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Radiation Protection in Radiotherapy

IAEA Training Material on Radiation Protection in Radiotherapy. Radiation Protection in Radiotherapy. Part 10 Good Practice in EBT Lecture 1 (cont.): Equipment design. 2. Features of safe design in practice. A General considerations B Kilovoltage radiation units C Telecurie units

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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 10 Good Practice in EBT Lecture 1 (cont.): Equipment design

  2. 2. Features of safe design in practice A General considerations B Kilovoltage radiation units C Telecurie units D Megavoltage units E Other irradiation units Part 10, lecture 1 (cont.): Equipment design

  3. A. General Safety Requirements • Radiation Protection Measures include • Protection of the patient during treatment • Equipment shielding • Collimation system • Patient comfort and control • Protection of others • Room shielding (this was covered in part 7) Part 10, lecture 1 (cont.): Equipment design

  4. Equipment shielding • Part of dose reduction strategy for patients • Dose to patient other than target due to scatter and leakage Part 10, lecture 1 (cont.): Equipment design

  5. Equipment shielding • X Ray equipment - only needed when machine is on • protects the patient during treatment • Telecobalt units - shielding needed all the time • protects patient and staff during set-up General design limit - leakage should be less than 0.1% of the primary radiation Part 10, lecture 1 (cont.): Equipment design

  6. Testing of shielding integrity of a linac head using film About 2t of lead Part 10, lecture 1 (cont.): Equipment design

  7. Collimation • Creates outlines of the radiation field which should conform to the target • Can be done by a variety of different measures depending on the treatment unit type • Always includes some leakage through the collimation - typically <2% of the primary beam Part 10, lecture 1 (cont.): Equipment design

  8. Collimation Customized blocks or prefabricated blocks in geometric shapes • Aim to limit field to the target only Part 10, lecture 1 (cont.): Equipment design

  9. Collimation • Applicators • electron beams • superficial beams • Movable jaws • Lead blocks • fixed shapes • customized • Multileaf collimator Part 10, lecture 1 (cont.): Equipment design

  10. Custom shielding may reduce the dose to critical organs • e.g. scrotal shields to reduce dose to scrotum due to scattered radiation Part 10, lecture 1 (cont.): Equipment design

  11. Patient comfort and control • The best collimation does not help if the patient is not stable • need good immobilization devices • need to put patient in a reasonably comfortable position (this is often difficult with very sick patients) • need to make them feel comfortable Part 10, lecture 1 (cont.): Equipment design

  12. Immobilization/set-up devices • There are innumerable systems - many of them home built and designed • A good mould room is essential - they are responsible for both, • immobilization and • block making Part 10, lecture 1 (cont.): Equipment design

  13. Immobilization/set-up devices • Head rests Part 10, lecture 1 (cont.): Equipment design

  14. Head and Neck Immobilization Head rests to fit Prone head rest All MedTec Part 10, lecture 1 (cont.): Equipment design

  15. Lateral Head position Part 10, lecture 1 (cont.): Equipment design

  16. Immobilization/set-up devices • The more accuracy is required, the more effort one must make e.g.: • Stereotactic head frame with repositioning accuracy better than 2mm Part 10, lecture 1 (cont.): Equipment design

  17. Immobilization/set-up devices • Immobilization shells for head • Vacuum bag for body immobilization Part 10, lecture 1 (cont.): Equipment design

  18. Various body immobilisation devices Body fix with external markers for set-up All MedTec Part 10, lecture 1 (cont.): Equipment design

  19. Belly board for prone position • Allows ‘belly’ to move into space • Some of the bowel can be moved out of the field Part 10, lecture 1 (cont.): Equipment design

  20. Vacuum bags Customized for every patient All MedTec Part 10, lecture 1 (cont.): Equipment design

  21. Immobilization/set-up devices • Board for set-up of breast patients Arm rest to get arm out of the treatment field Head rest Slope to straighten sternum in order to minimize lung dose Leg rest Part 10, lecture 1 (cont.): Equipment design

  22. … sometimes movement is difficult to control... • e.g. rectal and bladder filling in prostate treatment • determine location of the prostate prior to each treatment fraction using ultrasound Part 10, lecture 1 (cont.): Equipment design

  23. … sometimes movement is difficult to control... • e.g. lung motion due to breathing • determine motion and gate radiation beam External markers on the patient which can be tracked by a video system Part 10, lecture 1 (cont.): Equipment design

  24. Low cost solutions • Ask patients to • hold still • have reproducible bladder filling (e.g. always full or always empty) • provide dietary advise • breath shallow • Make patients feel comfortable and secure Part 10, lecture 1 (cont.): Equipment design

  25. A note on intercom systems • Need to be able to see the patient - is he/she comfortable? Is she/he moving? • Need to be able to talk to the patient • Need to be able to hear if the patient is in distress Part 10, lecture 1 (cont.): Equipment design

  26. B. Kilovoltage Equipment (10 - 150 kV) • Dose rate is approximately proportional to the nth power of the accelerating potential as kVn where 2 < n < 3 • Dose rate is approximately proportional to current (mA) • Therefore important that kV and mA are stable. • It is obviously important that the timer is accurate and stable Part 10, lecture 1 (cont.): Equipment design

  27. Kilovoltage Equipment (10 – 150 kV) • Dose control is achieved by a dual timer system as it is usually not practical to use a transmission ionization chamber • Interlocks should be present to prevent incorrect combinations of kV, mA, and filtration Part 10, lecture 1 (cont.): Equipment design

  28. Quick Question What are the fluctuations of the mains voltage in your hospital? What would be the consequence in dose if these would not be filtered out before generating the high voltage for the X Ray tube?

  29. Answer • A +/- 10% voltage variation is not uncommon due to loading of the net at different times of the day or heavy occasional uses on the same mains (e.g. a lift) • This translates into 40% dose variation which is unacceptable • Mains stabilization is a MUST Part 10, lecture 1 (cont.): Equipment design

  30. Kilovoltage Equipment (10 - 150 kV) • Leakage from the tube housing, the Air Kerma Rate (AKR) shall not exceed • 10 mGy h-1 at 1 metre from focus • 300 mGy h-1 at 5 cm from housing or accessory equipment • if the tube is designed to operate in the range 10 - 50 kV then a special housing is required with a maximum leakage of 1 mGy h-1 • Testing for hot spots should be carried out using film-wrap techniques Part 10, lecture 1 (cont.): Equipment design

  31. Patient shielding • May be done on the skin using lead sheets cut into customized shapes • Special shields may be used - e.g. eye shields Part 10, lecture 1 (cont.): Equipment design

  32. It is practical to use a transmission ionization chamber with this equipment and the primary dose control system should be an integrating dosemeter. The backup (secondary) dose control system can be either an independent integrating dosemeter or a timer Kilovoltage Equipment (150 - 400 kV)Orthovoltage irradiation units Part 10, lecture 1 (cont.): Equipment design

  33. Kilovoltage Equipment (150 - 400 kV) • Leakage from the tube housing, the Air Kerma Rate (AKR) shall not exceed • 10 mGy h-1 at 1 metre from focus • 300 mGy h-1 at 5 cm from housing or accessory equipment (including the beam collimation system such as cones) • Testing for hot spots should be carried out using film-wrap techniques Part 10, lecture 1 (cont.): Equipment design

  34. C. Telecurie units • 137-Cs or more importantly 60-Co • High activity in treatment head • Termination of exposure is usually by dual independent timers Part 10, lecture 1 (cont.): Equipment design

  35. Timers • Need two completely independent timers • One should count time up, one down Part 10, lecture 1 (cont.): Equipment design

  36. Gamma-ray equipment • The source should be sealed such that the container can withstand temperatures likely to be obtained in building fires. • Wipe tests should be carried out initially at installation and at regular intervals to check for surface contamination. This test need not be carried out directly on the source surface and can be carried out on a surface which comes into contact with the source during normal operation of the equipment. Part 10, lecture 1 (cont.): Equipment design

  37. Cobalt unit designs Part 10, lecture 1 (cont.): Equipment design

  38. Gamma-ray equipment • At commissioning, cross-sectional drawings of the head should be examined to identify possible locations where radiation leakage could be a problem. • Film wrap techniques can be used to identify positions of ‘hot’ spots. • Accurate integrated ionization chamber readings should be made at the location of any hot spots and also in a regular pattern around the head. Part 10, lecture 1 (cont.): Equipment design

  39. Gamma-ray equipment • Leakage from the head with the source in the Off position: the Air Kerma Rate (AKR) shall not exceed • 10 Gy h-1 at 1 metre from source • 200 Gy h-1 at 5 cm from housing or accessory equipment Part 10, lecture 1 (cont.): Equipment design

  40. Gamma-ray equipment • Leakage from the head with the source in the On position: the Air Kerma Rate (AKR) shall not exceed • 10 mGy h-1 at 1 metre from source or • 0.1% of the useful beam AKR • whichever is the greater Part 10, lecture 1 (cont.): Equipment design

  41. Gamma-ray equipment • The beam control mechanism shall be of the ‘fail to safety’ type and 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 10, lecture 1 (cont.): Equipment design

  42. Gamma-ray equipment • In case of failure of the automatic source return section of the beam control mechanism, it shall be possible to interrupt the exposure by other means, for example, a manual return system • It shall be possible to unload or repair the treatment head without exceeding the dose limit for occupational exposure recommended by regulation Mechanical source position indicator Part 10, lecture 1 (cont.): Equipment design

  43. Gamma-ray equipment • Collimation, patient immobilisation and blocking as described in first section of part 10 and the case of linacs. • Two particularities • No commercial MLC available (but several home built systems) • Due to large source size and wide penumbra: penumbra trimmers (collimation close to the patient can be employed) Part 10, lecture 1 (cont.): Equipment design

  44. Specific design for Co units • Penumbra trimmers - collimation close to patient reduces penumbra width Part 10, lecture 1 (cont.): Equipment design

  45. Beam stopper • Metal disk at the exit side: • reduces primary beam shielding requirements • may make set-up of patients more cumbersome Part 10, lecture 1 (cont.): Equipment design

  46. D Megavoltage units • Electron linear accelerators - linacs • Capable of X Ray (4 to 25MV) and electron (4 to 25MeV) irradiation Part 10, lecture 1 (cont.): Equipment design

  47. Linacs • Radiation exposure is usually controlled by two independent integrating transmission ionization chamber systems. • One of these is designated as the primary system and should terminate the exposure at the correct number of monitor units • The other system is termed the secondary system and is usually set to terminate the exposure after an additional dose, typically set around 0.25 Gy • Most modern accelerators also have a timer which will terminate the exposure if both ionization chamber systems fail Part 10, lecture 1 (cont.): Equipment design

  48. Linacs • Modern accelerators have a lot of treatment options as discussed in part 6, for example • X Rays or electrons (dual mode) • multiple energies • 2 X Ray energies • 5 or more electron energies • wedges • 3 or more fixed wedges • auto-wedge • dynamic wedge Part 10, lecture 1 (cont.): Equipment design

  49. Linacs • With such a large number of possible settings it is essential that interlocks be provided to prevent inappropriate combinations from being selected • It is also essential that the control console provide a clear indication of what functions have been set Part 10, lecture 1 (cont.): Equipment design

  50. A linac control example Active selection Parameter display Varian Part 10, lecture 1 (cont.): Equipment design

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