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THE COBALT BOMB

This seminar is presented as a part of weekly journal club and seminar regularly conducted at Apollo hospital,Kolkata Department of Radiation oncology.

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THE COBALT BOMB

  1. 1. THE COBALT BOMB!!! DR REJIL RAJAN
  2. 2. • Source of radiation is at a distance from the patientTELETHERAPY • Treatment machines incorporating γ ray sources for use in external beam radiotherapy ISOTOPIC TELETHERAPY
  3. 3.  Until 1951, all isotope machines produced were tele-radium units ( radium bomb ).  The source to skin distance was usually not greater than 10 cm in these machines.  Major drawbacks of these machines were-i) high risk of radiation hazard due to radon gas leak produced as a by product, ii) high cost of radium, iii) large self absorption, iv) low exposure rate constant and v) low output.
  4. 4. Bryant Symons radium "bomb" at Westminster Hospital, London, England, in the 1930s.
  5. 5.  For tele-cesium units the source to skin distance is 20cm to 40cm.  They have not been very popular because of relatively low exposure rate constant and low specific activity.
  6. 6. Dr. Harold E. Johns - Invented the cobalt-60 unit in the 1950s. First Tele-Cobalt machines –Saskatoon Cancer Clinic , Canada and the Victoria Hospital, London
  7. 7. PICKER MACHINE  Fixed head  SSD Technique
  8. 8. JANUS Co-60 machine. Ulrich Henschke, Bronx, New York Co-60 source of about 2000 Ci. Two specially designed rooms were built adjacent to each other Roman Mythology Two heads looking in opposite directions The gift to see both future and past
  9. 9. Iso-centric Cobalt-60 Teletherapy Machine, Atomic Energy of Canada, Ltd.  Isocentric  SSD : 60cm  Source activity :low  Output low  Time taken MORE
  10. 10. The main components of a teletherapy machine are: 1. radioactive source; 2. 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 5. a machine console.
  11. 11.  Cylindrical double-welded stainless-steel capsule  Diameter ranging from 1.0 to 2.0 cm  Discs / pellets  Positioned in the cobalt unit with its circular end facing the patient
  12. 12. • The Cobalt-60 source is produced by irradiating ordinary, stable 59Co with neutrons in a nuclear reactor. • The nuclear reaction is represented as • 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. 13.  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.
  14. 14. 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.
  15. 15. 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.
  16. 16.  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.
  17. 17.  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.
  18. 18.  Treatment Bed has motorized movements Horizontal Vertical Lateral  Table Top – 90 rotation to each side  Base - 110 rotation to each side
  19. 19. 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 +/- 180 4. OFF Shield 5. Treatment Mode 6. Wedge Filter 7. Tray Interlock 8. Timer
  20. 20.  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
  21. 21. • 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.
  22. 22. • 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.
  23. 23. 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.

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