Evaluation of extrapolation chamber response for surface and buildup dose assessment in radiotherapy photon beams...

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.

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.

EGSnrc applications were used for calculating the dose to the air cavity of two extrapolation chambers in a polystyrene phantom as a function of the electrode separation (also called gap), $s$ , between 0.1 and 10 mm. Doses to the phantom, $D_{\mathrm{pst}}$ , at depths corresponding to the effective point of measurement (EPOM) of the chambers were also calculated. Calculations were performed using clinical photon beams ( ${}^{60}\mathrm{Co}$ , 6, 10, and 25 MV) that were fully modeled using BEAMnrc. Calculated chambers' responses as a function of the gap were compared with experimental data from the literature. Variations of the replacement correction factor ( $P_{\mathrm{repl}}$ ) and the wall perturbation factor ( $P_{\mathrm{wall}}$ ) as functions of electrode separation and depth in the buildup region were also investigated.

Calculated %dose to the air in the chamber's cavity (% $D_{\mathrm{ch}}$ ) and measured % ionization from the literature at the polystyrene's surface (i.e., at a depth z = EPOM) agreed to within 2%-3% across all gap sizes and beam qualities. Differences between measured % ionization and calculated % $D_{\mathrm{pst}}$ at the phantom surface, were less than 4% (relative to $D_{\max }$ ). For small gaps  $\le 1$  mm, the calculated chamber response (% $D_{\mathrm{ch}}$ ) closely matched the calculated dose to the phantom material (% $D_{\mathrm{pst}}$ ), differing by less than 0.9% (relative to $D_{\max }$ ) with $\%D_{\mathrm{pst}}/\%D_{\mathrm{ch}}$ deviating from unity by 1%-6% depending on chamber's design and beam energy. At shallow depths (0.01 mm), $P_{\mathrm{repl}}$ exhibited the largest variation, increasing by 43% for gaps from 0.1 to 10 mm. In contrast, $P_{\mathrm{wall}}$ and stopping-power ratios varied by less than 2.0% and 1.2%, respectively. All correction factors remained approximately constant for small electrode separations ( $s\le 1$  mm). The Markus chamber exhibited maximum calculated and measured over-responses of 16.2% and 18.2% for ${}^{60}\mathrm{Co}$ and 3.0% and 3.4% for 25 MV, all within the first half of the buildup region.

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 $s\le 1$  mm are constant because perturbation effects are minimized. Extrapolation from larger gaps is unreliable due to increasing changes in $P_{\mathrm{repl}}$ 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.