AbstractID: 11397 Title: Improving buildup region measurement accuracy using a surface location method Purpose: To determine the depth of the inflection point in the buildup region of photon beam depth-ionization curves for 11 cylindrical ionization chambers of differing designs and investigate dependencies of the inflection point on beam energy, electron contamination, and chamber design. Method and Materials: Each ionization chamber is carefully aligned to the water surface using a precision alignment telescope with a custom, high-precision beam scanning system. Following alignment, depth-ionization curves are obtained including scanning the chamber beyond the air-water interface for 6, 10, and 25 MV photon beams. Measurements are made with and without a 1 mm lead foil in the beam for a 10×10 cm2 field. The inflection point in the measured depth-ionization data is determined by finding the maximum in its second derivative. Results: The location of each chamber-specific inflection point is invariant to changes in beam energy, electron contamination, and chamber orientation within the 0.5 mm measurement resolution. Eight chambers, whose outer diameters are within 0.7 mm of each other, exhibit inflection points that are within 0.5 mm of one another. The location of each chamber’s inflection point is most strongly impacted by its outer radius and corresponds with the shallowest depth at which the chamber is fully submersed in the water, within 0.5 mm. Conclusion: The buildup region inflection point for a given cylindrical chamber is invariant to the beam variations studied and occurs at a depth equal to the chamber outer radius, within measurement resolution, for the 11 ionization chambers studied. Using this dependence, the inflection point can be used as a robust offline check to identify the water surface location and to identify inter-setup/inter-user setup variations. Higher quality surface localization can ease beam modeling and improve the quality of treatment planning data.