SU‐E‐T‐246: Detector Selection for Small Field Measurements, a Comparative Study

Abstract

Purpose: To compare commercially available detectors for small field dosimetry. Methods: PTW (Freiburg, Germany) diode E, PTW microDiamond, and SunNuclear (Melbourne, FL) Edge detector were used to measure output factors, PDD, and profiles on a Varian iX linear accelerator (Varian Medical Systems, Palo Alto, CA) using 6MV and 18MV photon beams. The water tank was set to 100cm SSD. Centering profiles at 5cm and 20cm depth confirmed an offset <0.1mm. High resolution PDD (0.1mm) aligned the collecting volume at water surface. For setup and linearity verification, PDD and profiles were matched to annual measurements, which were performed with an A1SL ion chamber.Scans were taken for 6X and 18X, for 10cmx10cm, 10cmx2cm, 10cmx1cm, and 10cmx0.5cm fields. Profiles were measured at Dmax (6X=1.5cm, 18X=3.5cm), 5cm, 10cm, and 20cm depths. All fields used 10cmx10cm jaws with the MLC defining the field in the X‐direction. PDDs used 2mm step size and 0.2s discrete collecting time. Profiles used 1mm step size and a 0.4s discrete collecting time. Results: The PTW diode E and Edge detector exhibit centering stability, but all detectors require vertical shifts from Vendor defined vertical alignment. Output factor measurements reveal good agreement between detectors. The Edge detector demonstrates the sharpest penumbra and the closest consistent match to ion chamber measurements in the out of field regions for 6MV (3.2%). Each detector provides comparable curve quality for PDDs, with a maximum difference across detectors giving a standard deviation of 0.6% at 30cm depth, 18X, and 10cmx0.5cm. Profiles match closely across all detectors, depths, energies, and field sizes. The microDiamond detector produces the largest penumbra width, largest deviation from ion chamber dose in the tail region for 6MV, and exhibits centering instability. Conclusion: Based on our findings, the Edge detector was chosen for stereotactic measurements. The PTW diode E is an acceptable alternative.