Technical Note: Long-term monitoring of diode sensitivity degradation induced by proton irradiation.

PURPOSE To measure diode sensitivity degradation (DSD) induced by cumulative proton dose delivered to a commercial daily QA device. METHODS At our institution, six Daily QA 3 (DQA3, Sun Nuclear Corporation, Melbourne, FL) devices have been used for daily proton pencil beam scanning QA in four proton gantry rooms over a span of four 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. RESULTS The average dose delivered to the four inner DQA3 diodes was 104±5 Gy/year, and the rate of DSD was -5.1%±1.0/100Gy with the exception of one DQA3 device that had a significantly higher rate of DSD (-12%/100Gy). The R2 s of the linear fit to time-series plots were between 0.92 and 0.98. The DSD rates were not a constant but decreases with accumulated doses. The four center diodes, which received 40% of the cumulative dose received by inner diodes, had a DSD rate of -7.2%±0.9/100Gy. For our daily QA program, one year of DSD was equivalent to a 0.2 mm shift in spot position. CONCLUSIONS The DSD rate of DQA3 diodes determined by long-term proton daily QA data was about -5%/100Gy, which is more than 10 times greater than the reported DSD rate from photon irradiation. DQA3 diodes may be used for daily proton QA programs, provided that they are recalibrated at an appropriate frequency that should be determined specifically for different daily QA programs. This article is protected by copyright. All rights reserved.