SU‐FF‐T‐373: Investigation of Humidity Effects On Beam Monitor Performance in a Proton Clinical Gantry

Purpose: To investigate the effect of ambient humidity on performance characteristics of ionization chambers used as beam monitors in two clinical proton gantry systems, and to choose optimal electrodematerials.Method and Materials: Dose calibration results for two identical, vented beam monitors, with electrodes constructed of Mylar film with graphitecoating, and each installed in a gantry nozzle, were analyzed for stability over a period of 1 year. Additionally, two ion chambers of similar design, but constructed using Kapton for base layer and either i) gold‐plated copper or ii) graphite paint for electrodes, were fabricated for testing purposes. The test chambers were placed in a controlled humidity environment and their performance was evaluated in a proton beam under various humidity conditions. Results: Mylar‐based electrodes of the gantry beam monitors provide a dose calibration that varied within ±2% (maximum deviation from mean), over a period of 1 year while room's relative humidity (RH) varied from 20% to 70%, with brief excursions to 15% and 80%. Kapton‐based copper gold‐plated electrodes provided linear (±1%) response in dose rates up to 180 Gy/min, and were stable within ±1% in terms of measured charge per monitor unit, in the range of 10%–90% RH. The response of Kapton‐based carbon paint electrodes was linear (±1%) in dose rate up to 8 Gy/min and showed a slight rising trend (about 3% increase) in the 50% – 80% RH range. Conclusion: Treatment room humidity may affect the performance of Mylar‐based electrodes in a beam monitor with open‐to‐atmosphere design. When humidity of 70% RH or higher is possible, copper gold‐plated electrodes on Kapton show superior performance compared with graphite paint electrodes.