2 Brachytherapy Physics-Sources and Dosimetry

Brachytherapy Physics: Sources and Dosimetry

14

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

and for the 1-D formalism (point source approximation):

Terminology is at first glance quite different from the conven- tional formalism. First, according to the AAPM TG-43 defini- tion (19), the quantity S K is introduced as the air-kerma strength of the brachytherapy source. S K has units U = cGy·cm 2 ·h –1 and as stated previously, its measured value should be traceable to a primary standards dosimetry laboratory (PSDL). Numerically, S K equals K . ref expressed in μGy.m 2 .h -1 . The dose rate constant Λ has units of cGy h −1 U −1 such that

( r D

) , θ

θ

d

sin (

) θ

0

(2.21)

( ) r =

ϕ

an

) 0 , θr

D 2

(

All these dosimetry parameters are explained in greater detail in the 2004 AAPM TG-43U1 report (44) and in several text books (3, 9, 17, 49), with reporting criteria and good practice recom- mendations for dosimetry investigators obtaining these param- eters using measurements and/or Monte Carlo (MC) methods. Fig. 2.10 and Fig. 2.11 demonstrate great similarity in values of the parameters g(r) and F(r,θ) over a wide range of distances (with 20 cm being well outside the patient) and angles. This is ob- served for different source designs with the same radionuclide. Nevertheless, in the proximity of the sources and in the length direction (polar angle less than 10°) there are clear differences. 5.3 TG-43 dosimetry parameters It is important to note that the parameters used in the equations of the TG-43 formalism are specific for a given source mod- el. A user may decide to determine these parameters through measurement or otherwise (calculation), but this is not a recom- mended procedure. Measurement can for example be performed using TLD, but it is rather difficult to obtain accuracy in the or- der of 1-2% with this measurement technique as is required for a low uncertainty. A similar comment can be made on the use of Monte Carlo calculation techniques, which also requires an ex- perienced user of the MC techniques. The question is then how to obtain these parameters, or –in the case of acceptance of a cal- culation model in which the TG-43 formalism is followed- how to validate that the correct data are used. As with source strength verification before clinical use of a brachytherapy source, a user should validate before use the presence of a correct set of TG- 43 parameters provided by a vendor for a given brachytherapy treatment planning system. Often there are several dosimetry publications about a given brachytherapy source model, and thus there is potential for var- iable administration of dose with clinically-relevant ramifica- tions. To obviate this problem, the AAPM prepares consensus dosimetry datasets for use in brachytherapy treatment planning systems. Consensus datasets for LDR iodine-125 and palladi- um-103 sources have been issued by the AAPM TG-43U1 and TG-43U1S1 reports (44, 45). These datasets are incorporated by TPS software vendors as machine data , analogous to exter- nal beam treatment planning data. Potential for confusion about which data to use could cause variability in clinical practice and unnecessary variations in administered dose or even seri- ous dose-calculation errors. A brief description is provided of the main publicly-accessible archives or databases of dosimetry datasets to provide guidance on dosimetry parameter choice. To provide guidance on dataset choice, recommendations to medi- cal physicists are presented below: a) AAPM-recommended consensus data to be used directly from the TG-43U1 and TG-43U1S1 reports (44, 45), b) data to be taken from the joint AAPM/Radiological Physics Center (RPC) Source Registry if available, using data in the original publications to check for agreement (43), c) if a given source model is not posted on the joint AAPM/RPC Source Registry, this suggests that it may not have met the

r

θ

D (

)

0 0 ,

since the other dosimetry parameters take

=

Λ

K S

values of unity at the reference point, P( r 0 , θ 0 ).

Dose rate variations on the source transverse-plane are account- ed for by the geometry function, G , and radial dose function, g . The geometry function G P for the simple form of a point- source (in the point-source approximation) is the inverse-square

2

r

0

dependence of the dose rate:

G

=

P

r

and corresponds to the 1-D formalism as is most commonly used in brachytherapy treatment planning systems for LDR low-en- ergy photon-emitting sources such as iodine-125. However, the 2-D formalism approximates the distribution of radiation emissions as a line-segment L (line source approximation) and is most commonly used for HDR high-energy photon-emitting brachytherapy sources such as iridium-192 or cobalt-60. Here,

β . r . sin

θ if ≠ 0 °

θ ( )

L

=

(2.17)

G L

1

r 2 L 2 / 4 ( )

θ

if

= 0 °

The point-source radial dose function g p (r) (the equivalence of the use of the effective attenuation function ϕ(r) in the conven- tional formalism) is

2

( ) 0 r ( ) r

r

D

(2.18)

g r

( )

=

P

D

0 r

whereas the line-source radial dose function, g L (r)

) 0 , θr

( ) 0 0 , L θ G L G r ( ) 0 , θr

D

(

(2.19)

g r

( ) L

=

) 0 0 , θ r

D

(

For calculating dose rate off the transverse-plane where dose an- isotropy from brachytherapy sources comes into play, the 1-D anisotropy function ϕ an (r) or 2-D anisotropy function F(r,θ) is used. F(r,θ) is defined for the line-source approximation accord- ing to the following formula:

) , θr

( ) 0 , L θ G L G r (

D

(

(2.20)

F

( ) , θr

=

) 0 , θr

D

(

) , θr