Determination of dosimetric parameters for shielded ¹⁵³Gd source in prostate cancer brachytherapy

Prostate cancer is the most common cancer in men. In 2013, it was reported that the number of 2,850,139 men were living with prostate cancer in the United States.¹ The number of new cases of prostate cancer from the year 2009 to 2013, was 129.4 per 100,000 men per year and the number of deaths caused by prostate cancer was 20.7 per 100,000 men per year.¹ This cancer is observed with a higher prevalence in African-American men than in white men.² Brachytherapy is one of the common therapeutic methods for treatment of prostate cancer. This approach has shown successful outcomes in treatment of this cancer. The success of brachytherapy is due to advantages such as simple performance and reduced side effects of treatment compared to external radiation therapy and surgical removal of the tumor.^3-6 Brachytherapy is a treatment in which one or a number of covered radioactive sources are inserted at a short distance from the target. Brachytherapy is performed either as intracavitary procedure, in which the brachytherapy source is placed inside the natural body cavity, adjacent to the tumor. The second method is interstitial brachytherapy, in which brachytherapy seeds are implanted directly inside the tumor mass. With this treatment method, the required amounts of radiation dose can be delivered to the tumor with the rapid dose fall off to the healthy tissues around the tumor.

Originally, brachytherapy treatments were performed with ²²⁶Ra sources. Nowadays, the use of artificial radionuclides such as ¹²⁵I, ¹³¹Cs, and ¹⁰³Pd is rapidly increasing.^7-9 Presently, brachytherapy sources are widely used for treatment of various types of cancer patients. ¹⁵³Gd is a medium energy brachytherapy source emitting photons in the range of 40 to 100 keV. This isotope has a low dose rate and its half-life is 242 days. Interstitial rotating shield brachytherapy (I-RSBT) is a type of brachytherapy in which a shield is used to spare normal tissues from radiation damage. The rotating capability enables the user to select an emission angle to direct the radiation to a specified tumor. It can also be used in sequencing the rotating shield in dynamic rotational shield brachytherapy. With I-RSBT it is possible to have a significant dose reduction in urethra while delivering higher prostate organ dose. The main reason for the use of ¹⁵³Gd in I-RSBT method is that it requires less shielding within the source than other higher energy isotopes such as iridium.

The main goal of any radiation therapy treatment is to maximize the dose to the tumor while limiting the dose to adjacent normal tissue and organs. Therefore, shielding the sensitive organs around the target volume has always been one of the important issues. In brachytherapy of prostate and cervix, the sensitive organs such as the rectum and bladder should be protected to receive the radiation dose lower than their tolerance level. For the purpose (I-RSBT), the use of special cylindrically shielded source can reach the aim. The shield there is a platinum sleeve with an aluminium gap leaking the narrow source beam. The difficulty of manufacturing of a shielded source is in its limited size. Namely, the cylindrical source should be capable to enter the hollow brachytherapy needle with the diameter e.g. 1.6 gauge. Energetic sources (Ir-192) need more shielding thickness which prevents them entering the small diameter brachytherapy needles.

The report by task group No. 43 (TG-43) from American Association of Physicists in Medicine is known as the most common formalism for calculation of the dose distribution around brachytherapy sources. According to this protocol, dosimetric parameters around brachytherapy sources are obtained by experimental measurement or simulation techniques using the Monte Carlo codes in a uniform water phantom.^10-11 Monte Carlo is a computational technique for solving problems in various fields including physical and mathematical sciences. This method is based on random sampling to achieve the required results. Different Monte Carlo codes are currently used in medical radiation physics: MCNP, EGSnrc, GEANT4, SIMIND, etc. Monte Carlo N-Particle (MCNP) is specially designed for nuclear application of radiation transport but it also can be used for other fields, for example in medical physics applications. Monte Carlo codes are of high capabilities in particle transport physics lacking applications in practical medical physics dosimetric problems.^12-14 These methods can have desired results to assess dosimetric parameters such as air kerma strength, dose rate constant, radial dose function and anisotropy function of brachytherapy sources.^15,16 In a number of previous studies, dosimetric parameters of hypothetical brachytherapy sources were calculated prior to their fabrications in order to assess the feasibility of its fabrication. One of these sources is ¹⁵³Gd that has been introduced by Enger et al.¹⁷ Additionally, ¹⁵³Gd radionuclide has been recently used in nuclear medicine imaging systems e.g. linear scanners and quality assurance procedures such as phantom calibrations.

In radiotherapy the radiation dose destroys the tumor cells causing the healing of cancer and improving the patient’s quality of life. There is a need in prostate brachytherapy to reduce the dose to radiosensitive organs such as rectum and bladder reducing the dose side effects to these sensitive organs. Therefore, in a proper patient treatment plan the amount of absorbed dose to critical organs should be exactly specified and controlled.¹⁸

Interstitial rotating shield brachytherapy is a new type of high dose rate brachytherapy in which the radiation dose is delivered by shielded and rotating catheters. In I-RSBT a brachytherapy source has a well-defined shield with the capability of rotation around the source’s longitudinal axis to spare normal tissues. With I-RSBT the limitation that dose distributions should be symmetric around a brachytherapy source is resolved. Therefore, with this method there is the potential to deliver unrivalled dose distributions.¹⁹ In a recent study conducted by Adams et al.¹⁹, the interstitial rotating shield brachytherapy (I-RSBT) method was introduced for brachytherapy of prostate. In that study the method was presented to reduce the dose to rectum, bladder and urethra in I-RSBT with a ¹⁵³Gd source. They have demonstrated that brachytherapy with rotational shield has the potential to reduce the dose to organs at risk compared to traditional brachytherapy. The isodose curves around the shielded ¹⁵³Gd source were compared with the isodose curves of an¹⁹²Ir source. Additionally, they introduced the data related to an anonymous patient in a treatment planning system and compared dose volume histograms from conventional brachytherapy with ¹⁹²Ir source with those from ¹⁵³Gd based I-RSBT. However, in that study TG-43 dosimetric parameters of the source with shield were not investigated. Enger et al.¹⁷ introduced a hypothetical ¹⁵³Gd source and calculated TG-43 parameters for the source without rotational shield. In that study, the hypothetical ¹⁵³Gd source was introduced for potential use in I-RSBT. However, only TG-43 parameters were determined and the effect of rotational shield geometry on dosimetric aspects was not evaluated. In another study Ghorbani and Behmadi²⁰ calculated TG-43 parameters for the same hypothetical nonshielded ¹⁵³Gd source and compared them with dosimetric parameters of commercially available ¹⁹²Ir and ¹²⁵I sources. In the study by Ghorbani and Behmadi the effect of shielding was not considered, as it was not evaluated by the study of Enger et al.⁷ In the aforementioned studies TG-43 parameters of a shielded ¹⁵³Gd source were not evaluated.

Consequently the present study evaluates a ¹⁵³Gd source for use in the I-RSBT and its dosimetric parameters are obtained according to TG-43 formulation for use in prostate brachytherapy.

Air kerma strength per source activity (per GBq) for the non-shielded mode was obtained equal to 6.71 cGyh^-1cm²GBq^-1. Based on this value for the nonshielded ¹⁵³Gd source, 1 U is equal to 4.03 mCi. This value was found to be 0.04 cGyh^-1cm²GBq^-1 and 6.19 cGyh^-1cm²GBq^-1 for the Pt shield and Al window cases, respectively. The values of dose rate constant for the non-shielded mode was obtained as 1.20 cGyh^-1U^-1 and for the shielded mode for the Pt shield and Al window sides were achieved as 0.07 cGyh^-1U^-1 and 0.96 cGyh^-1U^-1, respectively. It should be noted that in calculation of dose rate constant for these three cases the dose rate at the reference point (at 1 cm and 90 degrees angle) in each case was divided by the air kerma strength value for the non-shielded source. It can be noted that in the shielded case the dose rate constant is reduced.

Radial dose function values for ¹⁵³Gd source are listed in Table 2. The first column there refers to the source without shield, the second column refers to the Pt shield side of the source and the third column refers to the Al window side of the source. The radial dose function values are plotted in the Figure 3B for the Pt shield side. Figure 4 shows shield the relative dose for the non-shielded mode of the source, the Pt shield and Al window sides of the shielded source. In this graph, for all three cases, the dose values have been normalized to the dose at 1 cm distance of the non-shielded source.

Anisotropy function values for the ¹⁵³Gd source the source were calculated for 0.5, 1, 2, 3, 5, 10 and 12 cm distances from the source at 19 different angles (ranging from 0 to 180 degrees) with angular intervals of 10 degrees. The results of anisotropy function are presented in Table 3 and Table 4 for the non-shielded and shielded cases, respectively. Figure 5A also shows anisotropy function values at different distances from the source for non-shielded mode. In Figure 5A anisotropy function for the non-shielded source is presented, in Figure 5B anisotropy function for the shielded source at Pt shield side and in Figure 5C anisotropy function for the shielded source at Al window side of the source is presented. In Figure 6 anisotropy function for three modes is presented. There are four graphs for distances of the evaluation point of the anisotropy function from the source: (A) 0.5 cm, (B) 2 cm, (C) 5 cm and (D) 12 cm distance.

Isodose curves (%) for the non-shielded source and the shielded source (Pt shield side) are plotted in Figure 7. In these plots the dose distributions were normalized to the dose at 1 cm distance from the source at transverse plane.

In this study TG-43 dosimetric parameters for a 153Gd source with Pt shield were calculated for use in I-RSBT of prostate brachytherapy. Air kerma strength value for the non-shielded 153Gd source was found to be 6.71 U per 1 GBq activity.

This means that 6.71 U of ¹⁵³Gd is equal to 27 mCi activity and 1U is equal to 4.03 mC activity. The results obtained in this study reveal that the ¹⁵³Gd dose rate constant is higher compared to the dose rate constant of ¹²⁵I source. This is due to the fact that average photon energy of ¹⁵³Gd is higher than that of ¹²⁵I (53.70 keV versus 28.37 keV). We found also that the simulated dose rate constant for non-shielded source is higher compared to the dose rate constant of shielded source. The reason for a higher dose rate constant in this case is the lower level of beam attenuation in non-shielded source compared to the shielded one. Furthermore I-RSBT source is two-part shielded (Pt shield and Al window Figure 1) where radiation attenuation at the Pt shield is higher compared to Al window. This effect can be explained by the values of the mass energy attenuation coefficients for Pt and Al, which are listed in Table 5. In the 153Gd photon energy range (40 keV to 100 keV), mass attenuation coefficient of Al is lower than Pt. Additionally there are also attenuations of the photons in the catheter and needle materials (Nitinol alloy of Ni and Ti elements) in the beam’s path (Figure 1). Finally the radiation beam attenuation on the Al window side of the source is lower compared to the other Pt shield side of the source which is the essence of the I-RSBT.

In Figure 3A it can be seen that the trend of radial dose function is falling with distance. According to this chart, at distances less than 1 cm, the red graph which corresponds to existence of Al window is higher than the blue graph (related to the source without shield). Beyond the 1 cm distance the value of radial dose function for the non-shielded mode is higher compared to Al window case. It is due to lower attenuation of photons in Al window material. The calculated treatment dose should normally include a factor accounting for the attenuation of the material. In a treatment situation the rotating source with Al gap is “directed” to the tumor side which means that effective emission dwell times are longer in that situation. Consequently the dose received by the tumor is relatively higher compared to the healthy tissue. As it can be seen in Figure 3B, radial dose function is increasing with radial distance, and this is due to the density of platinum. The distance could be plotted in terms of r×Pt density and the data be normalized to the dose rate at 1 cm physical distance from source center for the density corrected value. That way the trend of radial dose function may not be increasing.

In the calculation of radial dose function in Figure 3, each case was normalized to the dose value at 1 cm of itself. Based on Figure 3A the radial dose function in the Al window side is lower. This is due to higher photon attenuation in Al than in the water. Figure 3B, which corresponds to the Pt shield side, the radial dose function at the Pt shield side has increasing trend up to the distance of about 8 cm. In Figure 4 all the obtained dose values in three cases (non-shielded, Pt shield, Al window) were normalized to dose at 1 cm distance of the non-shield mode. With this calculation, there is evidence to have more accurate comparison. The graph in Figure 4 reveals the dependence of the source distance to the point of absorbed dose in the phantom. By increasing that distance the dose becomes lower for all three cases. Secondly, the dose in the case of Pt shield side is lower than at the Al window side. Additionally the dose at the Al widow source is lower than at the non-shielded source. These two effects are related to different absorption coefficients in Pt, Al and phantom. Pt has higher absorption coefficient than Al and Al has higher absorption coefficient than phantom (i.e water).

Figure 5A is related to the non-shielded source. Figure 5B is related to the Pt window source. In case of Pt window that side of the source receive lower dose. It indicates that Pt side should be turned to the side of organs at risk which means that central areas of the organs will receive lower dose. According to Figure 5A and C it can be mentioned that anisotropy at Al window mode is lower than at the non-shielded mode of the I-RSBT source. This is due to the attenuation of radiation in Al. This effect is similar for the comparison of anisotropy of Pt shield side versus non-shielded case. In directions other than 90º, beam traverses through longer path inside the needle and source, therefore the beam attenuation is more pronounced. The higher beam attenuation results to lower doses at these angles. Calculating the anisotropy function we normalized it to the dose at 90º. Thus the lower isotropy at these angles can be explained. The lower isotropy in I-RSBT source is like a disadvantage, because it causes non-uniformity of tumor dose. The peripheral parts of the tumor receive systematically lower doses than the central ones. In the Figure 6, anisotropy function is further analyzed. We compared different shielding effects of the I-RSBT source at four different distances in the phantom. We compared the source without shield, Pt shielded source and the source with Al window. They were compared at each of the four distances (0.5 cm, 2 cm, 5 cm, and 12 cm). As it can be concluded from the charts, at 0.5 cm from the source the anisotropy functions for the non-shielded mode and Al Window mode are relatively the same. This is because of the similar magnitude of self-absorption in both modes. Increasing the distance from the source there is a change of energy spectrum of photons in the phantom. The differences become greater by increasing the distance from the source. The reason for anisotropy function equality at a distance of 0.5 cm for these two modes is their almost equal self-absorption.

In Figure 7, isodose curves are plotted for both non-shielded and shielded cases. As it can be seen in the shielded source considerable dose reductions occur at the Pt shield side. Thus the organs at risk located near the shield side of the source receive significantly lower dose. Organs that are located at 1 cm distance from the source (Pt shield side) receive 5% dose. This relative dose value is reduced further to 1% when the distance is approaching 4 cm. It should be noted that the shield should be turned to the side of organs at risk, such as the rectum and bladder. In prostate cancer the total organ volume is tumor which is covered with many irradiation source dwell positions. Thus the optimization of treatment plan can compensate eventual cold spots from the individual source dwell positions. As it was mentioned in the study of Adams et al.¹⁹, by adjusting the number of needles and dwell positions as well as the number of rotations per dwell position, prostate dose can be controlled. It seems that in I-RSBT treatment of prostate cancer of Adams et al.¹⁹ the radiation sensitive organs at risk are protected receiving the relative lower dose compared to tumor dose. As a consequence some parts of the tumor can receive lower dose which has to be compensated extending the affected irradiation dwell times in the treatment plan calculations. Overall radiation time is thus prolonged using this irradiation technique. The I-RSBT could be promising also for some other sites in brachytherapy like gynecology or breast brachytherapy. It is interesting as a subject of further research.

Additionally, the dose distribution around the source in shielded mode is less symmetrical compared to unshielded source which could be accounted as a disadvantage (Figure 7). However this can be compensated in treatment planning. This leads to “Intensity Modulated Brachytherapy” which is considered as an innovation at this time. Only the future will show us the real possibilities of these new techniques. As it was aforementioned in the text, Adams et al.¹⁹ have introduced the interstitial rotating shield brachytherapy using a ¹⁵³Gd source for brachytherapy of prostate. In the study they used Computed Tomography imaging to offer clinical data for the shielded source positioning and rotation in the treatment planning process. They used 19 needles and 16 rotational angles per dwell position of the ¹⁵³Gd source rotating shield for annonimized prostate patient. The dose-volume histograms from conventional brachytherapy with ¹⁹²Ir source were compared with those from I-RSBT with ¹⁵³Gd source. Our study continues the scientific efforts adding TG-43 simulated dose parameters which enable Treatment Planning Systems to calculate the I-RSBT doses in phantoms and tissues.

The calculated TG-43 dosimetric parameters in this study for a combination of the ¹⁵³Gd source and the shielded needle could be used in treatment planning system. Dose rate constant and radial dose function with the shielded sources are considerably different than the non-shielded sources. Further studies will be required to justify our Monte Carlo simulation parameters as dose calculation tools on a real human RANDO phantom. From clinical point of view there are some issues relating the tumor dose uniformity which is related to dose volume histograms. The I-RSBT source has lower dose isotropy which can lead to lack of uniformity distribution in the tumor. This can be compensated with new Treatment Planning Systems based on the live Monte Carlo calculations. These are topics for the future research. In our opinion I-RSBT technique has realistic chances in the ″Intensity Modulated″ development of future Brachytherapy.