2 Brachytherapy Physics-Sources and Dosimetry

Brachytherapy Physics: Sources and Dosimetry

5

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

Table 2.1 Some properties of γ-ray-emitting sources discussed in this chapter. Only penetrating energies above 10 keV have been considered. The maximum specific activity is calculated for ideal pure point sources. Data were taken from the Table 5.2 of reference (3).

Photon energy range (MeV)

Effective energy, E eff (MeV)

Maximum specific activity (GBq mg -1 )

Γ δ . (μGy h -1 MBq -1 m 2 )

HVL (mm Pb)

Name

Symbol

Half-life, T ½

Z

A

Cesium Cesium Cobalt

Cs Cs Co Au

55 55 27 79 53 77 47 88 69 70

137 131

30.07 a 0.032-0.662 9.689 d 0.029-0.034

0.652 0.030 1.257 0.417 0.028 0.398 0.021

7.0

3.202

0.0771 0.0151 0.3059 0.0545 0.0348 0.1091 0.0361

0.030

3,809.13

60

5.27 a

0.347-2.159

12.0

41.91

Gold

198 125 192 103 226 170 169

2.695 d 0.069-1.088 59.49 d 0.027-0.035 73.81 d 0.061-1.378 16.991 d 0.020-0.497

2.8

9,055.12

Iodine Iridium

I

0.025

650.15 340.98

Ir

3.0

Palladium Radium Thulium Ytterbium

Pd Ra

0.008

2,763.13

1600 y

0.047-2.45

-

13.0 0.17 0.23

0.0366 221.07 893.29

-

Tm Yb

128.6 d 0.048-0.084 32.015 d 0.021-0.773

0.067 0.131

0.00053 0.0431

Note to the table: these data were taken from NIST Physical Reference Data and NUDAT vs. 2.0, both accessed in 2004 for the purpose of a book publication (3). Furthermore, data from ICRP 21 Figs. 50 and 51 were used (24). See Baltas et al. for the notes to the original table, the references, and further details (3).

Table 2.2 Some properties of the ß-ray emitting sources that contain yttrium-90, stron- tium-90 or ruthenium-106 radionuclides. Data were taken from Dutreix et al . (16) and Soares (50).

T period time

(

)

Maximum electron

2 1

Mass for 100 MBq (μg)

1/2

λ (s -1 )

Radio Nuclide

Half-life T ½

A

A

=

•

(actual)

calibratio (at

n)

(2.2)

Energy (MeV)

90 Y

64.1 h 3.01 10 -6

0.005

2.28

90 Sr / 90 Y *

28.2 y

7.60 10 -10

19.2

0.54, 2.28

In these formulae time period = (actual time – time at calibra- tion) . If the half-lives from the following Table 2.1 and Table 2.2 are converted and expressed in s, then λ values can easily be found using the equation (2.1) given above. Table 2.1 shows the γ-ray and Table 2.2 the ß-ray emitting radio- nuclides that are mentioned in the following chapters, and some of their properties. To correct for the decay of a radioactive source to obtain an ac- tual dose rate value at the time of application (or: the activity of that source), the initial dose rate at the time of calibration must be multiplied by a decay factor. For instance, for a time period of 24 days between the time of calibration of the source and the time of its clinical application, the decay factor for an iridium-192 source with a half-life of 74 days is 0.80 (see Table 2.3). For iridium-192 sources, the correction to the dose rate for decay is very close to 1% per day. For iodine-125 with a half-life of about 60 days, one needs to correct for the decay only slightly more than for iridium-192 (see Table 2.4). For a treatment duration in brachytherapy which is much short- er than the half-life of the given radionuclide, the calculation of treatment time can be performed assuming that the activity or the source strength remains constant. For sources such as ce- sium-137 with very long lives this will be the case. Due to the long half-life of 30 years, the activity of cesium-137 sources can adequately be corrected for decay twice a year only during its clinical use. For cobalt-60 with a T 1/2 of about 5 years, a monthly correction of about 1% is needed. When source decay is short compared to the duration of the application, it may be necessary to take into consideration the

106 Ru / 106 Rh*

372 d 2.19 10 -8

-

3.55

* the two nuclides are in radioactive equilibrium. λ = ln2 / T 1/2 , see text for a definition.

Table 2.3 Decay factor for iridium-192.

Iridium-192

Days

0

2

4

6

8

0.94 0.86 0.78 0.71 0.65 0.59

0.93 0.84 0.77 0.70 0.64 0.58

0

1.00 0.91 0.83 0.75 0.69 0.63

0.98 0.89 0.81 0.74 0.67 0.61

0.96 0.88 0.80 0.73 0.66 0.60

10 20 30 40 50

Table 2.4 Decay factor for iodine-125.

Iodine-125

Days

0

2

4

6

8

0.93 0.83 0.74 0.66 0.59 0.52

0.91 0.81 0.72 0.64 0.57 0.51

0

1.00 0.89 0.79 0.71 0.63 0.56

0.98 0.87 0.77 0.69 0.61 0.55

0.95 0.85 0.76 0.67 0.60 0.53

10 20 30 40 50