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

The data in Table 2.1 shows that the specific activity of radionu- clides is more or less inversely related to theier half-life. The val- ues show that, for example, the mass needed for a certain activity of iridium-192 with a half-life of 74 days is much smaller (by a factor of about 100) than for the same activity of cesium-137 with a half-life of 30 years. It is therefore not feasible to use ce- sium-137 as a similarly miniaturised source for brachytherapy applications. Note that, in order to be suitable for use in HDR brachytherapy, the specific activity is not the only factor that determines the ap- plicability of a given radionuclide. The yield , i.e. the number of useful photons per disintegration, the presence of beta emission, the energy of the emitted photons, and the radiochemical puri- ty also influence the suitability. Table 2.1 shows a significantly lower value of the specific activity of cobalt-60 vs. iridium-192 (41.91 vs. 340.98 GBq mg -1 ), but this is again compensated for by a factor of 3 because of the higher value of the gamma ray constant , Γ δ , of cobalt-60 (0.3059 vs. 0.1091 μGy h -1 MBq -1 m 2 ). As can be seen from the units, the gamma ray constant Γ δ is a measure of the emitted energy at a given reference distance from the source per unit of activity. Iodine-125 is mostly used in the form of seeds for permanent implants. The low photon energy is easily absorbed in layers of tissue, so that with deep seated implants (prostate) the risk of exposure to other individuals (caregivers, family members) is minimal. The photons emitted by palladium-103 have an even lower energy and this radionuclide has a shorter half-life. Appli- cations with palladium-103 therefore have a somewhat different radiobiological effect. The same holds for application of the ra- dionuclide cesium-131, which is not discussed here any further. Research of vendors is continuously directed towards new prod- ucts and new isotopes. Both ytterbium-169 and thulium-170 have elicited increasing interest. From Table 2.1 it is clear that the relatively low effective energy of the emitted photons is inter- esting from the point of view of radioprotection. Ytterbium-169 has a relatively short half-life which has implications for frequent source exchange, while thulium-170 has a relatively low value of the specific activity. The future will show if the application of these radionuclides in temporary applications (e.g., in HDR or PDR systems) is successful or not. Considering the overall numbers of patients treated with brachytherapy, the γ-ray-emitting sources are the most fre- quently used radionuclides. Tumor sites where brachytherapy using iridium-192 or cesium-137 is the treatment of choice or is part of this treatment are: uterine cervix, endometrium, vagina, urethra, oral cavity, oropharynx, bladder, prostate, oesophagus, bronchus, breast, lip and skin. For treatment of the prostate, large numbers of implanted iodine-125 or palladium-103 seeds are used, typically 60 to 80. For temporary applications for shallow lesions of the skin or the eye, a ß-emitter is sometimes used. ß-particles have a limited range when interacting with matter, depending on their energy. Their penetration in tissue is therefore limited in depth. Appli- cators employing the ß-emitters strontium-90 or ruthenium-106 are predominantly used to treat such shallow lesions, because the 50% isodose curve is at about 3 mm while these radionuclides are not used for lesions with extension over 5 mm in depth. Sources with a ß-emitter have been used for endovascular inter- vention techniques for prevention of vascular restenosis before chemically coated stents were introduced. Iodine-125 is intensively used in the form of seeds as well as to construct individual eye plaques. For distances from 1 up to

Table 2.5 Physical properties of radionuclides and their relevance for application in brachytherapy. From Table 5.3 in reference (3).

Physical property Radiation emitted

Relevance in brachytherapy

Source geometry and structure Determines if permanent or temporary implant or both are practical

Half-life, T

1/2

Specific activity Source size, dose rate Energy of emitted radiation Dose distribution within tissue, radiation protection requirements

Radiographic visibility/ localization, isotropy/anisotropy of dose distribution

Density and atomic number

20 mm, the dose distribution for a point source of iodine-125 is within 10% of the dose distribution from iridium-192 or ce- sium-137 sources. The low energy (<35 keV) allows gold or stainless steel foil to be used at the plaque edge to shield adja- cent structures (retina). The relative biologic effectiveness of io- dine-125, possibly from 1.2 to 1.4 compared with 250-kV X-rays, may be important for use with a radio-resistant tumor like mel- anoma. A summary of the physical properties discussed in this section of radionuclides and their relevance in brachytherapy is shown in Table 2.5.

4. SOURCE DOSIMETRY

4.1 Specification of source strength Several international organisations such as the ICRU (26,27), the AAPM in its report 32 (1), and some national organisations such as the BCRU (5), SFPH (51), NCS (38-40), DIN (14,15) have recommended the specification of the strength of γ-ray sources in terms of the quantity kerma rate to air at the point along the transverse axis of the source. The ICRU (27), ESTRO (17), IAEA (23) recommend the use of the Reference Air Kerma Rate for this purpose. In this chapter the acronym RAKR will be used occa- sionally for clarity. The RAKR , written as the symbol K . ref , is the kerma rate to air in air at a reference distance of 1 meter from the centre of the source, after correction for air attenuation and scattering. The quantity kerma , from kinetic energy released to matter , refers to the kinetic energy of charged particles, for example electrons and positrons, that have been liberated by uncharged particles such as by photons emitted by the brachytherapy source. Kerma does not include the energy that has been expended against the binding energies of these charged particles, even if this is usually a relatively small component. The name of the SI unit for kerma is gray (Gy), with 1 Gy = 1 J/kg. The quantity air kerma rate is the kerma per unit of time, K . ref , and is expressed in Gy s -1 or a multiple of this unit. The reference air kerma rate or RAKR is the air kerma rate at a defined reference distance, which is taken as 1 m in the reports cited above. For clarity, it is noted here that in AAPM task group reports, a slightly different approach was followed to specify the strength of brachytherapy sources, i.e. the air kerma strength , S K . S K was