The development of a cylindrical phantom for nanoDotTM calibration with multiple beam angles

Abstract

Background: The nanoDotTM dosimeter, an optically stimulated luminescent dosimeter (OSLD), is compact and precise, ideal for various applications like radiation dosimetry. The nanoDotTM requires calibration before use with the detector alignment perpendicular to the central beam axis. It exhibits angular dependence that may impact the calibration factor, requiring the fabrication of a specific cylindrical phantom for the calibration procedure. Objective: This study aimed to develop a cylindrical phantom for nanoDotTM calibration to facilitate dose measurements in composite fields with various beam angles and to evaluate the nanoDotTM calibration factors for different plans. Materials and methods: The cylindrical phantom was constructed using cast nylon material to accommodate the nanoDotTM or a cylindrical ionization chamber (IC). The novel phantom underwent validation for physical characteristics, including dimensions, density, and uniformity. Validation for the calibration factor, using cylindrical phantom (CF cylin) under standard conditions with a 10x10 cm² field size at 10 cm depth was conducted with 6 MV X-rays, comparing it with calibration factor using slab solid water phantoms (CFsolid). The CFcylin for different numbers of beams were determined and validated against a reference IC in various planning conditions. Furthermore, angular correction factors were determined for their application in the single-beam calibration factor. Results: The cylindrical phantom had dimensions of 20 cm in diameter and 30 cm in length, a density of 1.145 g/cm³ and good uniformity. As a result of single beam, the CF cylin, agreed well with CFsolid, showing a difference of -0.069%. The CFcylin increased with the number of beams, ranging between 1.179 and 1.242. Additionally, the angular correction factor increased as the number of beams increased, peaking at 1.058 with 9 beams. When comparing the results to the IC, it was observed that with an increase in the number of beams to 4 beams, the single-beam calibration factor exhibited a variation of more than 2%. However, when applying the CF cylin specific to the number of beams or correcting for the angular correction factor, the dose differences between nanoDotTM and IC measurements were within 2%. Conclusion: The developed cylindrical phantom is suitable for nanoDotTM calibration under single beam angle and in composite fields with various beam angles. The new calibration factors for specific numbers of beams allow for accurate dose measurements using nanoDotTM, thus reducing the dose difference from the IC to acceptable levels. Further studies should investigate its application in clinical situations.