SU‐GG‐T‐285: Experimental Techniques for the Determination of Relative Radiation Output From Small Stereotactic Radiosurgery Photon Beams

Abstract

Purpose: Accurate determination of radiation output from photon beams <3×3 cm2 has been hampered by the lack of an ideal dosimeter. Even with small volume ionization chamber, its relatively large aspect‐radio of active volume and the attached stem and electronic cable often introduce unintended error. The aim of this work was to investigate the utility of micro thermaluminescent dosimeter (TLD) in the experimental determination of output factor (OF) for small photon beams. Method and Materials: Output factor of 6MV high‐dose‐rate stereotactic photon beam with field sizes of 1×1 to 3×3 relative to 10×10 cm2 were measured. A 0.015 cm3 PTW PinPoint chamber with 2‐mm inner diameter and about 5‐mm long active volume and 1×1×1 mm3 micro TLD cubes were used. To minimize the volume‐averaging and stem effects, a new experimental setup that aligns the chamber's longitudinal axis to the beam's central axis was used. To minimize statistical uncertainties of TLD, a new approach that utilizes TLD's dose response rate measured for each field size was used for OF determination, instead of using the dose response for fixed beam‐on time. Results: Output factors measured by the TLD cubes (and PinPoint chamber) for 1×1, 2×2, and 3×3 cm2 beams were 0.753 (0.736), 0.859 (0.864), and 0.891 (0.901), respectively. The agreements were better than 2.5%. The TLD measured OF was consistently higher than that measured by PinPoint chamber for the 1×1 cm2 field. Conclusion: Micro‐TLD cube has the attributes of an ideal detector: tissue equivalent and free of stem and cable artifacts. The measured OF was comparable to that measured by a Pinpoint chamber. Volume‐averaging across the relatively larger chamber diameter could be responsible for the smaller chamber measured OF. Monte Carlo simulations using scoring volume equivalent to each detector's active volume are being performed to quantify this effect.