Tests of an electron monitor for routine quality control measurements of electron energies

Abstract

The depth dose for electrons is sensitive to energy and the AAPM Task Group 24 has recommended that tests be performed at monthly intervals to assure electron beam energy constancy by verifying the depth for the 80% dose to within +/- 3 mm. Typically, this is accomplished by using a two-depth dose ratio technique. Recently, a new device, the Geske monitor, has been introduced that is designed for verifying energy constancy in a single reading. The monitor consists of nine parallel plate detectors that alternate with 5-mm-thick absorbers made of an aluminum alloy. An evaluation of the clinical usefulness of this monitor for the electron beams available on a Varian Clinac 20 has been undertaken with respect to energy discrimination. Beam energy changes of 3 mm of the 80% dose give rise to measurable output changes ranging from 1.7% for 20-MeV electron beams to 15% for 6-MeV electron beams.

An automated procedure for the quality assurance of electron beam output and energy

“…This observation is also consistent with that in Ref. 5. As a comparison with the Geske monitor, the RCC system is still not as convenient to use.…”

“…This is due to the fact that the fall-off for the depth-ionization curve for low energy is steeper than that for the higher energy beams, a fact that has been discussed before in the literature. [3][4][5] More importantly, the sensitivity for the RCC system is higher than that for the Geske monitor. The reason for this is that the Geske monitor consists of nine chambers located at various depths, but the signals are summed together.…”

“…Other than the increased sensitivity mentioned earlier, the RCC system can obtain the measurement of the output level, i.e., the signal at d max , whereas for the Geske monitor alone a change in the output signal cannot be distinguished from a shift in beam energy. 5 With the RCC system individual parameters can be easily measured, as well as any custom-designed measurement condition, such as other two-depth ratios, off-axis measurements or even a complete depth-dose curve measurement. In fact, we have prepared a comprehensive set of aluminum disks to accommodate various measurement configurations.…”

“…The properties of the Geske monitor have been studied and reported in the literature. 5,6 In this work we report on an alternate method for electron beam energy monitoring that offers certain advantages over the earlier various methods.…”

Electron energy constancy verification using a double-wedge phantom

“…With these techniques, the constancy metric is expressed as a ratio of electrometer reading with and without the upstream wedge. More elaborate electron detector devices have been used to measure depth dose curves including a commercially available energy monitor ͑Geske 3405, PTW, Freiburg, Germany͒ consisting of nine parallel plate detectors that has been evaluated by several groups 8,9 and a detector device consisting of 12 scintillating fibers embedded into an acrylic phantom. 10 In this paper, a technique for measuring an electron energy constancy metric similar to R 50 using a double-wedge shaped phantom positioned on a Profiler™ diode array is described.…”