MO‐E‐T‐617‐06: Influence of Ion Chamber Response On In‐Air Profile Measurements in Megavoltage Photon Beams
Purpose: To investigate the influence of the ion chamber response including build‐up cap materials, on the measurements of in‐air off‐axis ratio (OAR) profiles in megavoltage photon beams using Monte Carlo (MC) stimulations with the EGSnrc system. Method and Materials: Two new techniques were developed for the calculations of OARs when the ion chamber is oriented horizontally or vertically. For a horizontally oriented chamber pre‐calculated tables of the response of an ion chamber inserted in a build‐up cap for different photon energies was used to compute the dose deposited in the air cavity on‐the‐fly within the BEAMnrc simulation. For a vertically oriented chamber the BEAMnrc code was modified so that it can be compiled into a shared library that serves as a particle source for the CAVRZnrc user code. With these BEAMnrc and CAVRZnrc changes the OAR could be calculated on the fly without intermediate phase‐space file generation. Results of the simulations were compared with experimental profiles from the 6, 10 25 MV photon beams from an Elekta Precise linac. Results: The calculated and measured in‐air profiles for all investigated beams and build‐up caps (brass, hevimet and two PMMA miniphantoms) are in a good agreement within the statistical and experimental uncertainties. The comparison between the calculated air‐kerma and OAR profiles shows 3–6% differences between air‐kerma and in‐air profiles measured with hevimet and brass caps and 0.5–1% differences for measurements with PMMA mini‐phantoms. Conclusion: The change of chamber response with distance from the central axis must be taken into account. For in‐air profiles measurements PMMA mini‐phantoms should be recommended over high‐Z material build‐up caps.