Plane-parallel ionization chamber response in the buildup region of obliquely incident photon beams.

Fixed-separation plane-parallel ionization chambers have been shown to overestimate the dose in the buildup region of normally incident high-energy photon beams. This work shows that these ionization chambers exhibit an even greater over-response in the buildup region of obliquely incident photon beams. This over-response at oblique incidence is greatest at the surface of the phantom and increases with increasing angle of beam incidence. In addition, the magnitude of the over-response depends on field size, beam energy, and chamber construction. This study shows that plane-parallel ionization chambers can over-respond by more than a factor of 2.3 at the phantom surface for obliquely incident high-energy photon fields.

[1]  J. A. Rawlinson,et al.  Design of parallel plate ion chambers for buildup measurements in megavoltage photon beams. , 1992, Medical physics.

[2]  D. Mellenberg.  Determination of build-up region over-response corrections for a Markus-type chamber. , 1990, Medical physics.

[3]  F. Khan,et al.  Measurement of dose in the buildup region using fixed-separation plane-parallel ionization chambers. , 1990, Medical physics.

[4]  B J Gerbi,et al.  The polarity effect for commercially available plane-parallel ionization chambers. , 1987, Medical physics.

[5]  J. S. Pruitt,et al.  Calibration of beta-particle-emitting ophthalmic applicators , 1987 .

[6]  A. Rubach,et al.  Dose build-up curves for cobalt-60 irradiation: a systematic error occurring with pancake chamber measurements. , 1986, Physics in medicine and biology.

[7]  B. Nilsson,et al.  Fluence perturbation in photon beams under nonequilibrium conditions. , 1986, Medical physics.

[8]  J. Purdy,et al.  "Measurements of surface dose using build-up curves" obtained with an extrapolation chamber. , 1975, Radiology.

[9]  J. Purdy,et al.  Build-up region of megavoltage photon radiation sources. , 1975, Medical physics.

[10]  Holt Jg,et al.  The extension of the concept of tissue-air ratios (TAR) to high-energy x-ray beams. , 1970 .

[11]  H. E. Johns,et al.  Currents induced in the dielectrics of fonization chambers through the action of highenergy radiation. , 1958, Radiation research.

[12]  J. Richardson.  Effect of chamber voltage on electron build-up measurements. , 1954, Radiology.