INTRODUCTION

1. INTRODUCTION • Brachytherapy is a method of treatment in which sealed radioactive sources are used to deliver radiation at a short distance by • Interstitial • Intracavitary • Intraluminal • surface application • high radiation dose can be delivered locally to the tumor with rapid dose fall-off in the surrounding normal tissue

2. RADIOACTIVE SOURCES • In the past, brachytherapy was carried out mostly with radium or radon sources. Currently, use of artificially produced radionuclides such as 137Cs, 192lr, 198Au, 125I, and 103Pd is rapidly increasing

3. Radium • Decay • A filtration of at least 0.5 mm platinum provided by the source case is sufficient to absorb all the a particles and most of the b particles emitted by radium and its daughter products. Only g rays are used for therapy.

4. Cesium-137 • supplied in the form of insoluble powders or ceramic microspheres, labeled with 137Cs, and doubly encapsulated in stainless-steel needles and tubes • T1/2 = 30 yr • Decay rate (0.5)1/30 = 0.977, ~ 2.2% / yr • Eg = 0.662 MeV • The b particles and low-energy characteristic x-rays are absorbed by the stainless-steel material, so that the clinical source is a pure g emitter • Gd = 3.26 Rcm2 mCi-1h-1

5. Cobalt-60 • main advantages : high specific activity, which allows fabrication of small sources required for some special applicators • T1/2 = 5.26 yr • usually fabricated in the form of a wire that is encapsulated in a sheath of platinum iridium or stainless steel • can be used to replace 226Ra in intracavitary applications

6. Iridium-192 • Iridium-192 (alloy of 30% Ir and 70% Pt) sources are fabricated in the form of thin flexible wires which can be cut to desired lengths. • Nylon ribbons containing iridium seeds 3 mm long and 0.5 mm in diameter, spaced with their centers 1cm apart, are also commonly used. • Both the wires and the seed ribbons are quite suitable for the afterloading technique • has a complicated g ray spectrum with an average energy of 0.38 MeV

7. Iridium-192 • T1/2 = 73.8 day • Decay rate (0.5)1/74.2 = 0.99, ~ 1.0% / day • Gd = 4.69 Rcm2 mCi-1h-1,Many values have been cited because different spectroscopic data.

8. specific activity • specific activity: A / mass A = l N mass = N / NA× Aw A / mass = l N / ( N / NA× Aw ) = l NA / Aw • Specific activity l / Aw  1 / ( T1/2×Aw )

9. Gold-198 • Seeds or "grains" consisting of a radioactive isotope of gold, 198Au, are used for interstitial implants • have beenused for permanent implants until 125I seeds gained more widespread acceptance • T1/2 = 2.7 days • emits a monoenergetic g ray of energy 0.412 MeV • b rays of maximum energy 0.96 MeV are also emitted but are absorbed by the 0.1-mm-thick platinum wall surrounding the seed. • A gold seed is typically 2.5 mm long with an outer diameter of 0.8 mm

10. Iodine-125 • wide use for permanent implants in radiation therapy • The advantages : • long half-life (59.4 days) • low photon energy, 35.5-keV g photon, Characteristic x-rays in the range of 27 to 35 keV (electron capture decay ) • Gd = 1.464 Rcm2 mCi-1h-1, forunfiltered point source

11. Palladium-103 • Their clinical applications are similar to those of 125I • T1/2 = 17 days • decays by electron capture with the emission of characteristic x-rays in the range of 20 to 23 keV (average energy 20.9 keV) and Auger electrons

12. P(r,) P(r0,0) b r r0=1 cm 2  1 L t Dose Calculation Formalism: AAPM TG-43Med. Phys. 22, 209-234 (1995)

13. P(r,) P0(r0,0) r r0=1 cm  Comparison of external beam and brachy dose calculations P0(0,0,dmax) P(x,y,d)

14. The “Air Kerma Strength” Sk  source d The “Air Kerma Strength” ,Sk, is a measure of the brachy source strength. It is defined as the product of the air kermarateK(d) at a distance d in free space, measured along the transverse bisector of the source, and the square of the distance, d. In practice, Sk is calibrated at d0 = 1 m in air by the standard lab, but verified by the user using a well-chamber. The unit of Sk is denoted as U. 1U = 1 mGy m2 h-1 = 1 cGy cm2 h-1

15. The “Dose Rate Constant”  The “Dose Rate Constant” , is defined as the dose rate to water at a distance of 1 cm on the transverse axis of a unit air kerma strength source in a water phantom. source r0 = 1 cm ‘’ includes the effects of source geometry, the spatial distribution of radioactivity within the source, encapsulation, and self-filtration within the source and attenuation and scattering in water.

16. The “Geometry Factor” G(r,) P(r,) b r 2  1 L The “Geometry Factor” G(r,), accounts for the variation of relative dose due only to the spatial distribution of activity within the source, ignoring photon absorption and scattering in the source structure.

17. The “Geometry Factor” G(r,) – for L = 3.0 mm (?)

18. The “radial dose function” g(r), source r r0 = 1 cm The “radial dose function” g(r), accounts for the effects of absorption and scatter in the medium along the transverse axis of the source. It depends on the source (photon energies, source design, filtration by the encapsulated material, and the source material).

19. The “radial dose function” g(r),

20. The “radial dose function” g(r),

21. The “anisotropy function” F(r,) source r  r The “anisotropy function” F(r,), accounts for the anisotropy of dose distribution around the source, including the effects of absorption and scatter in the medium. It gives the angular variation of dose rate at each distance due to oblique filtration through the source.

22. The “anisotropy function” F(r,)

23. Summary of the TG-43 Formalism Sk is the air kerma strength of the source L is the dose rate constant. G(r,) is the geometry factor g(r) is the radial dose function F(r,) is the anisotropy function

24. Point Isotropic Source Approximation If a large number of seeds are randomly oriented, or if the degree of anisotropy around a single source is small, the dose rate contribution from each seed can be approximated by the average radial dose rate by integrating over all solid angles.