Radiation detection materials for quality assurance and dose distribution assessment of small fields of high-energy X-rays

Abstract

This paper present a system for an integrated radiotherapy quality assurance (QA) protocol that maintains dose measurement accuracy through a dosimeter capable of accurate small-field dosimetry. The beam pulse data of a linear accelerator (LINAC) was obtained, and its conformity with the photoconductor material detection signal was evaluated. To determine the radiation detection material for dose measurement, unit-cell-type specimens were fabricated through sedimentation from four candidate materials (lead II iodide, PbI2; lead II oxide, PbO; mercury II iodide, HgI2; and a mixture of HgI2 and titanium dioxide, TiO2). Based on evaluations of dark current, output current, rising time (10%–90%), falling time, and response delay, the mixed TiO2and HgI2material exhibited the best characteristics. Accordingly, we fabricated 3×3 and 6×6 multi-pixel arrays capable of small-field dosimetry. The 3×3 multi-pixel array had a pixel size of 3 cm×3 cm; the thickness of the detection material was approximately 400 μm. LabVIEW was used to design the arrays. Analog signals generated in each pixel were converted to digital signals through an analog-to-digital converter, displayed in a waveform, and stored. The 6×6 multi-pixel array had pixels of diameter of 1 mm and pixel pitch of 1 mm; thickness of the detection material was approximately 485 μm. A LINAC was used to evaluate the arrays according to the detection characteristics and dose changes at acceleration voltages of 4, 6, 10, and 15 MV. Excellent reproducibility, linearity, and accuracy were confirmed by comparing the dose detector and response characteristics to actual cases. This study verifies the applicability of photoconductor-based solid-state detectors, which can replace existing dosimeters for small-field QA, and proposes a new protocol to simplify complex and difficult radiotherapy QA procedures, thereby increasing user convenience.