Details about brachytherapy basics and comparison between Cobalt 60 and Iridium 192 radioactive sources.

1. Sambasivaselli R
HDR BRACHYTHERAPY
Co-60 vs Ir-192

2. Brachytherapy
• short-distance treatment of malignant disease
with radiation emanating from small sealed
(encapsulated) sources.
• The sources are placed directly into the
treatment volume or near the treatment
volume

3. IMPLANTATION
• Sources are placed into body cavities close to
the tumour volume
Intracavitary
• Sources are implanted surgically within the
tumour volume
Interstitial
• Sources are placed over the tissue to be treated
Surface (mould)
• Sources are implanted into the target tissue
during sugery
Intraluminal
• A single source is placed into small or large
arteries
Intravascular

4. Treatment Duration
• Temporary
– Dose is delivered over a short period of time and
the sources are removed after the prescribed dose
has been reached
• Permanent
– Dose is delivered over the lifetime of the source
until complete decay

5. Source loading
• Hot loading
– The applicator is preloaded and contains
radioactive sources at the time of placement into
the patient
• Afterloading
– The applicator is placed first into the target
position and the radioactive sources are loaded
later, either by hand (manual afterloading) or by a
machine (automatic remote afterloading)

6. Dose Rate
•0.4 – 2 Gy/h
LDR
•2 – 12 GY/h
MDR
•>12Gy/h
HDR

7. HDR Brachytherapy
• The use of high-dose-rate (HDR) afterloading
brachytherapy is a highly widespread practice
today.
• It has proven to be a successful treatment for
cancers of the prostate, cervix, endometrium,
breast, skin, bronchus,oesophagus, head and
neck and several other types of cancer.

8. Choices of Sources
• Half life
• Specific activity
• Photon energy spectrum
• Source strength
• HVL for sheilding
• Inverse square law dose fall off

9. • Half life
– The half-life is the time it takes for the source
strength to decay to half of its initial value
– determines how long it can be used over a period
of time
– Co60 5.26yr, I 125 59.5d, Cs 137 30 y, Ir192 73.8d

10. • Specific activity
– The specific activity is the ratio of activity
contained within a unit mass of the source
– Strength of therpay source depends on specific
activity
– Limited by neutron flux field strength, source half
life, parent neutron cross section
– Ir 192 has high neutron cross section

11. • Photon energy
– Energy determines penetrability
– High energy higher doses to tissues at larger
distances
– Requires thicker shielding
– Intracavitary sources require radial dose fun which
do not fall rapidly. Co 60, Cs 137 and Ir 192 would
be suitable

13. Dosimetry
• TG 43 Formalism
– Dose distribution is described in terms of a polar
coordinate system with its origin at the source
centre.

15. • Dose rate constant
– defined as the dose rate to water at a distance of
1 cm on the transverse axis (reference point) per
unit air kerma strength in water
– DRC accounts for
• Effects of source geometry.
• Spatial distribution of radioactivity within source
encapsulation.
• Self-filtration within the source.
• Scattering in water surrounding the source

16. • Geometry factor
– geometric falloff of the photon fluence with
distance r from the source and also depends on
the spatial distribution of activity within the
source
– 1/r2 for point source
– β/L r sinθ for line source

17. • Radial dose function
– effects of attenuation and scatter in water on the
transverse plane of the source , excluding falloff
which is included by the geometry function .
– Radial dose function g(r) may also be influenced
by filtration of photons by the encapsulation and
source materials.

18. • Anisotrophy factor
– accounts for the anisotropy of dose distribution
around the source, including the effects of
absorption and scatter in water.
– unity on the transfer plane.
– decreases in directions off the transfer plane
• As r decreases.
• As approaches 0o or 180o.
• As the source encapsulation thickness increases.
• As the photon energy decreases.

19. Co-60 vs Ir-192

21. • Reference Air Kerma Rate 24 mGy/h
– Dose-rate in:
• 1 cm ~240 Gy/h and 2 cm ~ 60 Gy/h

22. Co-60 vs Ir-192
• Treatment time
• Half life
• Specific activity
• Energy
• Dosimetry parameters
• Absorption in tissues
• Attenuation in larger distances
• Shielding
• Economical factors

25. Half life
• 5.26y vs 73.8d
– Based on source operation time limited to one
half-life for Co-60 and three months for Ir-192, the
irradiation time on average is only 1.7 times
longer for the cobalt source

26. Specific activity
• The maximum specific activity of Co-60 (41.91 GBq/mg)
is much lower than that of Ir-192 (340.98 GBq/mg)
• Ir-192 sources have an initial activity of 370 GBq (10 Ci),
while Co-60 sources have only 74 GBq (2 Ci).
• The difference in source strength is smaller: 22,645 vs.
40,820 cGy·cm2/h.
• Thus the treatment time for the same treatment plan
with a Ir-192 source is about 1.8 times shorter than that
with a Co-60 source, both sources having their initial
source strength

27. Fig. 1. Schematic diagram of three HDR sources: (a) microSelectron HDR classic 192
Ir (Williamson and Li, 1995), (b) Varian HDR new 192
Ir (Angelopoulos et al., 2000) and
(c) BEBIG HDR new 60
Co source (Granero et al., 2007).

28. Energy

29. Energy

31. Absorption in tissues
• Dose by Co-60 to adipose tissue is 0.4% higher but
0.8% lower for the rectum.
• The largest difference is reported for lung tissue
(density 0.26 g cm-3) showing a 2.1% discrepancy
• The biological response difference to the different
energies of the 60Co and 192Ir sources are negligible

36. Dosimetry parameters
Anisotrophy factor

38. Dosimetry parameters
Radial dose function
• the initial fall of the radial dose function of the
iridium source is less steep than the cobalt
source.
• This relation reverses at distances greater than
22 cm as a result of the higher photon energy
of 60Co.
• although the integral dose in the patient is
slightly lower, higher room shielding is
required for the use of 60Co sources

40. Fig. 2. The radial dose functions of several HDR sources used in this study. (a)
The
radial dose function of microSelectron classic 1 92
Ir source in a 15 cm radius water
phantom was modified to that in a 40 cm radius water medium.
Acomparison of dose distributions of HDR intracavitary brachytherapy using
different sources and treatment planning systems
Dong-wook Park, Young Seok Kim, Sung Ho Park ×, Eun Kyung Choi, Seung Do Ahn,
Sang-wook Lee, Si Yeol Song, Jong Hoon Kim
Department of Radiation Oncology, Asan Medical Center, Ulsan University, 388-1, Pungnap-2dong, Songpa-gu, Seoul, Republic of Korea

41. Sheilding

44. Economical Factors
Alfredo Polo MD, PhD
Brachytherapy & Intraoperative Radiotherapy Unit Radiation Oncology Department
Ramon y Cajal University Hospital Madrid. Spain

45. Conclusion
• + less number of source exchanges
• + less problems with logistic
• + less paperwork
• + less amount of dosimetry
• + less costs
• + less pronounced dose dip
• = identical source geometry
• = comparable dose rate
• = comparable dose distribution
• = comparable absorption in tissue
• - more complex radiation protection

46. Conclusion
• Economic aspects make HDR Co-60 an option
to be considered for brachytherapy
applications, with the same technical
performance as in HDR Ir-192
• Within the treatment volume, both sources
give similar dose distributions, thus existing
optimizations and inverse planning tools give
similar results

47. Conclusion
• Quality assurance instrumentation used for HDR Ir-
192 is compatible with HDR Co-60.
• Currently, source calibration traceability is well
established (Accredited Laboratories to User chain)
for HDR Ir-192
• obvious logistical advantages and potential savings
will encourage other companies to make
miniaturized 60Co sources available in the near
future.

48. Thankyou