Measurement and calculation of the dose at large distances from brachytherapy sources: Cs‐137, Ir‐192, and Co‐60

Abstract

In a small number of special cases it may be necessary to estimate the radiation dose to organs far away from the target volume of a patient receiving radiotherapy, e.g. the dose to the gonads or to the fetus of a pregnant patient. Previously, for external beam radiotherapy a model was developed, which enabled this estimation with acceptable accuracy for the range of photon beam energies from cobalt-60 to 25 MV, independent of the make of the treatment machines, for generally used techniques and for a wide range of distances from the beam axis and field sizes [Van der Giessen and Hurkmans, Int. J. Radiat. Oncol. Biol. Phys. 27, 717-724 (1993); van der Giessen, ibid. 30, 1239-1246 (1994); to appear in Int. J. Radiat. Oncol. Biol. Phys.]. In this paper, results of measurements and Monte Carlo calculations are reported for evaluation of the dose at large distances from some brachytherapy sources. This has resulted in information on the behavior of the tissue attenuation factor T(r) for commonly used sources: Co-60, Ir-192, and Cs-137, in the range of distances up to 60 cm from the source. The influence of the size of the phantom was investigated. A mathematical model, described earlier by Kornelsen and Young, proved to yield the best fit to the results [Br. J. Radiol. 54, 136 (1981)]. Parameters for this model are derived for these isotopes. A few experiments in a Rando Alderson phantom are presented to support the measurements and the calculations.

Study of the dosimetric differences between 192Ir and 60Co sources of high dose rate brachytherapy for breast interstitial implant

“…In this study, doses to organs at risk, like the heart, lung, skin, ribs were marginally lower (0.4-30%) with 60 Co as compared to 192 Ir as all these are within 20 cm from the implant. Venselaar et al 7 mentioned the dose value at a distance further than 25 cm are marginally higher than 192 Ir, we did not consider this a major disadvantage because of logistical and financial benefits of 60 Co. The higher integral dose to the distant organs/body may not cause any immediate concern, but one has to keep in mind the late consequences.…”

“…The dose deposition differences around single 60 Co and 192 Ir sources (anisotropy, radial dose function and relative isodose curves) have been reviewed by Strohmaier 5 who found no advantage or disadvantage for 60 Co sources compared to 192 Ir. However, this review is based on the available work by Venselaar, 7 Richter et al 8 and Park et al, 9 who confined their analysis to point dose and qualitative isodose comparisons.…”

Comparison of 60 Co and 192 Ir sources in HDR brachytherapy

“…The relationship between the tissue attenuation factor T ( r ) and the radial dose function g ( r ) is discussed in a report of the AAPM Radiation Therapy Committee Task Group No. 43 [18]. For point source geometry and energies > 300 keV, there is a very close relationship between the two functions, so that g ( r ) equals T ( r ) normalized at the distance of r 0 = 1 cm.…”

A Review on Main Defects of TG-43

“…Williamson 1991, showed 5%-10% differences at distances greater than 5cm from a 192 Ir source comparing the dose in an unbounded liquid-water and a phantom of of nearly 20*20*20 cm 3 (Williamson, 1991 (Venselaar et al, 1996). They obtained significant differences in the dose between experiments with different phantom sizes.…”