The impact of water temperature on absolute dose calibration of photon beams

Abstract

The Task Group (TG) 51 protocol prescribes that dose calibration of photon beams be done by irradiating an ionization chamber in a water tank at pre-defined depths. Methodologies are provided to account for variations in measurement conditions by applying correction factors. However, the protocol does not completely account for the impact of water temperature. It is well established that water temperature will influence the density of air in the ion chamber collecting volume. The temperature, however, will also influence the size of the collecting volume via thermal expansion of the cavity wall, and the density of water. In this work the overall effect of water temperature on absolute dosimetry has been investigated. Dose measurements were made using a Farmer-type ion chamber for 6 and 23 MV photon beams with water temperatures ranging from 10 °C to 40 °C. A reference chamber was used to account for fluctuations in beam output between successive measurements. For the same beam output, the dose at the depth of calibration, determined using TG 51, was found to be dependent on the temperature of the water in the tank. The dose for both photon energies studied followed a similar trend reaching a maximum around 26 °C. For the 6 MV beam, the dose was found to increase 0.7% from 10 °C to 26 °C and then decrease 0.18% from 26 °C to 40 °C. Similarly, for the 23 MV beam the dose increased 0.95% from 10 °C to 26 °C and then decreased 1.25% from 26 °C to 40 °C. There is a measurable effect of water temperature on absolute dose calibration. To account for this effect, a reference temperature can be defined and a correction factor applied to account for deviations from this reference temperature during beam calibration. Such a factor is expected to be of similar magnitude to most of the existing TG 51 correction factors.