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

Surface dose assessment using extrapolation chambers can achieve accuracy within 1% when small electrode separations are used and depth is appropriately defined. The results confirm that the measured % ionization ratios for gaps s ≤ 1 s ≤ 1 mm are constant because perturbation effects are minimized. Extrapolation from larger gaps is unreliable due to increasing changes in P repl Pᵣₑₚₗ 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.