SU‐E‐T‐115: Dose Rate Dependence of OSLDs in Flattened and Flattening‐Filter Free Photon Beams

Abstract

Purpose: Optically stimulated luminescent dosimeters (OSLDs) are increasingly utilized for in vivo patient dosimetry. Current characterizations of OSLDs are limited to conventional dose rates and flat beams. The TrueBeam linac (Varian, Palo Alto, CA) is capable of producing flattening‐filter‐free (FFF) 6 MV beams with extremely high dose rates. The purpose of this work is to characterize OSLD dependencies on dose rate and presence/absence of flattening filters.Methods: OSLDs were placed at central axis of the TrueBeam accelerator on 5 cm solid water, covered with 1.5 cm bolus, and exposed to a range of doses (0, 100, 200, 300, 500, 800, 1100, and 2100 cGy) and dose rates (10, 200, 400, 600 cGy/min for conventional 6 MV, 400, 600, 1000, and 1400 cGy/min for 6 MV FFF). Twelve OSLDs were spaced 1 cm apart from left‐to‐right through the central axis to capture beam profiles of the flattened and FFF beam. OSLDs were read and corrected for dosimeter sensitivity and read‐out depletion. Linearity of counts with dose was assessed with regression. Profiles were compared with measurements from ion chamber array. Results: Comparable linearity up to 500 cGy and supralinearity from 500–2100 cGy was observed across all dose rates for both flattened and FFF beams. Slopes for all dose rates were within 2% for flattened beams and within 1% for FFF beams. Slopes for FFF beams were, on average, 3.7% higher than flattened beams. OSLD beam profiles approximated both flattened and FFF array profiles, but OSLDs read 5% greater than the array profile at the “horns” of the Purpose: In stereotactic radiosurgery (SRS), targets in the brain are located in CT or MR images and localized relative to external fiducial. Physicians need accurate assessments of the accuracy with which they can expect to hit targets in order to prescribe appropriate margins. We have developed a new tool which is set up for irradiation in a manner similar to a patient demonstrates the clinical accuracy of the imaging device. No commercial device currently exists which allows measurement of accuracy using both CT and MR planning images. Methods: The device contains radiosurgical targets visible by both CT and MR, with radiochromic film to capture the delivered dose distribution. The device consists of several thin acrylic plates holding non‐ferrous CT fiducial targets which pierce a piece of film. Adjacent to the film are channels filled with copper sulfate solution to provide MR fiducials. The assembly fits inside a water‐filled head phantom which can be secured to a SRS system. The phantom is imaged using CT or MR. Targets are determined using the treatment planning software and methods appropriate to the SRS device. Targeting accuracy is determined by measuring distances on the film between dose distribution centers, relative to the pinhole made by the CT fiducials. Results: Measurements of GK SRS accuracy with 4 mm and 8 mm collimation sizes show sub‐millimeter agreement for planned and delivered dose distributions with CT or MR imaging, in agreement with GK SRS manufacturer specifications. Conclusions: Assessing accuracy of dose delivery for GK or linac SRS can be limited by the use of one imaging modality for patient treatment. This phantom design enables the use of MR‐only, CT‐only, or MR‐CT‐combined image‐based assessment for GK or linac SRS approaches, providing physicists and radiation oncologists with basic accuracy information that is relevant to patient treatment.