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

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.