Improving the accuracy of entrance dosimetry measurements for dose modeling

Abstract

Due in part to measurement positional uncertainties, the entrance region of depth-ionization curves has challenged beam modelers - thus this region is often ignored in beam modeling despite the valuable dosimetric information contained within. In a previous work, for a single ionization chamber (IC), we demonstrated a method to reduce the positional uncertainty by scanning the IC beyond the water surface into the air and using the ionization inflection point to determine the water surface location. This study extends the previously developed surface location method to eleven additional cylindrical IC designs and investigates the functional dependencies of the inflection point. For the ICs studied, the relative in-air response is related to the IC's wall mass thickness. The inflection point is found to be invariant within measurement resolution (0.5 mm) to changes in beam energy, electron contamination, and field size. Inflection point locations are found to be strongly dependent on IC outer radius, with weaker dependencies on wall thickness and material composition. The inflection point determination technique is useful for accurately determining absolute IC position. Measurements taken with this improved positional accuracy should serve as more accurate inputs to treatment planning system dose calculation algorithms and as more accurate benchmarks for Monte Carlo beam model verification. © 2009 Springer-Verlag.