Isotopic Teletherapy Machines

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history of teletherapy and details of cobalt machine

Transcript of Isotopic Teletherapy Machines

  • 1. Isotopic Teletherapy MachinesDR ARNAB BOSEDept. of RadiotherapyNRS Medical College, Kolkata 1

2. Treatment machines incorporating ray sources foruse in external beam radiotherapy are calledisotopic teletherapy machines. For use in external beam radiotherapy, rays areobtained from specially designed and built sources thatcontain a suitable, artificially produced radioactive material.The parent source material undergoes a decay,resulting in excited daughter nuclei that attain groundstate through emission of rays ( decay).2 3. The important characteristics of radioisotopes inexternal beam radiotherapy are:1. High ray energy2. High specific activity3. Relatively long half-life4. Large specific air kerma rate constant( AKR )5. Must be available in large quantities3 4. For practical purposes, Co-60 and Cs-137 are the onlyisotopes which satisfy these requirements to a sufficientextent.4 5. Bryant Symons radium "bomb" at Westminster Hospital, London, England, in the 1930s.Until 1951, all isotope machines produced were teleradiumunits ( radium bomb ).The source to skin distance was usuallynot greater than 10 cm in these machines. Major drawbacks ofthese machines were high risk of radiation hazard due toradon gas leak produced as a by product, high cost of radium,large self absorption, low ray constant and low output. 5 6. Telecaesium Unitwith applicators For telecaesium units the source to skin distance is20cm to 40cm. They have not been very popular because ofrelatively low ray constant and low specific activity.6 7. The invention of the 60Coteletherapy unit by H.E. Johns inCanada in the early 1950sprovided a tremendous boost in thequest for higher photon energiesand placed the cobalt unit at theforefront of radiotherapy for anumber of years. The first two cobalt teletherapyunits were installed in Canada in 1951,at the Saskatoon Cancer Clinic andthe Victoria Hospital, LondonOntario.7 8. The development of nuclear reactors in the late 1940smade possible the production of small 60Co sources withspecific activities (in Curies per gram) high enough toproduce clinically acceptable dose rates of more than 1 Gray(Gy) per minute at a typical treatment distance of 80 cmfrom the source. These machines quickly became the standard ofradiotherapy because of their simplicity of design andoperation, low cost, and availability. 8 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; and5. a machine console.9 10. Sourcehead GantryMainframePatient supportassembly10 11. Hand control Control console 11 12. Source Natural cobalt is a hard, stable, bluish-gray, easilybreakable metal. Its atoms contain 27 protons, 32 neutrons,and 27 electrons. Non-radioactive cobalt can be found mixed with variousminerals in nature, and has been used to impart a blue colorto glass and ceramics for thousands of years. The well-known isotope of cobalt is unstable radioactiveCo-60. This isotope was discovered by Glenn Seaborgand John Livingood at California Berkeley University in1930.12 13. The Cobalt-60 source is produced by irradiatingordinary, stable 59Co with neutrons in a nuclear reactor.The nuclear reaction is represented as The resultant isotope 60Co is a radioactive one and itdecays to 6028Ni by means of emission. The maximumenergy of rays is 0.32 MeV.The nuclei of 60Ni will be in the excited states followingthis decay and the de-excite to the ground state byemitting two ray photons of energy 1.17 MeV and1.33 MeV in cascade.The decay half-life is 5.26 years and the average photonenergy is 1.25 MeV.13 14. These rays constitutethe useful treatment beam.The particles areabsorbed in the cobaltmetal and stainless steelcapsules. 14 15. The 60Co source, usually inthe form of solid cylinder,discs, or pellets, is containedinside a standard stainlesssteel capsule and sealed bywelding. The capsule is placedinto another steel capsule,which is again sealed bywelding.The double welded seal isnecessary to prevent anyleakage of the radioactivematerial.15 16. Source Housing A typical teletherapy 60Co source is a cylinder ofdiameter ranging from 1.0 cm to 2.0 cm and is positioned inthe 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 shieldingpurposes and device for bringing the source in front of anopening in the head from which the useful beam emerges.Also a heavy metal alloy sleeve is provided to form anadditional primary shield when the source is in the OFFposition.16 17. 17 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 position2. 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. 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 21. Typical source activities are of the order of500010 000 Ci (185370 TBq) andprovide a typical dose rate at 80 cm from the teletherapysource of the order of 100200 cGy/min. Often the output of a teletherapy machine is stated inRmm (roentgens per minute at 1 m) as a rough guide forthe source strength. Treatment Head has the capacity to take a source withan activity of 10 000 Roentgens per hour at a meter(RHm)(165 Roentgens per minute at a meter (Rmm)).21 22. When the source is in the beam OFF position, a lightsource appears in the beam on position above thecollimator opening, allowing an optical visualization of theradiation field, as defined by the machine collimators andany special shielding blocks. Some radiation will escape from the unit even when thesource is in the beam OFF position. The head leakagetypically amounts to less than 1 mR/h (0.01 mSv/h) at 1 mfrom the source. International regulations require that theaverage leakage of a teletherapy machine head be lessthan 2 mR/h (0.02 mSv/h) at 1m from the source.22 23. Collimators Collimators provide beams of desired shape and size. Collimators of teletherapy machines provide square andrectangular radiation fields typically ranging from 5 5 to35 35 cm2 at 80 cm from the source. The rotational movement of the collimator is continuous,and it can rotate 360about its own axis.The collimatorsystem can moveto any positionwhen the gantry is rotated.23 24. Gantry The gantry can rotate by 360. The rotational movementof 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 thepatient at a fixed SAD. They can be used either as fixedfield machines or rotation units. Modern teletherapy machines haveSADs of 80 or 100 cm.24 25. The axis of rotation of the three structures:GantryCollimatorCouchcoincide at a point known as the Isocenter. Isocentric Mounting1. 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 27. 27 28. Patient Support Assembly Treatment Bed has motorized movements1. Horizontal2. Vertical3. Lateral4. Table Top - 90o rotation to each side5. 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. Control Console Control Console is situated outside the bunker Interlocks present on the console for1.Air Pressure2.Door3.Head Lock -Treatment Head has a swivel movement of +/- 180o4. OFF Shield5.Treatment Mode6.Wedge Filter7.Tray Interlock8.Timer29 30. TimerThe prescribed target dose is delivered with the helpof two treatment timers: Primary Timer - the primary timer actually controls the treatment time. Secondary Timer - accounts for the sourcemovement from OFF to ONposition and again to OFFposition (shutter error). Source ON/OFF Indicator Red- ON Green- OFFAmber- TRANSIT 30 31. Treatment Beam Cobalt-60 ( 60Co) can be produced by placing cobalt-59in a strong neutron field, the nucleus absorbing a neutronto form 60Co. As soon as it is formed 60Co starts to undergoradioactive decay to nickel-60 with a half -life of 5.26 yrs. The emissions area particle with an energy of 0.31MeV(max) andtwo rays with energies of 1.17 MeV and 1.33 MeV.31 32. These rays constitute the useful treatment beam. The particles are absorbed in the cobalt metal and thestainless steel capsules resulting in the emission ofbremsstrahlung x-rays and a small amount of characteristicx-rays. However these x-rays of average energy 0.1 MeV donot contribute appreciably to the dose in the patientbecause they are strongly attenuated in the material of thesource and the capsule.32 33. The lower energy rays produced by the interaction ofthe primary radiation with the source itself, thesurrounding capsule, the source housing and the collimatorsystem are also contaminants of the treatment beam. The scattered components of the beam contributesignificantly ( approx. 10% ) to the total intensity of thebeam.33 34. Electrons are also produced by these interactions andconstitute electron contamination of the photon beam. Electron contamination can reverse the skin sparingeffects of cobalt60 treatment beam