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telecobalt machine.ppt
- 1. Powerpoint Templates Page 1 Powerpoint Templates Telecobalt machine
- 2. Powerpoint Templates Page 2 Telecobalt machine Main components of Telecobalt machine are Radioactive source Source housing includes beam collimator and source movement mechanism. Gantry Stand A patient support assembly(Couch) Beam stopper
- 3. Powerpoint Templates Page 3 Source Housing Housing for source is called Source head Contains Steel shell filled with lead for shielding purpose and device to bring the source to on position(opening in the head) Additional primary shield -Heavy metal alloy(copper- tungsten) sleeve in the source off position. Shutter- device which brings source in front of an opening from which the useful beam comes out. And shut off the beam when it is not needed. Four types of shutter mechanism basis of moving and Fixed source 1.Rotating wheel 2.Sliding window 3.Mercury Shutter
- 5. Powerpoint Templates Page 5 Moving source Moving source unit the source moves from a safe position to a position just behind the primary collimator. Sliding window and rotating wheel will come under this method Sliding window: Source mounted on a heavy metal drawer is moved horizontally by pneumatic pressure system through a hole running through the source head. In the on position source faces the aperture and in the off position source moves to its shielded location. In the off condition light source on the same drawer occupies the on position of the source.
- 6. Powerpoint Templates Page 6 Shutter Mechanisam
- 7. Powerpoint Templates Page 7 Rotating wheel Source is attached on a rotating wheel Off condition source is protected by approximately 20 HVL of lead When power is on the wheel is driven by a motor and its turning at the direction 180 where the diaphram .
- 8. Powerpoint Templates Page 8 Fixed source In the fixed source unit the cobalt source is stationary, the primary collimator rotates ,opening up a passage for gamma rays to emerge. Mercury Shutter: Mercury is allowed to flow into the container immediately below the source to shutt off the beam Moving Jaw: Source if fixed infront of the aperture and the beam can be turned on and off position by a heavy metal Jaws
- 9. Powerpoint Templates Page 9 Beam collimation and penumbra Collimators provide beams of desired shape and size Contains primary and secondary collimators Primary collimator is near to source which decides maximum angle of the exiting beam. Primary collimator made of tungsten alloy has a conically shaped hole and it is part of the shielding or shutter mechanisam. Secondary collimator Consists of two pairs of heavy metal blocks of tungsten to obtain square and rectangular field size. Secondary collimators are calibrated so that collimator reading will give corresponding field size at isocentre Collimators of telecobalt machine provides rectangular and square shaped radiation fields typically ranging from 5X5 to
- 10. Powerpoint Templates Page 10 The rotational movement of the collimator is continuous, and it can rotate 360属 about its own axis. The collimator system can move to any position when the gantry is rotated.
- 11. Powerpoint Templates Page 11 Field size Field size may be defined either geometrically or dosimetrically. Geometric field size: Defined as the projection on the plane perpendicular to the beam axis of the distal end of the collimator as seen from center of the source. It is correspond to the field size defined by the light localizer. Dosimetric or Physical field size:As per ICRU recommendation it is the Distance intercepted by the given isodose curve usually 50 % on a plane perpendicular to the beam axis at a stated distance from the source usually at isocentre and at
- 12. Powerpoint Templates Page 12 Penumbra-Beam edge 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. The penumbra width of Ideal external beam should be minimum. The physical penumbra is the penumbra measured in the dose profile. Physical penumbra corresponds the distance between 20% and 80% isodose line at dmax Components to physical penumbra: 1.Geometric penumbra 2.Transmission penumbra And scattered radiation from the patient.
- 13. Powerpoint Templates Page 13 Geometric penumbra Geometric penumbra is caused by source or focal spot having a finite size. The width of the penumbra =D(SSD-SDD)/SDD Factors that increase penumbra: Increase in SSD Increase in Source diameter Factors that decrease penumbra: Decrease in SSD& Source dia Increase in SCD (source to collimator distance) The distance between Pi and P0 is called geometric penumbra width
- 15. Powerpoint Templates Page 15 Transmission penumbra Transmission penumbra is due to photons emitted by the distal part of the source are partially attenuated by the collimator. These rays smear the geometric penumbra Transmission penumbra can be decreased when using diverging block in which block edges are parallel to the beam divergence but this will increase the geometric penumbra.
- 16. Powerpoint Templates Page 16 ODI and light field localizer Collimator also provided with optical distance indicator(ODI) and visual filed localizer. ODI used to measure the distance of the patient surface from the source. ODI consists of a light bulb coupled to optical device which displays the SSD Visual field localizer comprises a lamp and mirror ,which can show the size and position of the field in the patient skin. ODI represents the exact SSD
- 17. Powerpoint Templates Page 17 Gantry Gantry is attached to stand capable of rotating 360 degree 2 arms-Source head & beam stopper 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. Modern teletherapy machines have SADs of 80 or 100 cm
- 18. Powerpoint Templates Page 18 Treatment couch: Integral part of the machine Has motorized movements lateral,Longitudinal ,Rotational and vertical. Stand: Base of unit and its fixed in floor has an electrical and electronic circuits . Contains circular scale to indicate the position of gantry during treatment.
- 19. Powerpoint Templates Page 19 Control console: outside the treatment room connected to the machine by cables. Contains timer and on ,OFF switches with light indicator Prescribed target dose is delivered with help of two timers Primary timer: Actually controls the treatment time. Secondary timer: Accounts for the source movement from off to on position and again to off position Room Doors: Provided with inter locks: beam on when the door is closed
- 20. Powerpoint Templates Page 20 Beam Stopper Beam Stopper: It acts as counter weight and so absorbs exit radiation Reduces primary radiation however patient setup becomes difficult.
Editor's Notes
- #4: Radionuclide source can not be switched off like X ray tube can be turned on and off
- #10: Collimator confines the radiation to the appropriate size and direction.
- #14: Solid line shows the profile interpreted geometrically and dashed line shows real profile observed.
- #16: Penumbra of Co-60 beam is >1 cm
- #20: Supervised area area where the calculated exposure rates through barrier should be less than 1 msV/year