Design of parallel plate ion chambers for buildup measurements in megavoltage photon beams

Abstract

Dose measurements in the buildup region of megavoltage photon beams are most commonly made using parallel plate ion chambers having fixed electrode separation. Fixed-separation chambers generally do not read correctly under such beam conditions because of the contribution to the chamber signal of electrons from the side walls. In this work it is shown that the side wall error can be very large and published correction formulas are not accurate for all beam conditions and chamber geometries. The principal focus of this study has been to determine the design features of a fixed-separation chamber that has negligible side wall error. The approach has been to study, in beams of 60Co, 6 MV, and 18 MV, the response of a specially built ion chamber in which several chamber parameters could be independently varied. The study has shown that the side wall error is primarily dependent on the ratio of the electrode separation to the wall diameter as well as on the wall density and wall angle. Based on these findings the design of a fixed-separation chamber is described which reads to within about 1% of the correct dose. Guidelines are also provided for assessing the suitability of current commercial fixed-separation ion chambers for buildup measurements.

Surface dose variations in 6 and 10 MV flattened and flattening filter‐free (FFF) photon beams

“…In the first method, the chamber reading is corrected by a subtractive factor that depends on the beam energy, chamber geometry, and depth of measurement 15 , 16 , 17 , 18 . These factors are essentially independent of field size, but increase with decreasing energy.…”

Beam perturbation characteristics of a 2D transmission silicon diode array, Magic Plate

“…The Markus chamber is designed for the measurement of surface and buildup dose. Markus‐type chambers are known for their overresponse due to the large separation and their small guard ring, 44 , 45 and different methods have been developed to correct for this overresponse 46 , 47 , 48 , 49 . In this study, all measurements using the Markus ionization chamber were corrected using the Velkley correction as modified by Rawlinson to correct the overresponse of the Markus chamber.…”

Comparing dose in the build‐up region between compensator‐ and MLC‐based IMRT

“…The chamber readings were normalized to the readings obtained during the profiles matching procedure described above. The chamber readings were corrected using the Rawlinson method (11) to account for geometry and wall material density: $P\prime (d)=P(d)=\frac{C(E)l}{W}{\rho }^{0.8}{e}^{(\frac{‐4d}{d_{\text{max}}})}$where P / (d) is the corrected dose at depth d, P(d) is measured dose at d , the energy dependent factor C(E) is 0.27 for a 6 MV beam, (11) l is the plate separation, W is the inner wall diameter, ρ is the wall material density, d is the depth to the front surface of chamber, and $d_{\mathit{\text{max}}}$ is the depth of maximum dose.…”

“…An extrapolation chamber has been known to provide better dosimetric results (9) when compared to the parallel plate chambers, but it is bulky and time‐consuming to use. Velkey et al (10) and Rawlinson et al (11) have provided simple over‐response corrections for the parallel plate chambers.…”