A comparative study of detectors for relative dose measurements in kilovoltage small beams

Abstract

Introduction The XRAD225Cx is a small animal radiotherapy device using a medium energy beam (225 kVp) and small circular fields. In addition to the half-value layers and the absolute dose rate, the commissioning of this equipment requires relative dose measurements such as percentage depth dose (PDD), Output Factor (OF) and Tissue Maximum Ratio (TMR). The aim of this study was to compare four detectors to determine the optimal conditions to perform these relative measurements. Materials and methods RW3 material is known not to be waterequivalent at medium energy for absolute dose measurements. However the impact for relative dose measurements is negligible. To study the influence of the detector, four dosimeters (an IBA SFD diode, a PTW PinPoint 31014 microchamber, EBT3 films and a PTW-23342 plane-parallel chamber) were compared for PDDs, OFs and TMRs in water and/or RW3 depending on the dosimeter sealing. Measurements were performed in small fields (20, 15, 10, 8, 5 and 2.5 mm in diameter). OFs, PDDs, and TMRs were also computed with the GATE Monte Carlo model of this device. Results Regardless of detectors, simulated and measured OFs showed no differences down to a diameter beam of 8 mm. For the twosmallestbeams(5 and 2.5 mm),ionizationchambers yielded large discrepancies compared to SFD and EBT3 measurements and simulations. This is due to the size of the sensitive volume of chambers compared to beam diameter. For PDDs and TMRs, measurement accuracy depends on spatial resolution in depth of the detector. Therefore, Pin-Point chamber was not used. Plane ionization chamber, SFD diode and film measurements were closed to Monte Carlo computed results. SFD diode isknownto be energy-dependent, howevermeasurementswere satisfactory whatever the field diameter at 225 kVp. Conclusions For relative dose measurements such as PDD, OF and TMR, all studied detectors may be used down to a beam diameter of 8 mm. In smaller beams (5 and 2.5 mm), measurements should be performed with the SFD diode or EBT3 films. In particular for such small beams, EBT3 appeared to be very suitable.