearly dictate that there is a wide effect on dose distribution due to the presence of dry air and cortical bone inhomogeneities, and also dictate that source implant position from the surface puts a wide effect on dose distribution. Hence, it is advocated that these results should be put in TPS in order to ensure accurate HDR brachytherapy, especially in breast and intraluminal brachytherapy. The current planning systems of the HDR brachytherapy are lack of facility to take input of dose effect data of the presence of inhomogeneities and source position. Therefore, it may also be advocated that such HDR brachytherapy planning systems are needed to be made which may take the dose effect data of the presence of inhomogeneities and source position.
| > References|| |
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]