2 Brachytherapy Physics-Sources and Dosimetry

Brachytherapy Physics: Sources and Dosimetry

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

errors, for example when the effective activity is measured from dose rate in air by the vendor while the user of a given calcula- tion system has included another Γ δ value. The TG-43 defines instead a simple approach to the dose cal- culation with a unequivocal set of dosimetry parameters to be used. Its introduction has led to significant improvements in the standardisation of both dose calculation methodologies as well as dose rate distributions used for clinical implementation of brachytherapy. While dataset standardisation is a key element for acquisition of high-quality clinical results, dosimetry parameter datasets used in the AAPM TG-43 dosimetry formalism are obtained in a liq- uid water phantom with a fixed volume for radiation scattering. Consequently, the formalism does not readily account for several important dosimetric aspects, which still undermine accuracy of clinical application of the TG-43 formalism with these datasets. The mass-energy absorption coefficient, μ en / ρ , as a function of photon energy and atomic number in the conventional cal- culation formalism may be used to account for differences in absorbed dose between water and tissue. However, TG-43 pa- rameters are defined in water and therefore differences in tissue composition are not included. Similarly, the mass-attenuation coefficient as a function of photon energy and atomic number may be used to approximate differences in radiation attenuation between water and tissue, which is not included in TG-43. A situation that shows the problem directly is the effect of miss- ing tissue when sources are implanted near the body surface: there is no radiation scattered backwards that reduces the dose near the surface compared to a full scatter situation. So, a TG-43 approach lead to a too high dose estimate in the superficial lay- ers. As lower energy photons will show more scattering behav- iour, the effect is larger with these sources compared to high en- ergy photon emitting sources. Note that iridium-192 has a high effective energy (Table 2.1), but also many low energy photons in its complex spectrum. For multi-source clinical applications, inter-source shielding may be significant. Applicator radiation interactions and appli- cator shielding may detract from the accuracy of TG-43 based dose calculations. The photoelectric effect is largely responsible for the differences in comparison to water. The effect of phantom dimensions and volume has been analysed using Monte Carlo dosimetry and was reported in the literature as early as 1991. For high-energy photon-emitting brachythera- py sources such as iridium-192, dose differences greater than 5% are possible within 10 cm of the source. Finally, subtleties associated with dose calculation include the assumption of equivalence of absorbed dose and kerma. Fur- thermore, in brachytherapy dosimetry with current treatment planning systems, the dose contribution from betas emitted upon radionuclide disintegration is ignored. As a summary of the dosimetric concerns with TG-43, one can say that the absorbed dose in water is about –4% and –2% com- pared to tissue for low- and high-energy photons, respectively. Per centimeter, the attenuation of high-energy photons is about the same between water and tissue. However, there are signifi- cant differences in attenuation for low-energy photons, increas- ing as photon energy decreases. The presence of high- Z materials can substantially alter dose distributions for low-energy photons.

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Fig. 2.17 The active length of a uniform linear source that yields an isodose distribution in the region of interest equivalent to that from a uniform source line made up of n discrete spaced sources. (A) Definitions of active length, AL , equivalent active length, EL , and separation be- tween seeds, s . There is equivalence when AL = EL = n x s . (B) Isodose distribution for a 6 cm wire length and a seed ribbon consisting of 6 seeds, 4 mm long and 1 cm spacing

As a rule of thumb, equivalence in dose distribution exists at dis- tances from the axis of the source line more than half the inter- seed spacing s , as can be seen in Fig. 2.17.

5.5 Advantages and limitations in the use of the formalisms There are several advantages associated with the use of the TG- 43 formalism in brachytherapy dose calculations above the use of the conventional formalism discussed above (46). Although the conventional formalism is straightforward, there is, first, a possible confusion between the quantities of the source strength to be used; the real and the effective , assumed , or appar- ent activity. Secondly, we have, at least quantitatively, a poorly defined medium that is related to the value of the ϕ(r) func- tion. Its values differ for water and tissue and for materials with strongly differing composition such as bone and lung. Then, gamma ray constant Γ δ values must be taken from textbooks or other resources where it is easy to find large (~25%!) disparities in the tabulated values. This is apparent when trying, for exam- ple, to list Γ δ values from the literature for a radionuclide with a complex spectrum such as iridium-192. These flaws may lead to