SU‐F‐T‐319: The Impact of Radiation Beam Obliquity and Air Gap Thickness On Optically Stimulated Luminescent in Vivo Dosimetry for Radiation Therapy

Abstract

Purpose:Optically‐stimulated luminescent dosimeters (OSLDs) are increasingly utilized for in vivo dosimetry of complex radiation delivery techniques. Measured doses, however, underestimate planned doses for plans that utilize thermoplastic mask immobilization. The purpose of this work was to quantify the effect of beam obliquity and air gap span between the mask and backscatter material, on measured‐to‐planned OSLD dose agreement.Methods:A previously‐used thermoplastic mask was cut, reheated, and flattened to form a 33 by 9 cm2 stage approximately 2 mm thick. Two OSLDs were placed on the stage on 5 cm of solid water, covered with 50 by 50 by 5 mm3 square of bolus, and scanned in the CT simulator. Plans were created with 10 by 10 cm2 open fields using 4, 6, 10, and 15 MV photon beams at 0°, 45°, and 90° incidence. The isocenter was placed between the OSLDs at 5 mm depth. Dose was calculated and averaged for two OSLDs. Artificial air gaps of 3, 5, 10, and 20 mm were introduced in the plan and dose was recalculated for each energy/angle/gap combination. The experimental setup was replicated on a linear accelerator and air gaps were introduced by “bridging” the thermoplastic stage across solid water plastic of varying thickness. Fields were delivered as planned. OSLDs were read 12–15 hours after irradiation.Results:Measured‐toplanned percent differences were constant with increasing gap thickness for 0° and 45° beam angles. At 90° and 0 cm gap, planned dose underestimated measured dose by 10–23% for all energies. This discrepancy decreased linearly to 0% with a 20 mm gap. OSLD signal did not decrease more than 6% for any gap span and energy.Conclusion:With the exception of parallel beam incidence, beam obliquity and air gap thickness did not have a substantial effect on measured‐to‐planned dose agreement.