Radiation Protection in Radiotherapy Part 5 External Beam Radiotherapy
IAEA Training Material on Radiation Protection in Radiotherapy Radiation Protection in Radiotherapy Part 5 External Beam Radiotherapy Lecture 2: Equipment and safe design 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 Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 2 Contents 1. Superficial/orthovoltage equipment 2. Telecurie treatment units 3. Linear accelerators (linacs) 4. Other accelerator types 5. Associated equipment Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 3 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 Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 4 Photon percentage depth dose comparison Superficial beam Orthovoltage beam Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 5 Superficial and orthovoltage Superficial Orthovoltage • 40 to 120kVp • small skin lesions • maximum applicator size typically < 7cm • typical FSD < 30cm • beam quality measured in HVL aluminium (0.5 to 8mm) Radiation Protection in Radiotherapy • 150 to 400kVp • skin lesions, bone metastases • applicators or diaphragm • FSD 30 to 60cm • beam quality in HVL copper (0.2 to 5mm) Part 5, lecture 2: Equipment - superficial, telecurie 6 Superficial X Ray tube (Philips RT 100) • Manufacturers picture... X Ray tube Cooling water Target Applicator/ collimator Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 7 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. Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 8 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) Radiation Protection in Radiotherapy Inverse Square Law Part 5, lecture 2: Equipment - superficial, telecurie 9 On/off effect on off output time Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 10 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 Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 11 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. Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 12 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 Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 13 Operator control Radiation on indicator kV and mA indicator Dual timer Emergency off button Selection of filter Key for lock-up Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 14 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 Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 15 HVL Measurement • 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 : Relative response 10 (initial value = 9 50% of this = 4.5, thus HVL = 2.6 mm Al) Radiation Protection in Radiotherapy 1 0 1 2 3 4 mm Al Part 5, lecture 2: Equipment - superficial, telecurie 16 Orthovoltage units • 120 to 400kVp • conventional X Ray tube • Applications: • deeper skin lesions • bone metastasis Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 17 Orthovoltage Equipment (150 kVp) 400 • Different applicators and filters filters Applicators for different field sizes and distances Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 18 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 Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 19 Orthovoltage Equipment (150 - 400 kVp) • The Inverse Square Law is important • Depth dose dramatically affected by FSD Radiation Protection in Radiotherapy FSD 6cm, HVL 6.8mm Cu FSD 30cm, HVL 4.4mm Cu Part 5, lecture 2: Equipment - superficial, telecurie 20 Orthovoltage Equipment (150 kVp) 400 • Control console mA and kV control Dual timer On and emergency off button Filter and kV selection Radiation Protection in Radiotherapy 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 Radiation Protection in Radiotherapy 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 Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 24 Stamp to celebrate the 50th anniversary of 60-Co external beam radiotherapy Radiation Protection in 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 Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 26 Cobalt - 60 • Photon energy around 1.25MeV • 2 lines at 1.17MeV and 1.33MeV Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 27 Cobalt - 60 Radiation Protection in Radiotherapy 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 Radiation Protection in Radiotherapy 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 Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 30 Photon percentage depth dose comparison 60-Co beam Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 31 Control area of a 60-Co unit • Dual timer control • Patient monitoring • lead glass • video system Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 32 On/off effect on off output Shutter opens Shutter closes time Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 33 Gamma-ray equipment • A more recent Cobalt - 60 unit Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 34 Gamma-ray equipment • Source head and transfer mechanism Radiation Protection in Radiotherapy 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) Radiation Protection in Radiotherapy 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 Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 37 Cobalt source design Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 38 Source assembly Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 39 Gamma-ray equipment • Limited half life: 60Co 5.26years • Source change recommended every 5 years to maintain output Source transport container Treatment unit head Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 40 Picture of a Co source change Radiation Protection in Radiotherapy 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 Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 42 Cobalt unit head for a source in a rotating source draw Beam on indicator Radiation Protection in Radiotherapy 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 Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 44 Gamma-ray equipment • Geometric penumbra typically wide because source diameter is large (>2cm) Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 46 Gamma-ray equipment • Penumbra trimmer bars may be employed to reduce penumbra width Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 47 Gamma-ray equipment • Gantry rotation There should be two independent read-outs for all mechanical movements: 1. Electronic at console and or monitor in the treatment room 2. Mechanical Second non-isocentrical rotational axis for the 60-Co source Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 48 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 Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 49 Quick question Please estimate the dose to a staff member setting up patients at a 60-Co unit. 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) Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 51 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. Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 52 Film wrapping technique • Here shown with only a single film on a linac • In practice film can be wrapped around all the treatment head • This technique is useful also for other treatment units such as superficial and orthovoltage. Radiation Protection in Radiotherapy Never forget to mark and label the film Part 5, lecture 2: Equipment - superficial, telecurie 53 Gamma-ray equipment • 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. Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 54
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