LAUSR

A pencil-type ionization chamber commonly used in computed tomography (CT) dosimetry is usually calibrated free in air, but used inside a PMMA CT phantom. Measurements and Monte Carlo simulations are used to investigate the validity of chamber calibration in this situation where the body phantom modifies the x-ray spectrum impinging on the chamber. The results for the PTW 30009 chamber indicate that the additional uncertainty in CT dose index (CTDIw) measurement introduced by the phantom is ~1% or less. The pencil chamber used in CTDIw measurements should have a flat energy response down to at least 30 keV photon energy to avoid excessive increase in measurement uncertainty.