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Stereotactic Cone Output Factors of Flattening-Filter-Free Mode Photon Beams; a Plastic Scintillation Detector Measurement Study

J Kim*, D Kahn, X Qian, S Lu, A T Hsia, Z Xu, Stony Brook University Medical Center, Stony Brook, NY


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Tiny-volume diode detectors are most commonly used for stereotactic cone output factors, despite its known over-response in small field radiation beams, due to higher mass and electron densities than water. In this study, we present stereotactic cone collimator output factors of flattening-filter-free (FFF) photon beams measured with a near-water-equivalent plastic scintillation detector (PSD).
Methods: outputs of seven stereotactic cones, mounted on Edge and TrueBeam treatment machines (Varian Medical Systems, Palo Alto, CA) were measured for 6XFFF and 10XFFF photon beams using a micro PSD (1 mm dimeter x 1 mm length, Extradin Scintillator W2 1x1, Standard Imaging, Middleton, WI). The detector was placed at 5 cm depth with 95 cm source-to-surface distance in a stack of solid water phantoms. Measurements were repeated twice for reduced uncertainty. The measured outputs were compared to those of a diode detector (Edge, Sun Nuclear, Melbourne, FL) and derived diode correction factors.
Results: output factor difference between two machines were minimal (0.3% maximum). The average measured output factors for 6XFFF were 0.558 (4 mm cone), 0.611 (5 mm), 0.705 (7.5 mm), 0.766 (10 mm), 0.807 (12.5 mm), 0.835 (15 mm), and 0.853 (17.5 mm). The corresponding outputs for 10XFFF were 0.471, 0.530, 0.640, 0.717, 0.773, 0.815, and 0.845. The diode correction factors (PSD/diode) were respectively 0.91, 0.91, 0.94, 0.96, 0.98, 0.99, and 0.99 for 6XFFF and 0.90, 0.90, 0.92, 0.93, 0.95, 0.96, and 0.97 for 10XFFF.
Conclusion: measured PSD output factors were considerably lower than those of diode, especially for the 4 mm and 5 mm cones: 9 % for 6XFFF and 10% for 10XFFF. Small field dosimetry is challenging. Further cross-validation is needed with other high resolution detectors such as films and thermos-luminance detectors. Also, the cone output factors may vary from installation to installation.

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