the cobalt bomb

THE COBALT BOMB!!!DR REJIL RAJAN

TELETHERAP

Y

•Source of radiation is at a distance from the patient

ISOTOPIC TELETHE

RAPY

•Treatment machines incorporating γ ray sources for use in external beam radiotherapy

HISTORY

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.

ERA OF TELE-RADIUM AND TELE-CESIUM

Bryant Symons radium "bomb" at Westminster Hospital, London, England, in the 1930s.

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.

THE COBALT BOMBDr. Harold E. Johns - Invented the cobalt-60 unit in

the 1950s.

First Tele-Cobalt machines –Saskatoon Cancer

Clinic , Canada and the Victoria Hospital, London

COBALT-60 V/S RADIUM-226 AND CESIUM-137

PICKER MACHINE

Fixed head

SSD Technique

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 MythologyTwo heads looking in opposite directionsThe gift to see both future and past

Iso-centric Cobalt-60 Teletherapy Machine, Atomic Energy of Canada, Ltd.

Isocentric

SSD : 60cm

Source activity :low

Output low Time taken MORE

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.

COMPONENTS

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

COBALT – 60 SOURCE

DECAY

• 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.

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.

SOURCE HOUSING

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.

ON OFF

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.

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.

COLLIMATORS

The gantry can rotate by 360°. The rotational movement of the gantry is motorized and controlled in two directionscontinuously; its rotation speed can be adjusted.

Teletherapy machines are most often mountedisocentrically, allowing the beam to rotate about the patient at a fixed SAD. They can be used either as fixed field machines or rotation units.

GANTRY

Treatment Bed has motorized movements Horizontal Vertical Lateral

Table Top – 90 rotation to each side Base - 110 rotation to each side

PATIENT SUPPORT ASSEMBLY

Control Console is situated outside the bunkerInterlocks present on the console for 1. Air Pressure2. Door3. Head Lock -Treatment Head has a swivel movement of +/- 1804. OFF Shield5. Treatment Mode6. Wedge Filter7. Tray Interlock8. Timer

CONTROL CONSOLE

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

• 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.

TREATMENT BEAM

• 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.

COBALT 60 V/S LINAC

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.