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Isotopic Teletherapy Machines




                  DR ARNAB BOSE
                  Dept. of Radiotherapy
                ...
 Treatment machines incorporating γ ray sources for
use in external beam radiotherapy are called
isotopic teletherapy mac...
 The important characteristics of radioisotopes in
external beam radiotherapy are:

1.   High γ ray energy
2.   High spec...
 For practical purposes, Co-60 and Cs-137 are the only
isotopes which satisfy these requirements to a sufficient
extent.
...
Bryant Symons radium
 "bomb" at Westminster
 Hospital, London, England,
 in the 1930s.




Until 1951, all isotope machin...
Telecaesium Unit
with applicators




 For telecaesium units the source to skin distance is
20cm to 40cm. They have not b...
 The invention of the 60Co
teletherapy unit by H.E. Johns in
Canada in the early 1950s
provided a tremendous boost in the...
 The development of nuclear reactors in the late 1940s
made possible the production of small 60Co sources with
specific a...
 The main components of a teletherapy machine are:

1. a radioactive source;
2. a source housing, including beam collimat...
Source
                  head
                           Gantry

                                    Main
                ...
Hand control   Control console




                                 11
Source

 Natural cobalt is a hard, stable, bluish-gray, easily
breakable metal. Its atoms contain 27 protons, 32 neutrons...
 The Cobalt-60 source is produced by irradiating
ordinary, stable 59Co with neutrons in a nuclear reactor.
The nuclear re...
 These γ rays constitute
the useful treatment beam.

The β particles are
absorbed in the cobalt
metal and stainless steel...
The 60Co source, usually in
the form of solid cylinder,
discs, or pellets, is contained
inside a standard stainless
steel...
Source Housing
 A typical teletherapy 60Co source is a cylinder of
diameter ranging from 1.0 cm to 2.0 cm and is position...
17
 A number of methods have been developed for moving
 the source from OFF position to ON position-

1. Source mounted on a...
3. Mercury is allowed to flow into a container immediately
   below the source to shut OFF the beam.

4. Source is fixed i...
20
 Typical source activities are of the order of
5000–10 000 Ci (185–370 TBq) and
provide a typical dose rate at 80 cm from...
 When the source is in the beam OFF position, a light
source appears in the beam on position above the
collimator opening...
Collimators
 Collimators provide beams of desired shape and size.
 Collimators of teletherapy machines provide square an...
Gantry
 The gantry can rotate by 360°. The rotational movement
of the gantry is motorized and controlled in two direction...
The axis of rotation of the three structures:
Gantry
Collimator
Couch
coincide at a point known as the Isocenter.

 Isoc...
26
27
Patient Support Assembly
 Treatment Bed has motorized movements
1.   Horizontal
2.   Vertical
3.   Lateral
4.   Table Top...
Control Console
 Control Console is situated outside the bunker
 Interlocks present on the console for
1.      Air Press...
 Timer
      The prescribed target dose is delivered with the help
of two treatment timers:
       Primary Timer - the pr...
Treatment Beam

 Cobalt-60 ( 60Co) can be produced by placing cobalt-59
in a strong neutron field, the nucleus absorbing ...
 These γ rays constitute the useful treatment beam.

 The β particles are absorbed in the cobalt metal and the
stainless...
 The lower energy γ rays produced by the interaction of
the primary γ radiation with the source itself, the
surrounding c...
 Electrons are also produced by these interactions and
constitute electron contamination of the photon beam.

 Electron ...
 Beam characteristics for photon beam energy
        60Co, SSD = 80 cm



1. Depth of maximum dose = 0.5 cm
2. Increased ...
36
 Penumbra refers to the region at the edge of the beam
where the dose-rate changes rapidly as a function of
distance from...
 Penumbra width depends upon:
       1. Source diameter.
       2. SSD.
       3. Depth below skin.
       4. Source to d...
39
 Penumbra Trimmers consist of extensible, heavy metal
bars to attenuate the beam in the penumbra region.

 Increase the ...
Comparison with Linear Accelerator

 Several arguments have been put forward both for and
against Cobalt Units as well as...
LINAC   TeleCobalt




                     42
LINAC   TeleCobalt




                     43
LINAC   TeleCobalt




                     44
LINAC   TeleCobalt




                     45
 BHABHATRON - The indigenous Cobalt-60 Teletherapy
Machine has a capacity of 200 RMM source and its source
to iso-centre ...
BHABHATRON-II

Source Head Capacity:250 RMM
Minimum Collimator 3x3cm at
80cm Iso-centre
Maximum Collimator Field Size
35x3...
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Isotopic Teletherapy Machines

  1. 1. Isotopic Teletherapy Machines DR ARNAB BOSE Dept. of Radiotherapy NRS Medical College, Kolkata 1
  2. 2.  Treatment machines incorporating γ ray sources for use in external beam radiotherapy are called isotopic teletherapy machines.  For use in external beam radiotherapy, γ rays are obtained from specially designed and built sources that contain a suitable, artificially produced radioactive material. The parent source material undergoes a β decay, resulting in excited daughter nuclei that attain ground state through emission of γ rays (γ decay). 2
  3. 3.  The important characteristics of radioisotopes in external beam radiotherapy are: 1. High γ ray energy 2. High specific activity 3. Relatively long half-life 4. Large specific air kerma rate constant( ΓAKR ) 5. Must be available in large quantities 3
  4. 4.  For practical purposes, Co-60 and Cs-137 are the only isotopes which satisfy these requirements to a sufficient extent. 4
  5. 5. Bryant Symons radium "bomb" at Westminster Hospital, London, England, in the 1930s. Until 1951, all isotope machines produced were teleradium units ( radium bomb ).The source to skin distance was usually not greater than 10 cm in these machines. Major drawbacks of these machines were high risk of radiation hazard due to radon gas leak produced as a by product, high cost of radium, large self absorption, low γ ray constant and low output. 5
  6. 6. Telecaesium Unit with applicators  For telecaesium units the source to skin distance is 20cm to 40cm. They have not been very popular because of relatively low γ ray constant and low specific activity. 6
  7. 7.  The invention of the 60Co teletherapy unit by H.E. Johns in Canada in the early 1950s provided a tremendous boost in the quest for higher photon energies and placed the cobalt unit at the forefront of radiotherapy for a number of years.  The first two cobalt teletherapy units were installed in Canada in 1951, at the Saskatoon Cancer Clinic and the Victoria Hospital, London Ontario. 7
  8. 8.  The development of nuclear reactors in the late 1940s made possible the production of small 60Co sources with specific activities (in Curies per gram) high enough to produce clinically acceptable dose rates of more than 1 Gray (Gy) per minute at a typical treatment distance of 80 cm from the source.  These machines quickly became the standard of radiotherapy because of their simplicity of design and operation, low cost, and availability. 8
  9. 9.  The main components of a teletherapy machine are: 1. a radioactive source; 2. a source housing, including beam collimator and source movement mechanism; 3. a gantry and stand in isocentric machines or a housing support assembly in stand-alone machines; 4. a patient support assembly; and 5. a machine console. 9
  10. 10. Source head Gantry Main frame Patient support assembly 10
  11. 11. Hand control Control console 11
  12. 12. Source  Natural cobalt is a hard, stable, bluish-gray, easily breakable metal. Its atoms contain 27 protons, 32 neutrons, and 27 electrons.  Non-radioactive cobalt can be found mixed with various minerals in nature, and has been used to impart a blue color to glass and ceramics for thousands of years.  The well-known isotope of cobalt is unstable radioactive Co-60. This isotope was discovered by Glenn Seaborg and John Livingood at California Berkeley University in 1930. 12
  13. 13.  The Cobalt-60 source is produced by irradiating ordinary, stable 59Co with neutrons in a nuclear reactor. The nuclear reaction is represented as  The resultant isotope 60Co is a radioactive one and it decays to 6028Ni by means of β emission. The maximum energy of β rays is 0.32 MeV. The nuclei of 60Ni will be in the excited states following this decay and the de-excite to the ground state by emitting two γ ray photons of energy 1.17 MeV and 1.33 MeV in cascade. The decay half-life is 5.26 years and the average photon energy is 1.25 MeV. 13
  14. 14.  These γ rays constitute the useful treatment beam. The β particles are absorbed in the cobalt metal and stainless steel capsules. 14
  15. 15. The 60Co source, usually in the form of solid cylinder, discs, or pellets, is contained inside a standard stainless steel capsule and sealed by welding. The capsule is placed into another steel capsule, which is again sealed by welding. The double welded seal is necessary to prevent any leakage of the radioactive material. 15
  16. 16. Source Housing  A typical teletherapy 60Co source is a cylinder of diameter ranging from 1.0 cm to 2.0 cm and is positioned in the cobalt unit with its circular end facing the patient.  The housing for the source is called the ―source head‖.  It consists of a steel shell filled with lead for shielding purposes and device for bringing the source in front of an opening in the head from which the useful beam emerges. Also a heavy metal alloy sleeve is provided to form an additional primary shield when the source is in the OFF position. 16
  17. 17. 17
  18. 18.  A number of methods have been developed for moving the source from OFF position to ON position- 1. Source mounted on a rotating wheel inside the source head to carry the source from OFF to On position 2. Source mounted on a heavy metal drawer is moved horizontally by pneumatic system through a hole running through the source head. In the ON position the source faces the aperture for the treatment beam and in the OFF position the source moves to its shielded location and the light source mounted on the same drawer occupies the ON position of the source. 18
  19. 19. 3. Mercury is allowed to flow into a container immediately below the source to shut OFF the beam. 4. Source is fixed in front of the aperture and the beam can be turned ON and OFF by a shutter consisting of heavy metal jaws. 19
  20. 20. 20
  21. 21.  Typical source activities are of the order of 5000–10 000 Ci (185–370 TBq) and provide a typical dose rate at 80 cm from the teletherapy source of the order of 100–200 cGy/min.  Often the output of a teletherapy machine is stated in Rmm (roentgens per minute at 1 m) as a rough guide for the source strength.  Treatment Head has the capacity to take a source with an activity of 10 000 Roentgens per hour at a meter(RHm) (165 Roentgens per minute at a meter (Rmm)). 21
  22. 22.  When the source is in the beam OFF position, a light source appears in the beam on position above the collimator opening, allowing an optical visualization of the radiation field, as defined by the machine collimators and any special shielding blocks.  Some radiation will escape from the unit even when the source is in the beam OFF position. The head leakage typically amounts to less than 1 mR/h (0.01 mSv/h) at 1 m from the source. International regulations require that the average leakage of a teletherapy machine head be less than 2 mR/h (0.02 mSv/h) at 1m from the source. 22
  23. 23. Collimators  Collimators provide beams of desired shape and size.  Collimators of teletherapy machines provide square and rectangular radiation fields typically ranging from 5 × 5 to 35 × 35 cm2 at 80 cm from the source.  The rotational movement of the collimator is continuous, and it can rotate 360° about its own axis. The collimator system can move to any position when the gantry is rotated. 23
  24. 24. Gantry  The gantry can rotate by 360°. The rotational movement of the gantry is motorized and controlled in two directions continuously; its rotation speed can be adjusted.  Teletherapy machines are most often mounted isocentrically, allowing the beam to rotate about the patient at a fixed SAD. They can be used either as fixed field machines or rotation units.  Modern teletherapy machines have SADs of 80 or 100 cm. 24
  25. 25. The axis of rotation of the three structures: Gantry Collimator Couch coincide at a point known as the Isocenter.  Isocentric Mounting 1. Enhances accuracy. 2. Allows faster setup and is more accurate than older non isocentrically mounted machines. 3. Makes setup transfer easy from the simulator to the treatment machine. 25
  26. 26. 26
  27. 27. 27
  28. 28. Patient Support Assembly  Treatment Bed has motorized movements 1. Horizontal 2. Vertical 3. Lateral 4. Table Top - 90o rotation to each side 5. Base - 110o rotation to each side  Bedtop size l x w cm (in) - 235 x 46 (93x 18)  Patient weight capacity kg (lb) - 136 (300) 28
  29. 29. Control Console  Control Console is situated outside the bunker  Interlocks present on the console for 1. Air Pressure 2. Door 3. Head Lock -Treatment Head has a swivel movement of +/- 180o 4. OFF Shield 5. Treatment Mode 6. Wedge Filter 7. Tray Interlock 8. Timer 29
  30. 30.  Timer The prescribed target dose is delivered with the help of two treatment timers: Primary Timer - the primary timer actually controls the treatment time. Secondary Timer - accounts for the source movement from OFF to ON position and again to OFF position (shutter error).  Source ON/OFF Indicator – Red- ON Green- OFF Amber- TRANSIT 30
  31. 31. Treatment Beam  Cobalt-60 ( 60Co) can be produced by placing cobalt-59 in a strong neutron field, the nucleus absorbing a neutron to form 60Co. As soon as it is formed 60Co starts to undergo radioactive decay to nickel-60 with a half -life of 5.26 yrs.  The emissions are a β−particle with an energy of 0.31MeV(max) and two γ rays with energies of 1.17 MeV and 1.33 MeV. 31
  32. 32.  These γ rays constitute the useful treatment beam.  The β particles are absorbed in the cobalt metal and the stainless steel capsules resulting in the emission of bremsstrahlung x-rays and a small amount of characteristic x-rays.  However these x-rays of average energy 0.1 MeV do not contribute appreciably to the dose in the patient because they are strongly attenuated in the material of the source and the capsule. 32
  33. 33.  The lower energy γ rays produced by the interaction of the primary γ radiation with the source itself, the surrounding capsule, the source housing and the collimator system are also contaminants of the treatment beam.  The scattered components of the beam contribute significantly ( approx. 10% ) to the total intensity of the beam. 33
  34. 34.  Electrons are also produced by these interactions and constitute electron contamination of the photon beam.  Electron contamination can reverse the skin sparing effects of cobalt60 treatment beam, if severe.  Electron contamination is greater for very short diaphragm to skin distances and for large field sizes. 34
  35. 35.  Beam characteristics for photon beam energy 60Co, SSD = 80 cm 1. Depth of maximum dose = 0.5 cm 2. Increased penetration (10-cm PDD = 55%) 3. Beam edge not as well defined—penumbra due to source size 4. Dose outside beam low because most scattering is in forward direction 5. Isodose curvature increases as the field size increases 35
  36. 36. 36
  37. 37.  Penumbra refers to the region at the edge of the beam where the dose-rate changes rapidly as a function of distance from the beam axis.  Types: Transmission penumbra: Transmission through the edge of the collimator block. Geometrical penumbra : Finite size of the source.  Physical penumbra: Lateral distance between to specified isodose curves at a specific depth (90% & 20% at Dmax). Takes scattered radiation into account. 37
  38. 38.  Penumbra width depends upon: 1. Source diameter. 2. SSD. 3. Depth below skin. 4. Source to diaphragm distance (inversely) 38
  39. 39. 39
  40. 40.  Penumbra Trimmers consist of extensible, heavy metal bars to attenuate the beam in the penumbra region.  Increase the source to diaphragm distance, reducing the geometric penumbra.  Another method is to use secondary blocks placed close to the patient ( 15 – 20 cms). 40
  41. 41. Comparison with Linear Accelerator  Several arguments have been put forward both for and against Cobalt Units as well as Linear accelerators.  These arguments relate to physics, clinical advantages and more importantly, the cost consideration. 41
  42. 42. LINAC TeleCobalt 42
  43. 43. LINAC TeleCobalt 43
  44. 44. LINAC TeleCobalt 44
  45. 45. LINAC TeleCobalt 45
  46. 46.  BHABHATRON - The indigenous Cobalt-60 Teletherapy Machine has a capacity of 200 RMM source and its source to iso-centre distance is 80 cm. The system has unique user-friendly features and fully closable collimator for improved radiation safety. 46
  47. 47. BHABHATRON-II Source Head Capacity:250 RMM Minimum Collimator 3x3cm at 80cm Iso-centre Maximum Collimator Field Size 35x35 cm Automatic Collimator closure Arc Treatment Auto set up of Collimator Computerized Control Console Auto collision detection Computerised motorised wedge Asymmetric Collimator 47
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history of teletherapy and details of cobalt machine

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