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Traditionally “QA” applied to programmes of checking

Traditionally “QA” applied to programmes of checking radiotherapy equipment: accelerators, simulators etc . Strictly, they should be called QC (Quality Control) programmes QA has wider meaning and includes planning and management issues. Example: monthly output calibration.

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Traditionally “QA” applied to programmes of checking

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  1. Traditionally “QA” applied to programmes of checking radiotherapy equipment: accelerators, simulators etc. • Strictly, they should be called QC (Quality Control) programmes • QA has wider meaning and includes planning and management issues.

  2. Example: monthly output calibration. By itself it is a QC procedure intended to assure that the delivered dose complies with the calculations. In the framework of QA system there are additional requirements: • calibration factors, calibration forms are current and obsolete forms removed • work instructions and calibration protocol are available, up-to-date and consistent with national standards and obsolete documents removed • physics staff are trained • physics staff are available • physics staff are aware of their responsibilities and responsibilities of others • physics department negotiates necessary resources with the director of the department • physics department regularly reviews its strategies (including a dosimetry service) with the director of the department

  3. Most of these requirements are met in a good department, but with a QA system they are not left to chance. • This is typical UK approach (because of Co-60 calibration error) and is also adopted in the Netherlands. Radiotherapy is ISO 9000 certified. • ISO 9000 is a set of formal requirements to procedures, training, documentation management, review of activities etc.

  4. 3 steps in introducing of new equipment and its safe use: 1. Acceptance checks Mechanical Dosimetry Safety 2. Commissioning 3. Periodical checks Daily Monthly Annual

  5. Accelerator QC Checks Daily Output Constancy Light field (size, symmetry) Lasers vs. Optical Distance Indicator Monthly Gantry & Collimator indicators Gantry & Collimator axis rotation ODI readings Laser alignment Light field for several field sizes Light and radiation field coincidence Energy (TMR 20/10) Calibration of output Radiation field flatness and symmetry Room entrance interlock Audio and video monitors Emergency switches Annual

  6. Patient specific QA We want to assure that Deviation of delivered dose from calculated dose is within the tolerance 2) Radiation field is at the correct place Daily input of parameters is correct and no unapproved changes happen (especially important with modern multiple-field techniques ) For these purposes we have 1) In-vivo dosimetry 2) Portal imaging 3) Record and verify (R&V) system

  7. In-vivo dosimetry What may be a reason for incorrect dose delivery:          Wrong set-up         Calculation error (due to a mistake or problematic geometry)          Patient changes etc. In practice used for TBI, TSEI and some very problematic situations Instruments TLD Diodes Other (ion chambers, films)

  8. TLD • Irradiation – electrons are trapped in metastable states • More radiation – more electrons are trapped • By heating TLD, electrons are given enough energy to escape from traps • After escape, electrons revert to a stable state and emit an optical photon • Optical photons are detected by photomultiplier and light output is measured • Annealing is needed to free all trapped electrons

  9. TLD reading

  10. Diodes • Built-in electric field is created across p-n junction • During irradiation, electron-hole pairs are formed • The charge carriers move across depletion region • In this way a current is generated and may be detected

  11. Portal Imaging • Basic physical limitation of image quality at megavoltage energies Compton effect vs. Photoelectric Effect • Verification films Cassette Design; Kodak EC-L - significantly improved contrast • Digital imaging: E(lectronic) P(ortal) I(maging) D(evice) – EPID Dosimetric applications of EPIDs

  12. Video-based EPID

  13. Flat-panel EPID

  14. Record & Verify Systems • Ability to check and to record machine parameters is very attractive, but the acceptance of R&V systems was very slow for several reasons: 1) Data entry is time consuming 2) Frequent system malfunctions 3) Mistrust of computers • The situation is changing now because of:      1) Increase in sophistication of treatment techniques      2) Availability of computer assisted set-up (Jaws, Collimator, Gantry)      3) Greater awareness of safety

  15. Record & Verify Systems • Stores parameters, communicates with an accelerator and compares with the real parameters. • Stores parameters of all treatments so that can be checked later.

  16. Record & Verify Systems

  17. Summary for Treatment Verification • QA framework • QC of accelerators: acceptance, commissioning and periodical checks • Patient specific QA: In-vivo dosimetry TLDs, diodes – advantages and disadvantages Portal imaging Films, EPID Record and verify (R&V) systems

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