**9. Results and discussion**

**Table 2** illustrates the results obtained in this study for both the air-kerma strength and the dose rate constant for the two studied sources compared with the quoted results in previous studies. The value calculated for the air-kerma strength for the 60Co source was 3.030 0.002 (10<sup>7</sup> U Bq<sup>1</sup> ), this value agrees well with the value 3.039 0.004 (\*10<sup>7</sup> U Bq<sup>1</sup> ) obtained by (Anwarul et al., 2012) in their study [4]. Also, H. Badry et al., (2018) obtained the value 3.042 0.007 (\*10<sup>7</sup> U Bq<sup>1</sup> ) in their work [5], and Guerrero et al., (2014) found the value 3.046 0.0070 (\*10<sup>7</sup> U Bq<sup>1</sup> ) with a maximum difference of 1.6%. For the 192Ir source [3], the value obtained for the air-kerma strength, which is 1.092 0.004 (\*10<sup>7</sup> U Bq<sup>1</sup> ), was compared with the available quoted value from Perez-Calatayud et al., (2012) [1], the difference was within 0.1%.

For the constant of dose rate, we found for the 60Co a value of 1.092 0.001 cGy h<sup>1</sup> <sup>U</sup><sup>1</sup> . This result was compared with the published data quoted in **Table 2**, and we found a maximum difference of 0.5% with Granero et al. (2007) and Anwarul et al., (2012) [2, 4]. The result found for the 192Ir source was 1.109 0.004 cGy h<sup>1</sup> <sup>U</sup><sup>1</sup> , compared with the published data we found a maximum difference of 0.1% with the results obtained by Perez-Calatayud et al., (2012) and Granero et al. (2005) in their work [1, 6].

The radial dose function obtained for the 60Co source in this work (**Table 3**) was in good agreement with the obtained results in other studies using different Monte Carlo codes, especially, for distances (>1 cm) (**Figure 2a**). The **Figure 2b** represented the ratio gL(r) reference /gL(r) this work calculated to evaluate the deviation of our results from the published data. We observe for the distance greater than 1 cm a maximum relative difference of 0.94% compared with H. Badry et al., (2018) [5]. For the near distance to the source, a maximum relative difference of 6% was found compared with Guerrero et al., (2014) [3], 2.14% compared with H. Badry et al., (2018) [5], and 1.65% compared with Granero et al., (2007) [2]. These results can be attributed partially to the variety of the physics models for radiation transport used in each Monte Carlo code, on the one hand. On the other hand, it can be assigned to the differences in simulated geometries impact. For the 192Ir source (GI192M11), the obtained radial dose function in this work using MCNPX is presented in **Figure 3a**. The comparison with previous works, for the range of distance from 0.25 to 20 cm, was performed using the expression gL(r) reference/gL(r) this work presented in the **Figure 3b**. For the distance near to the source, we observe a maximum relative difference of 0.40% compared with D. Granero et al., (2005) [6]. For distances greater than 1 cm, the maximum relative difference found was 0.74%. The comparison in the case of the 192Ir was also made with the results obtained for the BEBIG Ir2. A852 source model. The maximum difference was found to be within 1.51% compared with D. Granero et al., (2008) [7] and 0.50% if the comparison is made with Belousov et al., (2014) [8].

The radial dose functions investigated using MCNPX for both 60Co and 192Ir were compared between each other by calculating the ratio gL(r) Co-60/gL(r) Ir-192 illustrated

#### **Figure 2.**

*(a) The curve of radial dose function for 60Co (Co0.A86) obtained with MCNPX compared with the published data for the same model source. (b) The curves of ratio gL(r) This work/gL(r) reference for 60Co (Co0.A86) obtained with MCNPX compared with previous studies for the same source model.*

in **Figure 4**. We observe that the radial dose function calculated for 60Co decrease faster than the 192Ir radial dose function**.** This difference between the two radial dose functions reached 10% for the distance of 9 cm. This makes the absorbed dose around the two sources different. In addition, regarding the slow decreases of radial dose function for the 192Ir source, we conclude that the 192Ir source could deliver a bit overdoses to the organs at risk more than the 60Co source, especially for tumors of high dimensions in gynecological applications.

The 2D along & away dose rates per unit of air-kerma strength were investigated for the BEBIG 60Co and 192Ir using the same geometry of detectors as mentioned before. The results obtained are tabulated (**Tables 4** and **5**), compared with the

**Figure 3.**

*(a) The curve of radial dose function for 192Ir (GI192M11) obtained with MCNPX compared with the available published data. (b) The curves of ratio gL(r) This work/gL(r) reference for 192Ir (GI192M11) obtained with MCNPX compared with the previous studies for the same source model, and the source model Ir2.A852.*

published data, the results are in good consistency. A comparison was made for the maximum dose rate per U, located in the distance 0.25 cm away in the transversal axis. For the BEBIG 60Co source, we obtain a maximum dose rate per U, which is 16.98 cGy h<sup>1</sup> U<sup>1</sup> agreeing well with value found by H. Badry et al., (2018) [5], which is 16.55 cGy h<sup>1</sup> U<sup>1</sup> . Furthermore, Granero et al., (2007) [2] found a value of 15.15 cGy h<sup>1</sup> U<sup>1</sup> . Otherwise, For the BEBIG 192Ir source, we found a value of 15.41 cGy h<sup>1</sup> U<sup>1</sup> in this study, and Granero et al., (2005) [6] obtained

#### *Dosimetry*

#### **Figure 4.**

*The curve of the ratio gL(r) 192-Ir/gL(r) Co-60, calculated to compare the differences between the results obtained for gL(r) for both sources Co0.A86 and GI192M11.*

15.50 cGy h<sup>1</sup> U<sup>1</sup> in their work for the same model. Moreover, Granero et al., (2006) [26] found in their study for Flexisource 192Ir HDR source model a value of 15.56 cGy h<sup>1</sup> U<sup>1</sup> . Also E. Reys et al., (2016) found 15.57 cGy h<sup>1</sup> U<sup>1</sup> in their work for the GammaMed HDR Plus 192Ir source model [24].

For the comparison made between the results obtained in this work for both BEBIG sources, we observe that the generated 2D along & away dose rates per unit of air-kerma strength for the near distance to the source are greater for the 60Co than for 192Ir. On the contrary, for distances greater than 1 cm, we observe that the values calculated for the 192Ir source are a little greater than for 60Co source, this difference increases by increasing the distance away in the transversal axis, **Figure 5**. Regarding the contribution of different dosimetric parameters in the treatment planning systems, this difference can be considered negligible within the agreement, concerning the clinic practice for the treated volume [12].

Outside of the treated volume, a study made by Venselaar et al. (1996) mentioned that the absorbed dose in peripheral organs at risk showed opposite behavior ( 192Ir doses >60Co doses) at distances near the treated volume in contrast to the behavior (192Ir doses <60Co doses) at larger distances [13]. In addition, recent study of dose delivered to organs has been calculated on a reference male phantom for a typical implant of the prostate in HDR brachytherapy using Monte Carlo method [14]. For the closest organs, equivalent delivered doses by 60Co were less (8–19%) than for 192Ir. However, increasing the distance beyond 10 cm, high equivalent doses were delivered by 60Co. The overall result is that effective doses per clinical absorbed dose from a 192Ir source are about 18% greater than from a 60Co source [14].

### **10. Conclusion**

In conclusion, the minor differences on the absorbed dose around the two sources observed in the radial dose function decrease and the 2D along & away dose rate per unit of air-kerma strength. For 60Co and 192Ir is considered negligible within the agreement by the specialists evaluated the use of 60Co in the afterloading devices as

*Comparative Dosimetric Study between 60Co and 192Ir BEBIG High Dose… DOI: http://dx.doi.org/10.5772/intechopen.102435*

#### **Figure 5.**

*A comparison between dose rate per unit of air-kerma strength of 60Co (Co0.A86) and 192Ir (GI192M11) sources in different away distances (a = 0.25 cm, b = 1 cm, c = 3 cm, d = 5 cm, e = 7 cm).*

192Ir equivalent. Their studies show that there are no significant differences between the two sources concerning the prescribing dose in a typical brachytherapy applications, neither in the treatment planning nor isodose distributions to target on the one hand. On the other hand, economic aspects make the 60Co an important option for clinics over the world. The recent introduction of miniaturized 60Co sources by Eckert & Ziegler BEBIG is considered as a mutation for this nuclide in HDR brachytherapy. The previous study announced that 60Co sources have potential logistical advantages and replacement intervals due to decay. One exchange of the 60Co source required 25 source exchanges for 192Ir, and this reduced operating costs, and makes 60Co a good option to be considered for applications in brachytherapy HDR, especially for the developing countries.

*Dosimetry*
