Quality Assurance and Quality Control
Quality assurance and quality control tests the system that keeps equipment, planning, and delivery within tolerance. Expect frequency, tolerance, baseline, action levels, and independent checks for linacs, imaging, TPS, and brachytherapy.
Programme design
QA is planned, documented, and reviewed. Baseline values are established after commissioning and acceptance; subsequent measurements are compared with baseline and trended. Tolerance is the permitted deviation; an action level triggers investigation, not automatic treatment stoppage unless patient safety requires it. Tests should be independent where possible: constancy checks, calibration checks, and end-to-end dosimetry answer different questions. TG-142 is a common framework for linac QA, with daily, monthly, and annual tests.
Typical linac checks
Daily: dose output constancy within about ±3%, laser alignment about 1 mm, optical distance indicator about 2 mm, door interlocks, audiovisual monitors, and couch/gantry/collimator motion. Monthly: output calibration within about ±1%, energy constancy, MLC positioning, imaging quality, and mechanical checks. Annual: comprehensive calibration, isocentre, MLC transmission, and dosimetry audit. TPS QA includes CT density conversion, dose calculation checks, and plan transfer tests. Brachytherapy QA includes source strength, afterloader timers, and treatment-channel checks.
Worked example
A 6 MV daily output check reads 0.97 Gy against a baseline of 1.00 Gy. The deviation is (0.97-1.00)/1.00×100=-3.0%, at the daily tolerance. The radiographer repeats after warm-up and checks the chamber set-up and temperature/pressure correction. If the repeat is 0.96 Gy (-4%), it exceeds tolerance: stop routine use, notify the physicist, and investigate before treating. The reasoning is that a 4% output error affects every patient; a single pass/fail label is less useful than the trend.
Traps
Do not recalibrate to make a daily check pass; investigate first. Do not confuse a tolerance with a target or with a legal limit. Do not skip QA because the machine was fine yesterday; drift and software changes are real. Do not use a patient plan as a constancy check without a known baseline. Do not treat annual checks as optional paperwork; isocentre and MLC errors can be clinically silent.