The document discusses quality assurance testing for a Varian 21EX linear accelerator used for radiation therapy. It describes various tests performed on the linear accelerator including absolute dose calibration using an ionization chamber and water phantom, measurements of tissue maximum ratios and beam transmission factors, and in-air measurements of collimator scatter factors and output factors. Mechanical tests are also described to check components like light and crosshair alignment, optical distance indicators, readout devices, and axes of rotation. Safety tests include checks of communication devices, beam termination systems, motion controls and limits, dosimetry systems, and backup safety mechanisms. Documentation and analysis of results against tolerance limits is emphasized.

1. Physics Quality Assurance Varian 21EX Linear Accelerator (LINAC)

2. Linear Accelerator Gantry Electrons are accelerated to energies of several million electron volts.

3. Linear Accelerator Block Diagram

4. Linear Accelerator Treatment Head The electrons strike a target to produce x-rays or they strike a scattering foil to treat directly with electrons.

5. Varian 21EX Linear Accelerator X-ray energies (used to treat target areas located more than a few cm below the skin surface 6 and 10 MV energies Electron energies (used to treat lesions on the surface of the skin) 6, 9, 12, 16, and 20 MeV energies

6. Fields can be shaped by using custom made blocks or multileaf collimators attached to the treatment head l

7. Linear Accelerator Quality Assurance Absolute dose calibration is performed according to the AAPM TG51 protocol using water phantom , ionization chamber and electrometer

8. This shows the ionization chamber in the water tank. It is connected to the electrometer located outside the room at the control console with a long cable. The TG51 protocol specifies the depth at which the calibration is performed.

9. Other measurements using the water phantom system Tissue maximum ratios (dose dependence on field size and depth) Transmission factors of beam shaping devices such as wedges, block trays and compensators

10. In-air measurements using a build-up cap on the ion chamber Collimator scatter factors Output dependence on gantry angle Inverse square factor Collimator transmission Dose rate dependence Output linearity MU end effect

11. LINAC Mechanical Tests Light field (used to set up patients) Properly centered Congruent with radiation field Does not “walk out” when collimator is rotated Accurate over full range of field sizes

12. LINAC Mechanical Tests (cont) Crosshairs (used to set up patients) Properly centered Do not “walk out” when collimator is rotated Optical distance indicator accuracy (used to measure the distance from the target to the patient – if this changes during the treatment course, patient may have gained or lost weight and a new calculation and/or treatment plan would be done) Readout devices Gantry, collimator and couch angles Field size for both independent jaws

13. LINAC Mechanical Tests (cont) Gantry axis of rotation – center of radiation field stable when gantry is rotated Collimator axis of rotation – center of radiation field stable when collimator is rotated

14. LINAC Safety Tests Communication devices Video Audio Beam termination Radiation off button Emergency off buttons Main power circuit breaker Time delay - when LINAC is powered up filaments must be warmed up Emergency couch pendant – couch can be moved out and down when all power is lost to LINAC Backup mechanical counter – displays dose given if power is lost during a treatment

15. LINAC Safety Tests (cont) Motions controlled by hand pendant Gantry rotation Collimator rotation Upper and lower jaws Couch rotation, vertical, lateral and longitudinal movements Safety checks Motion enable switch functional Limit switches functional Motor reversal can be initiated quickly Timer – terminates radiation if not stopped by preset MU Secondary dosimetry – terminates radiation if not stopped by primary MU setting

16. Other Tests of LINAC Components Multileaf collimator Digital portal imager

17. This device is used each day to check the calibration, energy, symmetry and flatness of each beam energy. If any of these parameters fall outside of limits set by the physicist, the reason must be determined and corrected before patient treatments can be performed.

18. Lasers on the walls and ceiling are used to set up the patient on the treatment couch. This device is used to check the accuracy of the laser positions.

19. Physics Quality Assurance Documentation and Analysis Tolerance limits are set for all parameters If any measurement exceeds the tolerance, adjustments are made by the physicist or engineer All measurements are carefully documented Inspectors from Florida Radiation Control review the documents at least once a year