WE‐C‐T‐617‐02: Direct Calibration of Ionization Chambers in Linac Photon Beams

Abstract

Purpose: Currently, the starting point for dosimetry in the radiotherapy clinic is an ion chamber calibrated at the standards laboratory in a beam. Conversion factors obtained from a protocol such as TG‐51 are then required to derive the dose in a linac beam. This paper details the measurement of these kQ factors in photon beams from a clinical linac. Method and Materials: The NRC primary standard water calorimeter was used to calibrate a set of NE2571 Farmer‐type ion chambers in 6, 10 & 25 MV photon beams from an Elekta Precise linac installed at NRC. A number of influence quantities — water purity, temperature sensors, temperature measurement system — were investigated in detail. In addition, significant effort was put into monitoring the stability of the linac to ensure that output variations did not significantly affect the measurements. Results: The kQ factors obtained were in good agreement with previous measurements using the NRC Vickers research linac reported by Seuntjens et al in 2000. This indicates no dependence on accelerator type and validates the use of %dd10x as a beam quality specifier for megavoltage photon beams. The calculated factors given in TG‐51 also agree well with these measured kQ factors. The standard uncertainty in the calibration of an ion chamber is estimated to be 0.45%, a significant improvement over using the calculated values. Conclusion: A clinical linac can be successfully used for primary standards dosimetry. Absorbed dose calibration factors for a set of NE2571 chambers have been obtained for 6, 10 & 25 MV photon beams. Factors for other chambers can be determined by direct comparison with these reference chambers in a water phantom.