2 Brachytherapy Physics-Sources and Dosimetry

Brachytherapy Physics: Sources and Dosimetry

17

THE GEC ESTRO HANDBOOK OF BRACHYTHERAPY | Part I: The basics of Brachytherapy Version 1 - 01/12/2014

as function of distance from the source for linear sources with lengths varying from 1cm tot 8cm, with the dose normalised at 1cm distance. With short lengths, the dose fall off is close to that of a point source, the inverse of the square of the distance (1/ r 2 ). With longer lengths, the fall off near the source approximates a fall off according to the inverse of the distance (1/ r ). As the dis- tance is increased, however, these effects of the long linear source approach those of the short linear source and of the point source and, therefore, its dose rate curve approaches the inverse square law. Two dimensional dose distribution, anisotropy Fig. 2.16 shows two 2-D representations of dose distributions for two source types with similar outer dimensions, on the bottom left side of the graph for a cesium-137 tube source, on the bottom right for radium-226, known for a large anisotropy effect. The upper part of the graph is calculated for a source of the same dimensions but with geometry effect only, with no account taken of anisotropy (16). Self absorption in the source material and in the capsule of the source is responsible for the deviation in dose delivery with distance at different angles around the source. The way the source is constructed is largely responsible for the effect of anisotropic dose delivery. In the TG-43 formalism the effects are taken into account in the anisotropy function F(r,θ) . Two dimensional dose distributions, stepping source and seed strings: equivalent length As stated previously, treatment planning systems calculate the dose distribution of a linear source by separating the source into a large number of small segments and then by superposition of the contributions from each segment. In practice, this is exact- ly what is done with dose delivery using a miniature stepping source technique such as used with pulsed or high dose rate afterloading equipment: the user has great freedom (although within some limitations depending on the design of the system) in the choice of dwell positions and dwell times of the source in each catheter. Another example is the technique of using seed ribbons as a replacement of a linear source of a given length. Commercial forms of stranded iridium-192 or iodine-125 seeds have been marketed. When using a stepping source technique or a combination of point sources, one can raise the question how to mimic the dose distribution of a continuously loaded line source. The active length, AL , of a uniform linear source (see the defi- nition and the examples for AL in the text and figure in section 3.3) that yields an isodose distribution in the region of interest equivalent to that of a uniform source line made up of discrete spaced sources is called the equivalent active length, EL , of the source line. An example is shown in Fig. 2.17. The equivalent active length EL (33) is the distance between the source centres, s , multiplied by the number of equal activity seeds (point sources, dwell positions), n :

Fig. 2.15 The dose in the transverse plane of a linear source as a function of distance to the source for lengths varying from 1-8cm. Dose is in all case normalised at a distance of 1cm.

Fig. 2.16 Anisotropic dose delivery from sources of the same outer dimensions. The upper part of the graph shows a dose distribution for a tube source calculated without a correction for scatter or absorption in the source material and capsule. At the bottom, the left side of the graph is for a cesium-137 tube source, the right side for a radium-226 tube. In the transverse plane the differ- ences are relatively small, but with self absorption at polar angle 0o, the influence is in some cases very large. See Dutreix et al. (16).

For lower energy photon emitting sources, the influence of scat- ter and absorption is considerably different, which is illustrated in the graphs shown in Fig. 2.13 with the radial dose function g(r) for a number of radionuclides. From these graphs it can be concluded that with high energy photon sources, the dose deposition near the source often can be approximated while ignoring the effects of absorbed radia- tion, as this is more or less compensated by the effects of scatter contributions. This is no longer true with low energy sources like iodine-125 and palladium-103. The spectrum of the most widely used iridium-192 radionuclide contains many low energy pho- tons, which explains the large contribution of scattered dose at distance, as illustrated in Fig. 2.14. Fig. 2.15 shows the influence of the physical form of a line- ar source on the dose distribution. The dose profile is shown

(2.22)

EL n s =

The relation between the source strength of the discrete sources and of the linear source is given by the formula

. K n • =

. K

(2.23)

seed

wire