Sci‐Sat AM: Radiation Dosimetry and Practical Therapy Solutions ‐ 06: Investigation of an absorbed dose to water formalism for a miniature low‐energy x‐ray source

Purpose: We present a formalism for calculating the absorbed dose to water from a miniature x‐ray source (The INTRABEAM system, Carl Zeiss), using a parallel‐plate ionization chamber calibrated in terms of air‐kerma. Monte Carlo calculations were performed to derive a chamber conversion factor (CQ) from reference air‐kerma to dose to water for the INTRABEAM. CQ was investigated as a function of depth in water, and compared with the manufacturer's reported value. The effect of chamber air cavity dimension tolerance was also investigated. Methods: Air‐kerma (Ak) from a reference beam was calculated using the EGSnrc user code cavity. Using egs_chamber, a model of a PTW 34013 parallel‐plate ionization chamber was created according to manufacturer specifications. The dose to the chamber air cavity (Dgas) was simulated both in‐air (with reference beam) and in‐water (with INTRABEAM source). Dose to a small water voxel (Dw) was also calculated. CQ was derived from these quantities. Results: CQ was found to vary by up to 15% (1.30 vs 1.11) between chamber dimension extremes. The agreement between chamber CQ was found to improve with increasing depth in water. However, in all cases investigated, CQ was larger than the manufacturer reported value of 1.054. Conclusions: Our results show that cavity dimension tolerance has a significant effect on CQ, with differences as large as 15%. In all cases considered, CQ was found to be larger than the reported value of 1.054. This suggests that the recommended calculation underestimates the dose to water.