SU‐E‐T‐71: An Investigation of Measurement Techniques for Small Field Dosimetry Using Commercially Available Detectors

Purpose: To identify and quantify the pros and cons of currently available commercial detectors and measurement techniques for small fields. Methods: Measurements are made for a TrueBeam STX (Varian Medical System) for 6X, 6XFFF, and 15XFFF beams. Four ion chambers, Iba cc01, cc04, cc13, and Exradin A16, and two diodes, Iba stereotactic diode (SD) and photon diode (PD), are used with an IBA blue phantom. For ion chambers, the recombination and polarity effects are evaluated based on the PDDs, while the out‐of field doses are compared with the diode measurements. The effects of finite detector size on penumbra are compared for field sizes from 3×3 to 10×10, while PDD are compared down to 1x1 cm2. Results: Recombination affects the PDDs by up‐to 0.5% and 3.1% at deeper depths for cc04 and cc13, respectively. Polarity effects for cc04 and cc13 are 2.0% and 0.3%, respectively for 40x40 cm2, while both cc01 and cc04 are within 0.5% for 10x10 cm2.The 3x3 and 5x5 cm2 PDD of cc04, cc01, A16 and SD are within 1% of each other but cc01, A16 and diodes over‐respond by 1– 2% for 10×10 cm2 compared to cc04. At 1×1 cm2, A16, cc01 and PD remains within 1% of each other while cc04 and SD show 1.3% over‐respond and 3.0% under‐respond at d20 relative to cc01. Relative to cc04, the relative out‐of‐field dose difference for PD, SD, cc01 and A16 are −0.4%, +0.8%, +0.5% and +0.3% respectively at dmax and 10×10 cm2. The penumbras measured by all the small detectors are between 0.6 to 1.0 mm sharper than cc04. Conclusion: No single detector is accurate over the entire range of field size. The effects of detector size, recombination and polarity effects, and sensitivity to scatter can be minimized by appropriate mixes of detector and measurement techniques. Research grant from Varian Medical Systems