Dose perturbations at interfaces: effect of ion chamber

Dose perturbations at interfaces between two dissimilar materials have been noted to be significant in kilovoltage beams and high atomic number media and can be divided into two parts: backscatter and forward scatter, known as BSDF and FDPF. The backscatter perturbation is extremely short range where as forward is relatively long range. The measurement of dose perturbation is extremely sensitive to the measuring devices. Large discrepancies have been noted among various investigators. Ion chambers are relatively simple and accurate detectors for dosimetry, which is attempted in this study, Especially designed parallel plate ion chamber having a window thickness of 0.9 μm along with other commercial chambers (5 μm-50 μm) were investigated for kilovoltage, Co-60 and megavoltage beams and at interfaces from aluminum to lead. Results show that the magnitude varies significantly among the ion chambers, There is some correlation between window thickness and magnitude but it alone cannot explain the large variations. The ion chamber measured dose perturbations are in the range of Monte Carlo simulation values, however, clear explanation cannot be given. It is concluded that a single system cannot provide accurate dose at interfaces. The different results obtained with various ion chambers indicate a chamber dependent fluence perturbation. In view of such observation, the geometry of the ion chambers may have to be included in the Monte Carlo simulations. This will be explored in future investigations with the first release of the EGSnrc Monte Carlo code