Comparison of (60)Co and (192)Ir sources in HDR brachytherapy

Abstract

This paper compares the isotopes (60)Co and (192)Ir as radiation sources for high-dose-rate (HDR) afterloading brachytherapy. The smaller size of (192)Ir sources made it the preferred radionuclide for temporary brachytherapy treatments. Recently also (60)Co sources have been made available with identical geometrical dimensions. This paper compares the characteristics of both nuclides in different fields of brachytherapy based on scientific literature. In an additional part of this paper reports from medical physicists of several radiation therapy institutes are discussed. The purpose of this work is to investigate the advantages or disadvantages of both radionuclides for HDR brachytherapy due to their physical differences. The motivation is to provide useful information to support decision-making procedures in the selection of equipment for brachytherapy treatment rooms. The results of this work show that no advantages or disadvantages exist for (60)Co sources compared to (192)Ir sources with regard to clinical aspects. Nevertheless, there are potential logistical advantages of (60)Co sources due to its longer half-life (5.3 years vs. 74 days), making it an interesting alternative especially in developing countries.

Keywords: 192Ir; 60Co; HDR brachytherapy; radionuclides.

Fig. 1

Encapsulated line source, illustrating calculation of the dose rate at point P(r,θ) = P(x,y) relative to the source centre. The source length and encapsulation thickness are denoted by L and t [15]

Fig. 2

Differences in absorbed dose of several tissues for a ⁶⁰Co source in comparison to a ¹⁹²Ir source [16]

Fig. 3

Anisotropy factor Φ(θ) for the ⁶⁰Co and ¹⁹²Ir source [16]

Fig. 4

Radial dose function of ⁶⁰Co and ¹⁹²Ir source [16]

Fig. 5

Radial dose functions for greater distances from the source in an infinite phantom [16]

Fig. 6

Dose distribution for ⁶⁰Co and ¹⁹²Ir source of a ring-shaped and linear applicator for the irradiation of a cervical carcinoma (upper half depicting ¹⁹²Ir, lower half ⁶⁰Co) [16]

Fig. 7

Radial dose functions of the different HDR sources. The radial dose function of the MicroSelectron classic ¹⁹²Ir source was modified to simulate a similar phantom condition as the ⁶⁰Co source [17]

Fig. 8

Anisotropy functions for the different HDR sources at the distance of (A) 0.5 cm, (B) 1 cm and (C) 5 cm [17]

Fig. 9

Results of the Monte Carlo calculations for the ¹⁹²Ir and ⁶⁰Co isotope in the range of 10 < r ≤ 60 cm. In the range of 0 ≤ r ≤ 10 cm, results are presented from the model of Meisberger. The lines represent a fit through the Meisberger and the Monte Carlo data points, according to the model of Kornelsen [19]

Fig. 10

Results of the measurements and the calculations for an infinite sized phantom, for ¹⁹²Ir, ¹³⁷Cs and ⁶⁰Co. Results in the range of 10-60 cm obtained by measurement are presented as points. Lines are used for the Monte Carlo results [19]

Fig. 11

Isodose curves in cGyh^–1 U^–1 for the old and the new Bebig HDR source. The background is the ratio between the kerma rate distribution [20]

Fig. 12

Isodose curves in cGyh^–1 U^–1 for the HDR and the PDR source [21]