Dose Distributions of an 192Ir Brachytherapy Source in Different Media

This study used MCNPX code to investigate the brachytherapy 192Ir dose distributions in water, bone, and lung tissue and performed radiophotoluminescent glass dosimeter measurements to verify the obtained MCNPX results. The results showed that the dose-rate constant, radial dose function, and anisotropy function in water were highly consistent with data in the literature. However, the lung dose near the source would be overestimated by up to 12%, if the lung tissue is assumed to be water, and, hence, if a tumor is located in the lung, the tumor dose will be overestimated, if the material density is not taken into consideration. In contrast, the lung dose far from the source would be underestimated by up to 30%. Radial dose functions were found to depend not only on the phantom size but also on the material density. The phantom size affects the radial dose function in bone more than those in the other tissues. On the other hand, the anisotropy function in lung tissue was not dependent on the radial distance. Our simulation results could represent valid clinical reference data and be used to improve the accuracy of the doses delivered during brachytherapy applied to patients with lung cancer.


Introduction
High dose rate (HDR) brachytherapy uses sealed radioactive sources to deliver radiation dose to a tumor over a short distance via intracavitary or interstitial placement. This methodology is designed to maximize the tumor dose while minimizing the dose to surrounding normal tissues. HDR brachytherapy is associated with a high dose gradient, with the dose decreasing rapidly away from the source. Several recent researches have studied the 192 Ir source referred to in the American Association of Physics in Medicine Task Group 43 (AAPM TG-43) report [1]. That report on dose parameter calculations was based on the assumption of a water environment. Several researchers used a dosimeter and Monte Carlo (MC) code to investigate the dose distribution around the source [2][3][4][5].
The development of smaller radiation sources has widened the application of brachytherapy to cancers of the nasopharynx, esophagus, bronchus, lung, and esophagus [6][7][8]. Guilcher et al. pointed out that brachytherapy is an effective and safe treatment option for patients with endobronchial carcinoma who cannot receive surgery or external beam radiotherapy (EBRT) [9]. Koutcher et al. indicated that patients with locally recurrent nasopharynx cancer who received EBRT combined with brachytherapy had fewer severe late side effects compared with those treated with EBRT alone [10]. Brachytherapy has demonstrated good clinical efficacy, but dose calculations still assume that the treatment environment comprises a water medium around the source.
The AAPM TG-43 report provided dose calculation formulas and dose parameters for brachytherapy. Although         2.1. 192 Ir Source. The photon energy spectrum of 192 Ir is quite complex, containing energies ranging from 0.0089 to 1.0615 MeV. 192 Ir has a relatively high atomic number ( = 77) and density ( = 22.42 g/cm 3 ), and the dose distribution around the source depends on its dimensions and outer encapsulation, as well as the treatment environment. The 192 Ir source used in this study had an active length of 3.5 mm and a diameter of 0.6 mm, as shown in Figure 1. It was encapsulated by a stainless steel outer cover with an outer diameter of 1.1 mm that was welded to a steel cable for attachment to a remote after-loading machine (microSelectron HDR, Nucletron, The Netherlands). For the purpose of dose calculation, the stainless steel cable extended 2 mm from the outer cover on the proximal side of the active source.

Monte-Carlo
Simulation. This study used the MCNPX 2.70 code to calculate the dose distributions of an 192 Ir source in water, bone, and lung tissue. The photon energy spectrum of 192 Ir was obtained from Brookhaven National Laboratory [13]. The simulation was divided into three parts. First, a 30 cm diameter spherical phantom was used in simulations for the radial dose function and anisotropy function; the spherical dimensions used in the simulations were identical to those used by Williamson and Li [11] and Karaiskos et al. [12], so that our results could be compared to the results calculated by those authors. The F6 tally was used to speed up the calculation, and the source was set at the center of the phantom. The F6 tally based on the assumption of an electronic balance exists in the tally region. The electron will lose its energy instead of undergoing electron transport at the position where a photon and electron collide. The radiation particles were removed from the simulation when they moved outside of the phantom. At least 10 8 particles were simulated, yielding 1 statistic errors of less than 3% for the total dose. Second, the phantom was simulated as a cylinder with a diameter of 25 cm and a height of 25 cm in order to closely approximate the experimental phantom, and the absorbed doses were recorded in a two-dimensional (2D) matrix. Third, the influence of phantom size on the radial dose function was investigated by simulating spherical water, lung, and bone phantoms with various radii.

Phantoms.
The compositions of bone and lung tissue used in this study are based on the ICRU 44 (International Commission on Radiation Units and Measurements Report 44) recommendation [14]. Cortical bone and inflated lung were adopted as bone and lung phantoms with densities of 1.92 and 0.26 g/cm 3 , respectively, while a polystyrene phantom with a density of 1.04 g/cm 3 was used to represent water.

Radiophotoluminescent Glass Dosimeter Measurements.
The radiophotoluminescent glass dosimeters used in this study (GD-301, Asahi Techno Glass Corporation, Shizuoka, Japan) had the following weight composition: 31.55% P, 51.16% O, 6.12% Al, 11.00% Na, and 0.17% Ag. The effective atomic number and physical density of the dosimeters were 12.04 and 2.61 g/cm 3 , respectively [15]. The GD-301 dosimeter is composed of a rod glass element measuring 1.5 mm in diameter and 8.5 mm in length, and it must undergo heat treatment at 70 ∘ C for 1 hour before reading with the Dose Ace FGD-100 reader. The active readout size of the GD-301 dosimeter of 1 mm makes it a suitable tool for measuring brachytherapy sources with high dose gradients [16].
To support the MC simulation results, the radial dose function and anisotropy function were measured using radiophotoluminescent glass dosimeters in phantoms representing three different tissue types. Slabs were sandwiched together to build 25 cm × 25 cm × 25 cm phantoms, in which a slot was milled in the center to accommodate the 192 Ir source. For GD-301 dosimeter measurements, concentric holes were drilled along polar angles of = 0-180 ∘ at radial distances of = 0.5,

Radial Dose Function and Dose-Rate Constant.
The radial dose function, ( ), accounts for the effects of photon absorption and scatter in the medium along the transverse axis of the source. MCNPX results of ( ) for a 30 cm diameter spherical phantom are presented in Figure 3(a) and Table 1 Table 2. Figure 3(b) presents the radial dose functions calculated for water, bone, and lung phantoms. As the depth increased, the radial dose function decreased more slowly for lung than for water due to the linear attenuation coefficient being small in lung tissue, whereas the function decreased faster in bone than in water due to the linear attenuation efficient being higher in bone than in water. Tables 1 and 2 also present the radial dose functions and dose-rate constants of bone and lung tissue to provide clinical reference data for dose calculations in various tissue types around an 192 Ir source. The ratios of the dose calculated by MCNPX code in lung tissue and bone to the dose in water are shown in Figure 4. Up to a depth of 11 cm the dose was less in lung tissue than in water; for example, at a depth of 2.8 cm, the lung/water dose ratio was 0.88. This shows that a treatment planning system (TPS) would overestimate the lung dose by 12%. Moreover, at a depth of 15 cm, the lung dose would be underestimated by 30%. These results also imply that, if a tumor is located in the lung, the tumor dose will be overestimated if the different tissue densities are not taken into account. At depths of less than 5 cm the dose rate in bone was similar to that in water, whereas at depths greater than 5 cm the bone dose would be overestimated by 47%.

Influence of Phantom Dimensions on the Radial Dose Function. Karaiskos et al. used MC calculations for an 192
Ir microSelectron source to examine the dose parameters in spherical water phantoms with different diameters. They found that the phantom dimensions significantly affected the radial dose functions near to the edges of the phantom, with deviations of up to 25% being observed. They did not observe that the anisotropy functions depended significantly on the phantom size. The present study simulated spherical water, bone, and lung phantoms with diameters ranging from 10 to 50 cm in order to evaluate how the phantom dimensions influence the radial dose functions in these tissues. Figure 5 shows the radial dose functions in water near the edge of the phantom, where deviations of up to 23% are evident and the results are similar to those of Karaiskos et al. The deviations in the radial dose functions in bone and lung tissue were 26.7% and 6.5%, respectively. These results show that the backscattered components vary with the material densities, being maximal in bone and minimal in lung tissue. Moreover, the backscattering in lung tissue does not differ significantly with the phantom size.

Anisotropy Function Comparison.
The anisotropy function, ( , ), accounts for the anisotropy of the dose distribution around the source due to the geometry structure of the source and the encapsulation. Our MCNPX results of ( , ) for a 30 cm diameter spherical phantom are presented in Figure 6 and Table 3. The calculated results were compared with the MC calculations of Williamson and Li; the differences were within 4.6% for = 1 cm and < 5 ∘ , within 2.5% for = 1 cm and 5 ∘ < < 180 ∘ , and within 2% for > 1 cm. Moreover, our results agree with the MC calculations of Karaiskos et al. within 2.7% for 15.7 ∘ < < 178 ∘ and within 5% for < 5 ∘ and = 179 ∘ . These comparisons indicate that our calculation results are highly consistent with the MC results of Williamson and Li and Karaiskos et al.
The anisotropy functions in the three tissue types are shown in Figure 7. Figure 7(a) plots our calculation results for 192 Ir in lung tissue as an anisotropy function, with the data showing that ( , ) is independent of the radial distance. However, ( , ) was closer to 1 in bone tissue for all polar angles when radial distance increases, as shown in Figure 7(b). Figure 7(c) presents our anisotropy functions in the three tissue types at = 5 cm for comparison. The anisotropy function increases due to the increasing contribution of scattered component in medium which compensate for the attenuation of primary radiation. ( , ) in bone resembles a point source due to the scattered components being larger than those in water and lung tissue. This study used MCNPX code to calculate the dose parameters in bone and lung tissue for clinical reference; the anisotropy functions in bone and lung tissue are listed in Table 3.   Figures 8 and 9. For the radial dose function, the maximum difference between the measured value and the MCNPX results for water was 11.4% at = 0.5 cm. This difference was due to the high dose  This study also used MCNPX code to calculate the 2D dose distribution for the three tissue types, as shown in  and that the rate of dose attenuation was the fastest among three different types of tissue (due to the linear attenuation coefficient).

Conclusions
This study used MCNPX code to calculate the 192 Ir dose distributions in water, bone, and lung tissue. The dose parameters for water were highly consistent with the MC results of Williamson and Li and Karaiskos et al. For > 3 cm, the results from the MCNPX code and GD-301 dosimeter measurements for bone and lung tissue were highly consistent. The results demonstrate that the dose distribution of HDR brachytherapy differed in water, bone, and lung tissue. The TPS, which currently does not take into account such differences in tissue density, thus, overestimated the dose by up to 12% in lung tissue near the source. Moreover, the magnitude of the attenuation and scatter would vary with the tissue density. The radial dose functions would depend not only on the phantom size but also on the phantom density.    The dose-rate constant, radial dose function, and anisotropy function have been calculated for the 192 Ir microSelectron source in water, bone, and lung tissue. These dose parameters can be used as clinical reference data and to improve the accuracy of the doses delivered during HDR brachytherapy.