Ion Chamber Collection Efficiencies for Proton Spot Scanning Calibration

Abstract

Charge accumulation was measured under calibration conditions in the spread-out Bragg peak (SOBP) using the calibration bias as well as a range of voltages from 10V to 500V and a Farmer-style ion chamber. Collection efficiency was determined by extrapolating to infinite voltage. Similar measurements were taken in an identical dose distribution with a much shorter spot duration. The impact of each of the three models on calibration was then quantified using the TRS-398 protocol. The collection efficiency for the standard calibration was determined to agree well with the prediction of a continuous beam recombination correction. The standard calibration field was found to persistently agree with a continuous beam recombination correction for much lower operating biases. The collection efficiency result for the short spot duration field did not agree with either the continuous or pulsed-beam correction. Using the incorrect recombination model under the standard calibration conditions resulted in a 0.5% calibration difference. We have determined that our spot scanning system would be most appropriately calibrated using a recombination correction with continuous beam model. Physicists responsible for the calibration of such systems are advised to take measurements described here to correctly identify the applicable recombination model for their clinics.Comment: Submitted December 16, 2015 to Medical Physic