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To measure diode sensitivity degradation (DSD) induced by cumulative proton dose delivered to a commercial daily quality assurance (QA) device.

At our institution, six Daily QA 3 (DQA3, Sun Nuclear Corporation, Melbourne, FL, USA) devices have been used for daily proton pencil beam scanning QA in four proton gantry rooms over a span of 4 years. DQA3 diode counts were cross‐calibrated using a homogenous field with a known dose of 1 Gy. The DSD rate (ΔR%/100 Gy) was calculated using linear regression on time‐series plots of diode counts and an estimate of cumulative dose per year based on the cross‐calibration. The effect of DSD on daily QA spot position measurements was quantified by converting DSD to baseline spot position shift.

The average dose delivered to the four inner DQA3 diodes was 104 ± 5 Gy/year, and the rate of DSD was ‐5.1% ± 1.0/100 Gy with the …