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Radiation Protection in Radiotherapy

Radiation Protection in Radiotherapy

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Radiation Protection in Radiotherapy

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  1. IAEA Training Material on Radiation Protection in Radiotherapy Radiation Protection inRadiotherapy Part 5 External Beam Radiotherapy Lecture 2: Equipment and safe design

  2. Objectives • To review physics and technology of external beam radiotherapy equipment • To understand the design and functionality of the equipment including auxiliary equipment • To appreciate the role of international standards such as IEC 601-2-1 for equipment design Part 5, lecture 2: Equipment - superficial, telecurie

  3. Contents 1. Superficial/orthovoltage equipment 2. Telecurie treatment units 3. Linear accelerators (linacs) 4. Other accelerator types 5. Associated equipment Part 5, lecture 2: Equipment - superficial, telecurie

  4. 1. Superficial and Orthovoltage • “conventional” X Ray tube with electrons accelerated by an electric field • stationary anode (in contrast to diagnostic tubes which have a rotating anode to allow for a smaller focal spot) • filtration important Part 5, lecture 2: Equipment - superficial, telecurie

  5. Photon percentage depth dose comparison Superficial beam Orthovoltage beam Part 5, lecture 2: Equipment - superficial, telecurie

  6. Superficial 40 to 120kVp small skin lesions maximum applicator size typically < 7cm typical FSD < 30cm beam quality measured in HVL aluminium (0.5 to 8mm) Orthovoltage 150 to 400kVp skin lesions, bone metastases applicators or diaphragm FSD 30 to 60cm beam quality in HVL copper (0.2 to 5mm) Superficial and orthovoltage Part 5, lecture 2: Equipment - superficial, telecurie

  7. X Ray tube Cooling water Target Applicator/ collimator Superficial X Ray tube (Philips RT 100) • Manufacturers picture... Part 5, lecture 2: Equipment - superficial, telecurie

  8. Use of cones essential • Large focal spot and close treatment distance (Focus to skin distance FSD often 10cm or less) means the beam MUST be collimated on the skin • Cones are highly suitable to do this. Additional shielding can be achieved using lead cutouts on the skin as detailed in part 10 of the course. Part 5, lecture 2: Equipment - superficial, telecurie

  9. Output in superficial beam depends on: • On/off effect • Strong dependence on FSD --> applicator length significantly affects output • Electron contamination from the applicator (significant for skin dose around 100kVp) Inverse Square Law Part 5, lecture 2: Equipment - superficial, telecurie

  10. On/off effect on off output time Part 5, lecture 2: Equipment - superficial, telecurie

  11. Kilovoltage Equipment (10 - 150 kVp) • Filters are used to remove unwanted low energy X Rays (which only contribute to skin dose) Interlocks must ensure that the correct filter is in place Part 5, lecture 2: Equipment - superficial, telecurie

  12. Kilovoltage Equipment (10 - 150 kVp) • Dose rate is approx. proportional to kVpn where 2 < n < 3 • Dose rate is approx. proportional to electron current (mA) • Therefore it is important that kVp and mA are stable. • It is also obviously important that the timer is accurate and stable - and that the on/off effect is accounted for. Part 5, lecture 2: Equipment - superficial, telecurie

  13. Kilovoltage Equipment (10 - 150 kVp) • Dose control is achieved by a dual timer system - one should count time up, one should count time down from a pre-set treatment time • Interlocks must be present to prevent incorrect combinations of kVp, mA, and filtration Part 5, lecture 2: Equipment - superficial, telecurie

  14. Operator control Radiation on indicator kV and mA indicator Dual timer Emergency off button Selection of filter Key for lock-up Part 5, lecture 2: Equipment - superficial, telecurie

  15. Beam Half Value Layer (HVL) • Possibly the most important test to characterize beam quality • Checks whether there is sufficient filtration in the X Ray beam to remove damaging low energy radiation • Need not only a radiation detector, but also high purity (1100 grade) aluminium - most Al has high levels of high atomic number impurities e.g. Cu Part 5, lecture 2: Equipment - superficial, telecurie

  16. HVL Measurement Relative response • Be careful of beam hardening (semi-log plot is not a straight line) • The second HVL is typically larger than the first • Use points either side of half initial value • Calculate HVL : (initial value = 9 50% of this = 4.5, thus HVL = 2.6 mm Al) mm Al Part 5, lecture 2: Equipment - superficial, telecurie

  17. Orthovoltage units • 120 to 400kVp • conventional X Ray tube • Applications: • deeper skin lesions • bone metastasis Part 5, lecture 2: Equipment - superficial, telecurie

  18. Orthovoltage Equipment (150 - 400 kVp) • Different applicators and filters filters Applicators for different field sizes and distances Part 5, lecture 2: Equipment - superficial, telecurie

  19. Orthovoltage units • Uses mostly cones • More recently also a diaphragm with light field has been introduced. Care must be taken to: • ensure correct distance • account for wide penumbra due to large focal spot Part 5, lecture 2: Equipment - superficial, telecurie

  20. Orthovoltage Equipment (150 - 400 kVp) • The Inverse Square Law is important • Depth dose dramatically affected by FSD FSD 6cm, HVL 6.8mm Cu FSD 30cm, HVL 4.4mm Cu Part 5, lecture 2: Equipment - superficial, telecurie

  21. Orthovoltage Equipment (150 - 400 kVp) • Control console mA and kV control Dual timer On and emergency off button Filter and kV selection Part 5, lecture 2: Equipment - superficial, telecurie

  22. Orthovoltage Equipment (150 - 400 kVp) • It is possible to use a transmission ionization chamber as the primary dose control system instead of treatment time • The backup (secondary) dose control system can be either an independent integrating dosimeter or a timer • Alternatively, two independent timers are used - this is the most common scenario Part 5, lecture 2: Equipment - superficial, telecurie

  23. 2. Telecurie units • Very high activity source (>1000Ci) • Virtually all 60-Co • Some older units using 137-Cs Part 5, lecture 2: Equipment - superficial, telecurie

  24. Stamp to celebrate the 50th anniversary of 60-Co external beam radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie

  25. Telecurie units • 137-Cs • Photon energy 0.66MeV • Relatively large source to relatively low specific activity • Medium FSD (around 60cm) • No isocentric mounting - similar to orthovoltage equipment in set-up • Not sold anymore and should not be in use Part 5, lecture 2: Equipment - superficial, telecurie

  26. Cobalt - 60 • Photon energy around 1.25MeV • 2 lines at 1.17MeV and 1.33MeV Part 5, lecture 2: Equipment - superficial, telecurie

  27. Cobalt - 60 Part 5, lecture 2: Equipment - superficial, telecurie

  28. Cobalt - 60 • Photon energy around 1.25MeV • Specific activity large enough for FSD of 80cm or even 100cm • Therefore, isocentric set-up possible Part 5, lecture 2: Equipment - superficial, telecurie

  29. Cobalt - 60 equipment • Isocentric set-up allows movement of all components around the same centre • collimator • gantry • couch Part 5, lecture 2: Equipment - superficial, telecurie

  30. Photon percentage depth dose comparison 60-Co beam Part 5, lecture 2: Equipment - superficial, telecurie

  31. Control area of a 60-Co unit • Dual timer control • Patient monitoring • lead glass • video system Part 5, lecture 2: Equipment - superficial, telecurie

  32. On/off effect on off output Shutter opens Shutter closes time Part 5, lecture 2: Equipment - superficial, telecurie

  33. Gamma-ray equipment • A more recent Cobalt - 60 unit Part 5, lecture 2: Equipment - superficial, telecurie

  34. Gamma-ray equipment • Source head and transfer mechanism Part 5, lecture 2: Equipment - superficial, telecurie

  35. Gamma-ray equipment • Other source drawer transfer mechanisms Moving jaws Rotating source draw Mercury shutter (employed in the first 60-Co unit in 1951) Part 5, lecture 2: Equipment - superficial, telecurie

  36. Gamma-ray equipment Source assembly: • The source must be sealed so that it can withstand temperatures likely to be obtained in building-fires • Dual encapsulation is recommended to avoid leakage Part 5, lecture 2: Equipment - superficial, telecurie

  37. Cobalt source design Part 5, lecture 2: Equipment - superficial, telecurie

  38. Source assembly Part 5, lecture 2: Equipment - superficial, telecurie

  39. Gamma-ray equipment • Limited half life: 60-Co 5.26years • Source change recommended every 5 years to maintain output Source transport container Treatment unit head Part 5, lecture 2: Equipment - superficial, telecurie

  40. Picture of a Co source change Part 5, lecture 2: Equipment - superficial, telecurie

  41. Gamma-ray equipment • Mechanical source position indicator • Essential to: • indicate if source is out of safe • often coupled with mechanical device to push source back if stuck Part 5, lecture 2: Equipment - superficial, telecurie

  42. Cobalt unit head for a source in a rotating source draw Beam on indicator Part 5, lecture 2: Equipment - superficial, telecurie

  43. Gamma-ray equipment • The beam control mechanism shall be a ‘fail to safety’ type.This means the source will return to the Off position in the event of: • end of normal exposure • any breakdown situation • interruption of the force holding the beam control mechanism in the On position, for example failure of electrical power or compressed air supply Part 5, lecture 2: Equipment - superficial, telecurie

  44. Gamma-ray equipment • Geometric penumbra typically wide because source diameter is large (>2cm) Part 5, lecture 2: Equipment - superficial, telecurie

  45. Gamma-ray equipment • Penumbra trimmer bars may be employed to reduce penumbra width Part 5, lecture 2: Equipment - superficial, telecurie

  46. Gamma-ray equipment There should be two independent read-outs for all mechanical movements: 1. Electronic at console and or monitor in the treatment room 2. Mechanical • Gantry rotation Second non-isocentrical rotational axis for the 60-Co source Part 5, lecture 2: Equipment - superficial, telecurie

  47. Gamma-ray equipment • Leakage from the head with the source in the Off position • max. 10 Gy h-1 at 1 meter from source • max. 200 Gy h-1 at 5 cm from housing • This can contribute a significant proportion of the maximum permissible dose to staff Part 5, lecture 2: Equipment - superficial, telecurie

  48. Quick question Please estimate the dose to a staff member setting up patients at a 60-Co unit.

  49. Annual dose to staff • Assume: • 200 days, 8hours per day working time per year • 10% of this time in treatment room • 3 Gy h-1 typical dose averaged over all locations of the staff member in the treatment room • Dose = 200 x 8 x 0.1 x 3 Gy  0.5mGy/year (half of dose limit for general public) Part 5, lecture 2: Equipment - superficial, telecurie

  50. Gamma-ray equipment • At commissioning, drawings of the head should be examined to identify locations where radiation leakage could be a problem. • Accurate ionization chamber readings should be made at the location of any hot spots and also in a regular pattern around the head. • Film wrap techniques can be used to identify positions of ‘hot’ spots. Part 5, lecture 2: Equipment - superficial, telecurie