Measurement of dose in the buildup region using fixed-separation plane-parallel ionization chambers

Accurate measurement of dose at the surface of a phantom and in the buildup region is a difficult task but one that is important for the proper treatment of patients. The instruments of choice for these measurements are extrapolation chambers but few institutions have these instruments at their disposal. As a result, fixed-separation plane-parallel ionization chambers are most commonly used for this purpose. Recent papers have re-emphasized the inaccuracies in the measurement of dose in the buildup region of normally incident photon beams when using fixed-separation plane-parallel ionization chambers. Data for Co-60, 6-, 10-, 18-, and 24-MV photon beams are presented that show the magnitude of this over response in the buildup region for several commercially available plane-parallel ionization chambers versus results obtained using both an extrapolation chamber and LiF thermoluminescent detectors. Differences in the percent depth dose at the surface of a phantom of greater than 19% were found for one of the chambers. All chambers over responded in the buildup region to some degree based upon their internal dimensions. The appropriateness of published corrections for these chambers is evaluated and guidelines for the accurate measurement of dose in the buildup region are presented.