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11 Cards in this Set
- Front
- Back
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how does pair production occur |
incident xray photon interacts directly with nucleus of tissue atom, positron and electron are produced. electron undergoes many interactions, positron gets annihilated, rrleasing two xray photons |
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interactions in MeV range |
compton scattering and pair production dominate |
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why are xrays in MeV range not suitsble for imaging |
there is little difference in attenuation between tissue types |
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what is the absorbed dose |
the amount of energy deposited per kilogram of tissue. described in Grays (Gy), where 1Gy = 1J/kg |
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typical doses in Gy |
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how does radiotherapy work |
high energy xrays are generated by a linesr accelerator, and precisely target the tumour. damages the dna within the cancer cells, making them unable to reproduce, and causing the cells to die. if healthy cells get hit, they can repair more easily |
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types of treatment |
based on location, size, and type of cancer. for more superficial tumours, can use a kilovoltage electron bean or xray unit for deeper tumours, MeV xrays are needed |
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the processTM |
1. patient immobilisation. organs need to be reproducibly positioned. done with thermoplastic moulds and plastic blocks, etc 2. planning xray ct scan. combines 3d imging anf simulations og the beam to make sure positioned accurately. 3. simulations. multileaf collimators are used to chnage shape of beam to suit tumour 4. treatment in fractions. usually treated daily for 37 days. in ablative therapy(where theyre trying to kill the tumour), fewer fractions are used with higher doses |
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quality assurance |
mechanical isocentre and radiation isocentre must be aligned to within 1mm. light field positioner must be accurate to 2mm |
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how to check the radiation dose |
using an ionisation chamber in a water tank - btec human. can be impractical, use 'water equivalent phantom' instead. percentage depth dose is measured. inplane and crossplane profiles measure the width nd flatmess of the field |
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what is the skin sparing effect |
dose decreases after a maximum distance due to attenuation of the beam. the energy of the beam will be chosen based on the depth of the tumour. as photon beam energy imcreases, surface dose decreases |
