Linac Output Drift, MLC Error, and VMAT QA Failure
A 6 MV TrueBeam is calibrated with TG-51 at 1.000 cGy/MU for a 10 x 10 cm field at dmax, 100 cm SSD. Over three consecutive mornings, the daily output check reads 0.985, 0.984, and 0.985. The reference chamber temperature is 22.0 C and pressure 760 mmHg; the vault is at 21.5 C and 758 mmHg. The monthly MLC picket fence test shows leaves 5-10 in bank A displaced by 1.2 mm; all other leaves are within 0.3 mm. A VMAT prostate plan that previously passed gamma at 3%/2 mm with 98% now passes at 91%. The physicist who performed the annual calibration left, and the electrometer was replaced two months ago but the ion chamber has not been recalibrated. The machine has 14 patients scheduled today.
1. What is the significance of the 1.5% output drop, and what immediate checks and decision rules apply?
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A 1.5% drop is within the daily TG-142 tolerance of 3%, but it is a consistent trend and should not be ignored. First I would verify the temperature-pressure correction: at 21.5 C and 758 mmHg, the correction is about 0.997, which does not explain a 1.5% drop. I would repeat the measurement with an independent chamber and electrometer, check MU linearity and beam energy, and compare against the monthly and annual tolerance of 2% and 1%. If the drop is confirmed and exceeds 2%, I would recalibrate before treatment. If it remains between 1% and 2%, I would investigate the cause, including the recent electrometer replacement, and document a plan for correction. Treatment should not proceed if output is outside 3% or if the cause is unexplained.
2. The MLC error and gamma failure are related. What is your action before treating the 14 patients?
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A 1.2 mm leaf displacement in one bank can cause a VMAT gamma failure, especially in small fields and dynamic segments. A drop from 98% to 91% at 3%/2 mm is clinically significant. I would pause VMAT treatments, recalibrate the MLC bank using the manufacturer procedure, repeat the picket fence test, and require all leaves to be within 1 mm before resuming. I would then repeat the VMAT QA and require a pass rate at or above the department threshold, typically 95%. If the machine cannot pass, I would treat only patients whose plans are not sensitive to MLC position, or refer/transfer as clinically appropriate. The 14-patient schedule does not justify treating with a known MLC calibration error.
3. The ion chamber has not been recalibrated after electrometer replacement. How does this affect TG-51 and what should you do?
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TG-51 depends on the ion chamber calibration factor NDW and a calibrated electrometer. Replacing the electrometer changes the effective calibration chain, so the output reading may be biased even if the chamber itself is unchanged. The 1.5% drop could be real or could be an artifact of the new electrometer. I would stop using the local calibration as the sole reference, cross-check with a second calibrated chamber-electrometer system, and send the chamber and electrometer for calibration or recalibrate against a reference system. Until then, I would not make a large output adjustment based only on this reading. The correction should be documented and the annual calibration repeated.