Click here to


Are you sure ?

Yes, do it No, cancel

When a Good Monitor Chamber Goes Bad: Diagnosing Atmospheric Communication of a Sealed Monitor Chamber

T McCaw*, B Barraclough , M Belanger , A Besemer , D Dunkerley , Z Labby , University of Wisconsin, Madison, WI


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To identify the cause of daily output variations up to ±2% on a Varian TrueBeam STx.

Methods: Daily changes in output relative to baseline, measured with an ionization chamber array (DQA3) and the amorphous silicon flat panel detector (IDU) on the TrueBeam, were compared with daily temperature-pressure corrections (PTP) determined from sensors within the DQA3. Additionally, output measurements were performed using a Farmer-type ionization chamber over a five-hour period, during which the monitor chamber was initially heated using a heat gun, then cooled using air conditioning and a fan. The measured ionization was corrected for ambient temperature and pressure. The temperature of the monitor chamber was measured using an infrared non-contact thermometer.

Results: The RMS difference between percentage output change measured with the DQA3 and IDU was 0.64% over all measurements. Over a seven-month retrospective review of daily changes in output and PTP, only a loose correlation (R²=0.25) was observed between output and PTP for the first five months; for the final two months, daily output changes were linearly correlated with changes in PTP, with a slope of 0.82 (R²=0.89). Corrected ionization measurements during controlled heating and cooling of the monitor chamber differed from expected values for a sealed monitor chamber by up to 4.6%, but were consistent with expectation for an air-communicating monitor chamber within uncertainty (1.3%, k=2). Following replacement of the depressurized monitor chamber, there is no correlation between daily percentage change in output and PTP (R²=0.03).

Conclusion: Daily output variations measured with two independent systems on a TrueBeam STx were found to highly correlate with atmospheric conditions during a period in which the monitor chamber was suspected to be communicating with the atmosphere. Additional measurements acquired during controlled temperature variation of the monitor chamber confirmed atmospheric communication of an originally sealed chamber.


Linear Accelerator, Quality Assurance, Monitor Chambers


TH- External beam- photons: Quality Assurance - Linear accelerator

Contact Email