Evaluation of extrapolation chamber response for surface and buildup dose assessment in radiotherapy photon beams using Monte Carlo simulations

BACKGROUND: Surface dose assessment is essential in radiotherapy, but accurately measuring doses in the buildup region of megavoltage beams presents significant challenges. Extrapolation chambers are frequently regarded as the most suitable detectors for this purpose. PURPOSE: To establish under what conditions extrapolation chambers can be used to measure surface doses and to use Monte Carlo calculations to prove that measured surface-to-maximum ionization ratios correspond to absorbed-dose ratios. A secondary purpose is to understand why surface measurements with a Markus fixed parallel-plate chamber are inaccurate. METHODS: ) as functions of electrode separation and depth in the buildup region were also investigated. RESULTS: and 3.0% and 3.4% for 25 MV, all within the first half of the buildup region. CONCLUSIONS: and extrapolation is only needed to establish where % ionization ratios become constant for small gaps and are hence accurate. Surface measurements with fixed parallel-plate chambers are inaccurate if the gap size is too large.