Equipment and Quality Assurance
Equipment and Quality Assurance tests what a linear accelerator, CT simulator, and brachytherapy unit contain and how each is verified at daily, monthly, and annual intervals. Questions are pinned to AAPM task group tolerances and to the clinical consequence of a failed test.
Linac hardware
- RF power from a magnetron (typical low energy) or klystron (high energy) feeds the waveguide. Electrons strike a tungsten target to produce photons.
- Removing the flattening filter on FFF machines raises dose rate but leaves a peaked, non-flat profile that changes how flatness and output are checked.
- Sealed transmission ion chambers monitor dose, dose rate, and flatness, and terminate the beam when limits are exceeded.
- MLC, jaws, gantry, collimator, and couch each carry mechanical tolerances; isocenter is the coincidence of those axes.
TG-142 checkpoints
- Daily: output constancy within 3 percent, laser alignment within 2 mm, distance indicator within 2 mm, door interlock and beam-off function.
- Monthly: output constancy within 2 percent, photon energy constancy within 2 percent, beam profile constancy, MLC leaf position within 1 mm.
- Annual: calibration to TG-51 with a 1 percent tolerance plus a full mechanical and safety review.
Simulator and brachytherapy
- CT simulator: HU constancy within about 5 HU, laser and couch accuracy, mechanical checks per TG-66.
- HDR: source strength assay within 3 percent of the certificate, well-chamber constancy before each treatment day, timer accuracy, and emergency source-retraction drills.
Worked example
At commissioning, 6 MV output measured 1.000 cGy/MU at dmax, 100 cm SSD, 10 x 10 cm. A monthly check reads 0.973 cGy/MU. The deviation is (1.000 - 0.973)/1.000 = 2.7 percent. The monthly tolerance is 2 percent, so this is not simply documented and continued: the physicist checks chamber placement, temperature and pressure, and warm-up, then investigates the machine. The annual calibration tolerance of 1 percent is tighter still.
Traps
- Quoting annual tolerances for daily tests or daily tolerances for annual calibration.
- Assuming a passing output check also confirms energy and profile.
- Judging FFF flatness with flattened-beam criteria.
- Treating interlocks, warning lights, and audio-visual monitors as maintenance items rather than QA items.